Cardiac beta-receptor variation in rat strains selectively bred for differences in susceptibility to stress

The radioligand 3H-DHA was used to estimate the density and affinity of cardiac beta-receptors in rat strains selectively bred for differences in response to stress. Maudsley Reactive rats selected for heightened reactivity to stress had a greater density of beta-adrenergic binding in cardiac membranes than rats of two genetically distinct Maudsley Non-Reactive strains selected for decreased reactivity to stress, and compared with one of these Non-Reactive strains the MNR/Har, increased affinity for 3H-DHA. Together with previous findings the present results demonstrate a negative correlation between estimates of basal sympathetic activity on the on hand, and post-synaptic beta-receptors in heart on the other, that are consistent with the notion that these receptor alterations have occurred as a result of long-term differences in pre-synaptic release of transmitter. The Maudsley strains may, therefore, provide a useful model for the study of beta-adrenergic receptors as a physiological locus for regulation of end-target responsiveness to sympathetic stimulation.     

Adrenergic-cholinergic interactions in left atria: interaction of carbachol with alpha- and beta-adrenoceptor agonists

The purpose of the present investigation was to determine the nature of the functional interaction of muscarinic agonists with cAMP-generating and cAMP-independent agonists in left atria. Negative inotropic responses of rabbit isolated left atrial strips to the muscarinic agonist carbachol were measured in the absence and presence of equi-active inotropic doses of the beta-adrenoceptor stimulant isoproterenol (Iso), the mixed alpha- and beta-adrenoceptor stimulant phenylephrine (PE) plus 1 microM timolol to block the beta-receptor mediated component of its response, and elevated extracellular Ca2+. Carbachol produced dose-dependent negative inotropic responses in left atrial strips, which were much greater than control in the presence of either Iso, or PE plus timolol. However, carbachol responses were of a similar magnitude to the control in the presence of elevated extracellular Ca2+. In the presence of timolol, PE had no significant effect on cAMP levels in left atrial strips, and inotropic responses to carbachol alone and in combination with PE plus timolol were accompanied by significant increases in cGMP levels but no change in cAMP levels. Carbachol attenuated Iso-induced increases in cAMP levels, but decreases in left atrial tension were proportionally greater than the decreases in cAMP levels produced by carbachol in the presence of Iso. These results suggest that the antiadrenergic effects of muscarinic receptor stimulation may occur by a different mechanism in left atria than has been previously reported in ventricular muscle. While the nature of this mechanism is unknown, it may involve antagonism by muscarinic agents of both alpha- and beta-adrenoceptor mediated increases in Ca2+ influx.     

Brain Adrenoceptor Binding Sites in Mice Susceptible (DBA/2J) and Resistant (C57 Bl/6) to Audiogenic Seizures

We have examined if the age-related susceptibility of DBA/2J mice to audiogenic seizures is the result of an abnormality in the number or sensitivity of brain adrenoceptors. The binding of alpha 1-, alpha 2-, and beta-adrenoceptor ligands to membranes prepared from whole brain or regions of brain of DBA/2J mice was measured at various ages, corresponding to the periods before, during, and after the maximal sensitivity to audiogenic seizures. For comparison, we have studied concurrently age-matched C57 Bl/6 mice, a strain resistant to audiogenic seizures at all ages. There was no difference in the binding of alpha 2- or beta-adrenoceptor ligands to whole brain membranes between the two strains of mice at any age. The maximal number of alpha 1-adrenoceptor binding sites was lower in whole brains of DBA/2J mice than of C57 Bl/6 mice at all ages studied except 13-15 days of age. The differences were small (maximally 17%) but were statistically significant at 21-23 days of age, the time of maximal sensitivity of DBA/2J mice to audiogenic seizures. No difference between the two strains was found in the number or affinity of alpha 1- or alpha 2-adrenoceptor binding sites at any age in any of the brain regions studied. The age-related susceptibility of DBA/2J mice to audiogenic seizures is not the result of an abnormality in number or sensitivity of alpha 2- or beta-adrenoceptor binding sites, but a reduced number of alpha 1-adrenoceptor binding sites may be involved.     

The relationship between beta-adrenoceptor regulation and beta-adrenergic responsiveness in hepatocytes. Studies on acquisition, desensitization and resensitization of isoproterenol-sensitive adenylate cyclase in primary culture

The role of beta-adrenoceptor regulation in the mechanisms controlling beta-adrenergic responsiveness in hepatocytes was explored, using primary monolayer cultures. When plated in vitro, these cells gradually acquire a strong catecholamine-sensitive adenylate cyclase activity and an enhanced ability to bind the beta-adrenoceptor ligand [125I]iodocyanopindolol (125ICYP). Examination of the time course showed that the increase in the number of 125ICYP binding sites was detectable within 1-2 h of culturing and slightly preceded the elevation of isoproterenol-responsive activity. Then the responsiveness rose steeply and between about 5-24 h it closely followed the increase in beta-receptor binding. Addition of isoproterenol (10 microM) to cells after 20 h of culturing caused a rapid homologous desensitization of the adenylate cyclase (50% after about 5 min). This was paralleled by a down-regulation of beta-adrenoceptors measured both in membrane particles and in total cell lysates. Removal of isoproterenol led to a resensitization of the adenylate cyclase, which was rapid and protein-synthesis-independent after a brief (10-min) desensitization, or slow and cycloheximide-sensitive after prolonged (4-h) exposure to the agonist. In both cases an up-regulation of the 125ICYP binding paralleled the recovery from refractoriness. In contrast, no concurring changes in 125ICYP binding were measured when the beta-adrenoceptor-linked adenylate cyclase activity was enhanced by pretreatment with pertussin toxin (islet-activating protein, IAP) or was desensitized by exposure of the cells to glucagon or 8-bromo-cAMP; however, these modulations of the adenylate cyclase were nonselective, since the pretreatments with IAP, glucagon or 8-bromo-cAMP affected both isoproterenol-sensitive and glucagon-sensitive activities. The results suggest that, in hepatocytes, regulation at the beta-adrenoceptor level is a major determinant for both short-term and long-term selective changes of the beta-adrenergic responsiveness.     

Differences in central noradrenergic and behavioural responses of Maudsley non-reactive and Maudsley reactive inbred rats on exposure to an aversive novel environment

The present experiments compared the noradrenaline and behavioural responses of inbred Maudsley reactive (MR) and non-reactive (MNRA) rats when they are exposed to the light or dark arena of a light/dark shuttle-box. Behavioural scores confirmed that both strains of rats perceived the light arena to be more aversive than the dark one. Using in vivo microdialysis, exposure to the light, but not the dark, arena was found to increase noradrenaline efflux in both the frontal cortex and the hypothalamus of MNRA and MR rats. However, whereas the increase in the frontal cortex of both strains and the hypothalamus of MR rats was transient, the hypothalamic response in MNRA rats was maintained throughout exposure to the test zone. Strain differences in activity/visit and time/visit were evident but it was not possible to discern whether this could be attributed to the strain difference in the hypothalamic noradrenaline response. Nevertheless, it remains possible that, by comparison with MR rats, the prolonged noradrenaline response in the hypothalamus of MNRA rats could contribute to their well-documented, greater resistance to aversive environmental stimuli.     

Strain and sex differences in the cardiopulmonary adaptation of rats to high altitude

On chronic exposure to hypoxia, the commercially available Hilltop (H) strain of male Sprague-Dawley rats develops severe pulmonary hypertension, right ventricular hypertrophy (RVH), and polycythemia. These signs of chronic mountain sickness are associated with a high mortality rate. In contrast, the Madison (M) strain of Sprague-Dawley rats remains healthy with significantly less severe cardiopulmonary and hematological responses. Breeding experiments under locally controlled conditions were undertaken to determine if the differences between the two strains were genetically determined and to look for possible sex differences. Following 30 to 50 days exposure to a simulated altitude of 18,000 ft, the first generation of male H rats exhibited a higher right ventricular peak systolic pressure (RVPP), a more pronounced RVH, and a greater degree of polycythemia than the male M rats. The H rats had a mortality rate of 40% in contrast to a rate of 0% in the male M rats. The first generation of female H rats also developed a higher RVPP, a greater RVH, and more severe polycythemia than that in the female M rats. There were no differences in RVPP or RVH between the males and females of either strain. Females of both strains tolerated the hypoxic exposure with a 0% mortality rate. The data suggest that the differences between the males of H and M strains in their cardiopulmonary and hematological responses and in their susceptibilities to chronic hypoxia are genetic in nature. They further suggest that the female resistance to hypoxia is not due to milder cardiopulmonary responses. Perhaps female rats tolerate RVH better than male rats, at least of the H strain.     

Effects of insulin on adrenoceptor binding and the rate of catecholamine-induced lipolysis in isolated human fat cells

The mechanisms by which insulin inhibits catecholamine-induced lipolysis in fat cells are unknown. In this study the possible role of an interaction between insulin and the adrenoceptors on human fat cells was investigated. Insulin inhibited, in a dose-dependent fashion, the specific binding of hydrophobic as well as hydrophilic nonselective beta-receptor radioligands but had no effect on the binding of alpha 2-selective radioligands. The results of saturation experiments and competition-inhibition experiments under both equilibrium conditions and nonequilibrium conditions revealed that insulin reduced the total number of beta-adrenergic binding sites (maximum effect 25%) without changing the beta-adrenoceptor affinity. This insulin effect was rapid and reversible; one-third of the effect occurred within 1 min of incubation and it was completely reversed within 30 min after withdrawal of insulin. It could be mimicked by a polyclonal rabbit insulin receptor antibody but not by insulin mimickers acting distal to the initial interaction between the hormone and its specific insulin-receptor binding site. The beta-adrenoceptor binding to a plasma membrane-enriched fraction decreased at the same time as it increased to a microsomal enriched fraction after insulin treatment, indicating a redistribution of beta-adrenoceptors in the cell. In lipolysis experiments performed under conditions like those in the binding experiments, insulin inhibited the rate of lipolysis with a lag period of 3 min. Furthermore, the hormone caused a dose-dependent maximum 10-fold shift to the right of the dose-response curve for isoprenaline-induced lipolysis without changing the amplitude of the curve. This effect of insulin was specific for the beta-adrenergic receptors system, since insulin markedly decreased the amplitude of the dose-response curve for parathyroid hormone-induced lipolysis. In addition, the effect of insulin on isoprenaline-induced lipolysis could be mimicked by long-lasting fractional inactivation of the beta-adrenoceptors. The dose-response relationships for the inhibitory effects of insulin on beta-adrenoceptor binding and the lipolytic sensitivity to isoprenaline were almost identical. Half-maximum and maximum effects occurred at about 5 and 100 microunits/ml of insulin, respectively. In conclusion, the exposure of human fat cells to physiological insulin doses is followed by a rapid and dose-dependent translocation of beta-adrenoceptors from the exterior to the interior of the cell and a subsequent dose-dependent decrease in the lipolytic sensitivity to beta-adrenergic agonists, without a change in maximum lipolysis.(ABSTRACT TRUNCATED AT 400 WORDS)     

Hippocampal glucocorticoid receptor and behavior: a correlative study in rats and mice

A correlation has been demonstrated between binding capacity for [3H]corticosterone in the hippocampus and the performance in passive and active avoidance in the rat, and impaired behavior in adrenalectomized rats by exogenous corticosterone is restored. On this basis we have studied the possible correlation between strain-dependent behavioral differences and the glucocorticoid binding capacity in the hippocampus in mice and rats. In Naples high- (NHE) and low-excitability (NLE) rat strains, genetically selected on the basis of divergent locomotor activity upon forced exposure to a spatial novelty situation, no differences were found in glucocorticoid maximal binding capacity while both strains had a lower capacity than Naples random-breed (NRB) control rats. However, the intra-strain correlative analysis of hippocampal glucocorticoid receptors number and behavioral scores demonstrated that motor and emotional indexes of arousal to novelty were positively correlated in NLE-and negatively in NHE- while no correlation was present in NRB rats. Using two inbred strains of mice, C57BL/6 and Balb/c, extensively investigated for learning abilities, the lower active avoidance score of C57BL/6 was associated with a lower binding capacity for [3H]corticosterone in the hippocampus. Altogether the above results support the involvement of the hippocampal glucocorticoid receptor in the modulation of some adaptive behavioral responses, while do not prove that genetic differences in behavior rest on parallel differences in binding capacity for glucocorticoid hormone.     

Esmolol: effects on isoprenaline- and exercise-induced cardiovascular stimulation in conscious dogs

Esmolol, a recently developed ultra-short acting beta-adrenoceptor blocking agent, was evaluated in 12 conscious chronically instrumented dogs with intact autonomic reflexes. The significance of its beta 1-adrenoceptor selectivity was examined at various cardiovascular activation levels established by either incremental isoprenaline infusion or graded treadmill exercise. The observed parameters were heart rate, systolic and diastolic arterial blood pressure, left ventricular dp/dtmax, and left ventricular end-diastolic pressure. Intravenous infusion of esmolol (25 and 250 micrograms.kg-1.min-1) led to a dose-dependent reduction of the isoprenaline-induced increase in positive dp/dtmax. The concomitant increase in heart rate was suppressed to a lesser extent. Characteristically of a beta 1-selective agent, esmolol had only a slight effect on the isoprenaline-induced reduction in diastolic blood pressure. The impact of esmolol on exercise-induced hemodynamic activation was much smaller. Exercise-induced increase in positive dp/dtmax was more sensitive to beta-adrenoceptor blockade than the concomitant increase in heart rate. Diastolic blood pressure was not influenced significantly. beta-Adrenoceptor blockade was virtually reversed within 20 min of discontinuation of esmolol infusion.     

Differential pre- and postsynaptic effects of desipramine on cardiac sympathetic nerve terminals in RHF

Right heart failure (RHF) is characterized by chamber-specific reductions of myocardial norepinephrine (NE) reuptake, beta-receptor density, and profiles of cardiac sympathetic nerve ending neurotransmitters. To study the functional linkage between NE uptake and the pre- and postsynaptic changes, we administered desipramine (225 mg/day), a NE uptake inhibitor, to dogs with RHF produced by tricuspid avulsion and progressive pulmonary constriction or sham-operated dogs for 6 wk. Animals receiving no desipramine were studied as controls. We measured myocardial NE uptake activity using [(3)H]NE, beta-receptor density by [(125)I]iodocyanopindolol, inotropic responses to dobutamine, and noradrenergic terminal neurotransmitter profiles by glyoxylic acid-induced histofluorescence for catecholamines, and immunocytochemical staining for tyrosine hydroxylase and neuropeptide Y. Desipramine decreased myocardial NE uptake activity and had no effect on the resting hemodynamics in both RHF and sham animals but decreased myocardial beta-adrenoceptor density and beta-adrenergic inotropic responses in both ventricles of the RHF animals. However, desipramine treatment prevented the reduction of sympathetic neurotransmitter profiles in the failing heart. Our results indicate that NE uptake inhibition facilitates the reduction of myocardial beta-adrenoceptor density and beta-adrenergic subsensitivity in RHF, probably by increasing interstitial NE concentrations, but protects the cardiac noradrenergic nerve endings from damage, probably via blockade of NE-derived neurotoxic metabolites into the nerve endings.     

Endothelial nitric oxide synthase overexpression attenuates myocardial reperfusion injury

Previous studies indicate that deficiency of endothelial nitric oxide (NO) synthase (eNOS)-derived NO exacerbates myocardial reperfusion injury. We hypothesized that overexpression of eNOS would reduce the extent of myocardial ischemia-reperfusion (MI/R) injury. We investigated two distinct strains of transgenic (TG) mice overexpressing the eNOS gene (eNOS TG). Bovine eNOS was overexpressed in one strain (eNOS TG-Kobe), whereas the human eNOS gene was overexpressed in the other strain (eNOS TG-RT). Non-TG (NTG) and eNOS TG mice were subjected to 30 min of coronary artery occlusion followed by 24 h of reperfusion, and the extent of myocardial infarction was determined. Myocardial infarct size was reduced by 33% in the eNOS TG-Kobe strain (P < 0.05 vs. NTG) and by 32% in the eNOS TG-RT strain (P < 0.05 vs. NTG). However, postischemic cardiac function (cardiac output, fractional shortening) was not improved in the eNOS TG-Kobe mouse at 24 h of reperfusion [P = not significant (NS) vs. NTG]. In additional studies, eNOS TG-Kobe mice were subjected to 30 min of myocardial infarction and 7 days of reperfusion. Fractional shortening and the first derivative of left ventricular pressure were measured in eNOS TG-Kobe and NTG mice, and no significant differences in contractility were observed (P = NS) between the eNOS TG mice and NTG controls. Left ventricular end-diastolic pressure was significantly (P < 0.05 vs. NTG) reduced in the eNOS TG-Kobe strain at 7 days of reperfusion. The cardioprotective effects of eNOS overexpression on myocardial infarct size were ablated by Nomega-nitro-l-arginine methyl ester (300 mg/kg) pretreatment. Thus genetic overexpression of eNOS in mice attenuates myocardial infarction after MI/R but fails to significantly protect against postischemic myocardial contractile dysfunction in mice.     

Sex- and strain-dependent hepatic microsomal ethylmorphine N-demethylation in mice: the roles of type I binding and NADPH-cytochrome P-450 reductase

The roles of type I binding and NADPH-cytochrome P-450 reductase in ethylmorphine demethylation were investigated in two strains of mice, using sex differences in these activities as a tool. In the CPB-SE strain, females metabolize ethylmorphine faster than males. Sex differences in cytochrome P-450 content and endogenous NADPH-cytochrome P-450 reductase activity were too small to account for this. On the other hand, the differences in the magnitudes of type I spectra and ethylmorphine-induced enhancement of cytochrome P-450 reduction were considerable larger than those in the rates of demethylation. All parameters, except endogenous cytochrome P-450 reduction, were modified in a similar way by testosterone pretreatment: in females they were depressed to the male level, whereas in males they remained unchanged. Castration had no effect in females and enhanced the activities in males. The CPB-V strain exhibited little or no sex differences in ethylmorphine demethylation, cytochrome P-450 content and endogenous cytochrome P-450 reduction. Testosterone pretreatment had little or no influence on these activities. Type I binding and reductase stimulation, however, showed sex differences, comparable to those observed in the CPB-SE strain, which were also abolished by testosterone. A relationship between reductase stimulation and type I binding was observed, which was, apparently, independent of sex or strain. It is concluded that androgen primarily influences the amount of cytochrome P-450-substrate complex formed, but that the reduction of this complex is not rate-limiting in the demethylation of ethylmorphine.     

Measurement of strain in the left ventricle during diastole with cine-MRI and deformable image registration

The assessment of regional heart wall motion (local strain) can localize ischemic myocardial disease, evaluate myocardial viability, and identify impaired cardiac function due to hypertrophic or dilated cardiomyopathies. The objectives of this research were to develop and validate a technique known as hyperelastic warping for the measurement of local strains in the left ventricle from clinical cine-magnetic resonance imaging (MRI) image datasets. The technique uses differences in image intensities between template (reference) and target (loaded) image datasets to generate a body force that deforms a finite element (FE) representation of the template so that it registers with the target image. To validate the technique, MRI image datasets representing two deformation states of a left ventricle were created such that the deformation map between the states represented in the images was known. A beginning diastolic cine-MRI image dataset from a normal human subject was defined as the template. A second image dataset (target) was created by mapping the template image using the deformation results obtained from a forward FE model of diastolic filling. Fiber stretch and strain predictions from hyperelastic warping showed good agreement with those of the forward solution (R2=0.67 stretch, R2=0.76 circumferential strain, R2=0.75 radial strain, and R2=0.70 in-plane shear). The technique had low sensitivity to changes in material parameters (deltaR2= -0.023 fiber stretch, deltaR2=-0.020 circumferential strain, deltaR2=-0.005 radial strain, and deltaR2=0.0125 shear strain with little or no change in rms error), with the exception of changes in bulk modulus of the material. The use of an isotropic hyperelastic constitutive model in the warping analyses degraded the predictions of fiber stretch. Results were unaffected by simulated noise down to a signal-to-noise ratio (SNR) of 4.0 (deltaR2= -0.032 fiber stretch, deltaR2=-0.023 circumferential strain, deltaR2=-0.04 radial strain, and deltaAR2=0.0211 shear strain with little or no increase in rms error). This study demonstrates that warping in conjunction with cine-MRI imaging can be used to determine local ventricular strains during diastole.     

Myocardial responsiveness to isoproterenol and calcium: a comparison of SD and F344 rats

1. Inotropic effects of isoproterenol and extracellular Ca2+ were compared in left atrial muscle isolated from F344 and SD rats. Preparations from the F344 strain were more sensitive to the actions of both agents. 2. The chronotropic action of isoproterenol was not different in right atria isolated from the two strains. 3. This suggests that the strain-related difference in responsiveness to the inotropic effect of isoproterenol is not caused by heterogeneity in the beta-adrenoceptor/adenylate cyclase system but rather by variations in excitation-contraction coupling.     

Ureases of extreme halophiles of the genus Haloarcula with a unique structure of gene cluster

We searched for urease activities in 71 strains of extreme halophiles by a urea-phenol red-agar plate method. Positive strains were further investigated by measuring the ammonia released from urea in cell-free extracts. Only 4 strains of the genus Haloarcula, Har. aidinensis, Har. hispanica, Har. japonica, and Har. marismortui were finally shown as the urease producers. A partially purified urease from Har. hispanica was a typical halophilic enzyme in that it showed maximum activity at 18-23% NaCl and lost the activity irreversibly in the absence of NaCl. Partial genes (1596 bp) of the urease encoding from upstream of the beta subunit down to the N-terminal 139 amino acids of the alpha subunit, were PCR amplified from the four strains, as well as from five urease-negative Haloarcula strains. Strains of other genera, which were urease-negative, did not yield PCR products. The deduced amino acid sequences of the beta subunit and partial alpha subunit were similar to each other (92-100% similarities) and to those from other organisms. Analysis of the draft genome sequence of Har. marismortui, however, suggested that the order of the genes encoding the three subunits (with the total number of amino acids of 834) and four accessory proteins was beta-alpha-gamma-UreG-UreD-UreE-UreF. This order is quite unique, since in other microorganisms the order is gamma-beta-alpha-UreE-UreF-UreG-UreD in most cases. No open reading frames were detected in the PCR-amplified upstream of the beta subunit, suggesting that all Haloarcula species have the same unique structure of the urease gene cluster.     

Sex differences in left ventricular function and beta-receptor responsiveness following prolonged strenuous exercise.

Sex differences in neuroendocrine and metabolic responses to prolonged strenuous exercise (PSE) have been well documented. The aim of this investigation was to examine sex differences in left ventricular function and cardiac beta-receptor responsiveness following a single bout of PSE. Nine male and eight female triathletes were examined during three separate sessions: before, immediately after, and 24 h following a half-ironman triathlon using dobutamine stress echocardiography. Steady-state graded infusions of dobutamine were used to assess beta-receptor responsiveness. Slopes calculated from linear regressions between dobutamine doses and changes in heart rate and contractility for each participant were used as an index of beta-receptor responsiveness. Despite no change in preload, fractional area change decreased from baseline after the race in both men and women, with a greater decrease in men [men: 54.1% (SD 2.1) to 50.7% (SD 3.4) vs. women: 55.4% (SD 2.7) to 53.3% (SD 2.5); P < 0.05]. The amount of dobutamine necessary to increase heart rate by 25 beats/min [men: 29.6 microg x kg(-1) x min(-1) (SD 6.6) to 42.7 microg x kg(-1) x min(-1) (SD 12.9) vs. women: 23.5 microg x kg(-1) x min(-1) (SD 4.0) to 30.0 microg x kg(-1) x min(-1) (SD 7.8); P < 0.05] and contractility by 10 mmHg/cm2 [men: 20.9 microg x kg(-1) x min(-1) (SD 5.1) to 37.0 microg x kg(-1) x min(-1) (SD 11.5) vs. women: 22.6 microg x kg(-1) x min(-1) (SD 6.4) to 30.7 microg x kg(-1) x min(-1) (SD 7.2); P < 0.05] was greater in both men and women postrace. However, the amount of dobutamine required to induce these changes was greater in men, reflecting larger beta-receptor alterations in male triathletes following PSE relative to women. These data suggest that following an acute bout of PSE, male triathletes demonstrate an attenuated chronotropic and inotropic response to beta-adrenergic stimulation compared with female triathletes.     

Modulation of {beta}-adrenoceptor signaling in the hearts of 4-wk simulated weightlessness rats.

The modulation of beta-adrenoceptor signaling in the hearts of hindlimb unweighting (HU) simulated weightlessness rats has not been reported. In the present study, we adopted the rat tail suspension for 4 wk to simulate weightlessness; then the effects of simulated microgravity on beta-adrenoceptor signaling were studied. Mean arterial blood pressure (ABP), left ventricular pressure (LVP), systolic function (+dP/dtmax), and diastolic function (-dP/dtmax) were monitored in the course of the in vivo experiment. Single rat ventricular myocyte was obtained by the enzymatic dissociation method. Hemodynamics, myocyte contraction, and cAMP production in response to beta-adrenoceptor stimulation with isoproterenol or adenylyl cyclase stimulation with forskolin were measured, and Gs protein was also determined. Compared with the control group, no significant changes were found in heart weight, body weight and ABP, while LVP and +/-dP/dtmax were significantly reduced. The ABP decrease, LVP increase, and +/-dP/dtmax in response to isoproterenol administration were significantly attenuated in the HU group. The effects of isoproterenol on electrically induced single-cell contraction and cAMP production in myocytes of ventricles in the HU rats were significantly attenuated. The biologically active isoform, Gsalpha (45 kDa) in the heart, was unchanged. Both the increased electrically induced contraction and cAMP production in response to forskolin were also significantly attenuated in the simulated weightlessness rats. Above results indicated that impaired function of adenylyl cyclase causes beta-adrenoceptor desensitization, which may be partly responsible for the depression of cardiac function.     

Ventilatory and hematopoietic responses to chronic hypoxia in two rat strains.

Hilltop (H) and Madison (M) strains of Sprague-Dawley rats exhibit strikingly different susceptibilities to the effects of chronic altitude exposure. The H rats develop greater polycythemia, hypoxemia, and pulmonary hypertension. We studied ventilation, pulmonary gas exchange, tissue oxygenation, and hematologic adaptations in the two rat strains during a 50-day exposure to a simulated altitude (HA) of 5,500 m (18,000 ft). There were no strain differences among the variables we studied under sea level (SL) conditions. Within the first 14 days of hypoxic exposure, the only significant strain differences were that erythropoietin (EPO) rose much higher and erythroid activity was greater in the H rats, even though arterial Po2 and PCo2 (Pao2 and PaCo2, respectively), renal venous PO2 (Prvo2), and ventilation (VE) were equivalent in the two strains during this time. By day 14 at HA, the H rats had significantly higher erythroid activity, hematocrit (Hct), and EPO levels, significantly lower PaO2 and PrvO2, but equivalent VE and PaCO2. These changes persisted for the remainder of the exposure, except that the Hct continued to rise and the increase was greater in H rats. Despite the greater O2-carrying capacity of H rats in the later stages of hypoxic exposure, PaO2 and PrvO2 were significantly lower in H rats. There were no strain differences at either SL or HA in ventilatory responses to hypercapnia or hypoxia, in blood O2 affinity or 2,3-diphosphoglycerate, in extrarenal production of EPO, or in EPO clearance. We conclude that early in the hypoxic exposure the H rats produce more EPO at apparently equivalent levels of hypoxia, and this is the first step in the pathogenesis of the maladaptation to HA manifest by H rats. We find no consistent evidence that differences in VE contribute to the variable susceptibility to hypoxia in the two rat strains.     

DNA adducts in relation to lung tumour outcome are not markers of susceptibility following a single dose treatment of SWR, BALB/c and C57BL/6J mice with N-nitrosodiethylamine

The formation of DNA adducts by the covalent binding of genotoxic chemicals to DNA represents a valuable marker for assessing exposure to carcinogens but as yet the role of DNA adducts as a biomarker of carcinogenic susceptibility still needs to be clearly ascertained. To address this question an animal study was instigated using mice (SWR (high), BALB/c (intermediate) and C57BL/6J (low)) varying in their susceptibility to lung carcinogenesis. Groups of animals from each strain were dosed with a single intraperitoneal injection of saline or N -nitrosodiethylamine (NDEA) at 15 or 90 mg kg^-1 body weight. Lung and liver tissues were removed at different time points following dosing. Further groups of mice dosed with the same regime had urine samples collected 24 h post dosing and were then left up to 18 months to allow for the development of tumours. Immunoslot-blot analysis was used for the determination of N-7 ethylguanine (N-7EtG) and O⁶ ethylguanine (O⁶EtG) adduct levels in the DNA from the tissues and gas chromatography-mass spectrometry (GC-MS) was used to determine N-3 ethyladenine (N-3EtA) adduct levels in the urine samples. Levels of alkyltransferase (ATase) were also determined in the tissues. The results showed that the DNA adduct levels and persistence were similar across the three strains of mice following dosing with 15 and 90 mg kg^-1 NDEA. High levels of adducts were observed in the urine of the BALB/c strain, implying an increased metabolic or repair capacity in this strain. However there were no differences in the levels of ATase in the lung and liver of the three strains of mice following dosing with 15 mg kg^-1 NDEA. The incidence of tumours in C57BL/6J mice was lower compared with the other two strains and showed a dose dependent increase. The results from this study show that the differences in susceptibility to lung carcinogenesis between the three strains of mice do not appear to be linked to the formation of the two adducts detected. These results imply that dosing with NDEA resulted in toxicity which may have led to cell death and induction of tumours by compensatory cell proliferation. Although these results do not allow decisive conclusions to be drawn concerning the relationship between total levels of DNA adducts and differences in carcinogenic susceptibility for the three strains of mice it is clear that the increased presence of a DNA adduct in the target tissue increases the likelihood of tumour development.     

A Three-Dimensional Analytical (Rheological) Model of the Human Left Ventricle in Passive-Active States: Nontraumatic Determination of the In Vivo Values of the Rheological Parameters

In this paper a three-dimensional continuum model of a mammalian left ventricle is formulated. The stresses in the model satisfy the conditions of zero stress on the outer (epicardial surface-representing) boundary. The strains of the model are obtained from the actual dynamic geometry measurements (obtained from cineangiocardiography). Since the left ventricular muscle is incompressible, the dilatational strain is zero and hence the (three-dimensional) deviatric stress components are related to the corresponding strain components by Maxwell and Voigt rheological model analogues of one-dimensional systems; the parameters of the model are series and parallel elastic (SE, PE) elements and the contractile element (CE) (representing the sarcomere). The incorporation of the rheological features of the cardiac muscle into the three-dimensional constitutive equations (for the three-dimensional continuum model of the left ventricle) is a feature of this paper. A procedure is presented to determine the parameters of the constitutive equations (i.e., the SE, PE, and the parameters of the force-velocity relation for the CE) for the left ventricle of a subject from data on the dimensions and chamber pressure of the left ventricle. The values of these parameters characterize the rheology of the left ventricular muscle of the subject. In order to demonstrate clinical application of the analyses, in vivo data of the subjects' left ventricular pressure and dimensions are obtained, and the analyses are applied to the data to determine (for each subject) the values and characteristics of the elastic elements and CEs.