Evaluation of the pentose phosphate pathway from 14CO2 data. Fallibility of a classic equation when applied to non-homogeneous tissues.

Two cyclic nucleotide phosphodiesterase (PDE) activities were identified in pig aortic endothelial cells, a cyclic GMP-stimulated PDE and a cyclic AMP PDE. Cyclic GMP-stimulated PDE had Km values of 367 microM for cyclic AMP and 24 microM for cyclic GMP, and low concentrations (1 microM) of cyclic GMP increased the affinity of the enzyme for cyclic AMP (Km = 13 microM) without changing the Vmax. This isoenzyme was inhibited by trequinsin [IC50 (concn. giving 50% inhibition of substrate hydrolysis) = 0.6 microM for cyclic AMP hydrolysis in the presence of cyclic GMP; IC50 = 0.6 microM for cyclic GMP hydrolysis] and dipyridamole (IC50 = 5 microM for cyclic AMP hydrolysis in the presence of cyclic GMP; IC50 = 3 microM for cyclic GMP hydrolysis). Cyclic AMP PDE exhibited a Km of 2 microM for cyclic AMP and did not hydrolyse cyclic GMP. This activity was inhibited by trequinsin (IC50 = 0.2 microM), dipyridamole (IC50 = 6 microM) and, selectively, by rolipram (IC50 = 3 microM). Inhibitors of cyclic GMP PDE (M&B 22948) and of low Km (Type III) cyclic AMP PDE (SK&F 94120) only weakly inhibited the two endothelial PDEs. Incubation of intact cells with trequinsin and dipyridamole induced large increases in cyclic GMP, which were completely blocked by LY-83583. Rolipram, SK&F 94120 and M&B 22948 did not significantly influence cyclic GMP accumulation. Dipyridamole enhanced the increase in cyclic GMP induced by sodium nitroprusside. Cyclic AMP accumulation was stimulated by dipyridamole and trequinsin with and without forskolin. Rolipram, although without effect alone, increased cyclic AMP in the presence of forskolin, whereas M&B 22948 and SK&F 94120 had no effects on resting or forskolin-stimulated levels. These results suggest that cyclic GMP-stimulated PDE regulates cyclic GMP levels and that both endothelial PDE isoenzymes contribute to the control of cyclic AMP.     

Modulation of positive inotropy and metabolic coronary dilatation by myocardial alpha-adrenoceptors

Cardiac hyperactivity and its consequent metabolically induced coronary vasodilation (MCD) were studied in isolated, perfused, electrically paced rat hearts. The alpha-adrenoceptor agonists, phenylephrine and methoxamine, produced a concentration-dependent inhibition of the inotropic responses to noradrenaline, dobutamine, isoprenaline, tyramine, and glucagon, while relatively potentiating their MCD reactions. This inhibition was unrelated to the alpha-agonists' known inotropic action and was not affected by catecholamine depletion of the heart. Withdrawal of the alpha-agonists or administration of the alpha-adrenoceptor antagonists phentolamine, phenoxybenzamine, or prazosin returned the inotropic and MCD reactions to normal. Neither the MCD response to electrically induced tachycardia nor the inotropic reactions produced by calcium chloride were affected by alpha-adrenoceptor agonists or antagonists. Alone, alpha-adrenoceptor antagonists were shown to potentiate the inotropic responses to noradrenaline and isoprenaline while the MCD was relatively diminished. The responses to glucagon were unaltered by alpha-antagonists. We postulate that myocardial reactivity to sympathetic stimulation can be modulated through alpha-adrenoceptors by the inhibition of processes that mediate cardiostimulation at post-beta-adrenoceptor sites, together with facilitation of those leading up to MCD. Accordingly, this modulation would act to prevent ischaemic damage to the heart by acting to limit the inotropic responses to increasing sympathetic stimulation while maximizing the blood supply to the myocardium.     

The inotropic and chronotropic responses of the guinea pig and dog myocardium to isoprenaline and salbutamol.

The relative effects of isoprenaline and salbutamol on the inotropic and chronotropic responses of the denervated myocardium of the chloralose anesthetized dog and of the isolated guinea pig atrium, and the inotropic response of the isolated dog papillary muscle were studied. Both the in vivo dog heart and the in vitro guinea pig atrium displayed a similar relative response pattern to isoprenaline and salbutamol with regard to their inotropic and chronotropic responses. However, a comparison of the relative inotropic responses of the dog heart in vivo and in vitro showed that in vitro, salbutamol has a much lower affinity and efficacy for the adrenergic receptors than isoprenaline.     

Species-specific hypoglycemic activity of triphenylphosphoranylideneacetophenones.

The species-specific hypoglycemic activity of two 2-triphenylphosphoranylideneacetophenones is described. 2-Triphenylphosphoranylideneacetophenone (SK&F 45359) and 2-triphenylphosphoranylidene-m-trifluoromethyl-acetophenone (SK&F 62775) were hypoglycemic in various rat models, but failed to exhibit hypoglycemic activity in other species.     

Effects of the thromboxane receptor antagonist SK&F 88046 in the canine, monkey and human coronary vasculature

U-46619, a stable "functional" thromboxane/endoperoxide receptor agonist, produced potent contractile responses in isolated canine, rhesus monkey and human left circumflex coronary arteries (EC50 = 9.11 x 10(-9)M, 1.98 x 10(-8)M and 3.50 x 10(-9)M, respectively). Canine intrapulmonary veins were also contracted potently by U-46619 (EC50 = 1.22 x 10(-9)M). SK&F 88046, a thromboxane A2 (TxA2) end-organ receptor antagonist, blocked the vasoconstrictor effects of U-46619 in the canine circumflex artery (KB = 1.33 x 10(-8)M), canine intrapulmonary vein (KB = 1.46 x 10(-9)M), monkey circumflex artery (KB = 8.47 x 10(-8)M), and human circumflex artery (KB = 8.49 x 10(-7)M). SK&F 88046 was 10-60 times more potent in the canine and rhesus monkey coronary vasculature than in the human coronary preparations. Intracoronary administration of U-46619 to anesthetized, open chest dogs produced a dose-related decrease in left circumflex coronary artery blood flow which resulted in decreases in left ventricular developed pressure, left ventricular positive and negative dP/dt, ascending aortic blood flow, and an increase in left ventricular end-diastolic pressure. The decrease in coronary blood flow and the hemodynamic changes were either attenuated or completely inhibited by i.v. administration of SK&F 88046 (2.5 mg/kg + 0.05 mg/kg/min or 5.0 mg/kg + 0.1 mg/kg/min). SK&F 88046 was compared to two other TxA2 receptor antagonists in canine isolated intrapulmonary veins. SQ 29,548 was approximately 2-times more potent than SK&F 88046 as an antagonist of U-44619 mediated contractions (KB = 7.0 x 10(-10)M). In contrast, BM 13.177 was 150-fold less potent (KB = 2.19 x 10(-7)M) than SK&F 88046. Thus, the present study demonstrates species variability in response to TxA2 agonists and antagonists and reconfirms the relative importance of species selection in studying these agents.     

Inhibition of the cardiac inotropic effect of phenylephrine by a tetramine disulfide with irreversible alpha-adrenoceptor blocking activity.

The newly synthesized alpha-adrenoceptor blocking drug BHC (N,N'-bis[6-(10-methoxybenzyl-amino)-a-hexyl]cystamine) was found to block irreversibly the positive inotropic effect of the sympathomimetic drug phenylephrine on the isolated rat left atrium. BHC was used to test the adrenoceptor interconversion hypothesis which claims that low temperature converts inotropic beta-adrenoceptors in rat atrium and frog ventricle to alpha-adrenoceptors. There was no evidence of adrenoceptor 'interconversion.' In the rat atrium low temperature did not increase the BHC antagonism of phenylephrine and did not cause BHC to inhibit the inotropic effect of noradrenaline or isoprenaline. In the perfused frog heart low temperature did not lead BHC to inhibit the inotropic effect of phenylephrine, adrenaline, or isoprenaline.     

SK&F 96365, a novel inhibitor of receptor-mediated calcium entry.

A novel inhibitor of receptor-mediated calcium entry (RMCE) is described. SK&F 96365 (1-(beta-[3-(4-methoxy-phenyl)propoxy]-4-methoxyphenethyl)-1H- imidazole hydrochloride) is structurally distinct from the known 'calcium antagonists' and shows selectivity in blocking RMCE compared with receptor-mediated internal Ca2+ release. Human platelets, neutrophils and endothelial cells were loaded with the fluorescent Ca2(+)-indicator dyes quin2 or fura-2, in order to measure Ca2+ or Mn2+ entry through RMCE as well as Ca2+ release from internal stores. The IC50 (concn. producing 50% inhibition) for inhibition of RMCE by SK&F 96365 in platelets stimulated with ADP or thrombin was 8.5 microM or 11.7 microM respectively; these concentrations of SK&F 96365 did not affect internal Ca2+ release. Similar effects of SK&F 96365 were observed in suspensions of neutrophils and in single endothelial cells. SK&F 96365 also inhibited agonist-stimulated Mn2+ entry in platelets and neutrophils. The effects of SK&F 96365 were independent of cell type and of agonist, as would be expected for a compound that modulates post-receptor events. Voltage-gated Ca2+ entry in fura-2-loaded GH3 (pituitary) cells and rabbit ear-artery smooth-muscle cells held under voltage-clamp was also inhibited by SK&F 96365; however, the ATP-gated Ca2(+)-permeable channel of rabbit ear-artery smooth-muscle cells was unaffected by SK&F 96365. Thus SK&F 96365 (unlike the 'organic Ca2+ antagonists') shows no selectivity between voltage-gated Ca2+ entry and RMCE, although the lack of effect on ATP-gated channels indicates that it discriminates between different types of RMCE. The effects of SK&F 96365 on functional responses of cells thought to be dependent on Ca2+ entry via RMCE were also studied. Under conditions where platelet aggregation is dependent on stimulated Ca2+ entry via RMCE, the response was blocked by SK&F 96365 with an IC50 of 15.9 microM, which is similar to the IC50 of 8-12 microM observed for inhibition of RMCE. Adhesion and chemotaxis of neutrophils were also inhibited by SK&F 96365. SK&F 96365 is a useful tool to distinguish RMCE from internal Ca2+ release, and to probe the role of RMCE in mediating functional responses of cells. However, SK&F 96365 is not as potent (IC50 around 10 microM) or selective (also inhibits voltage-gated Ca2+ entry) as would be desirable, so caution must be exercised when using this compound.     

The indirect negative inotropic effects of the P1-receptor agonist, L-phenylisopropyladenosine, in guinea-pig isolated cardiac preparations: comparison with cromakalim.

The possible mechanisms of the indirect negative inotropic responses to the P1-receptor agonist, L-phenylisopropyladenosine (L-PIA) were evaluated in electrically paced (2 Hz, 5 ms pulse width, voltage 50% above threshold) left atria and papillary muscles of guinea pigs. The responses were compared in naive tissues (direct effects) or after prestimulation with submaximal concentrations of either cAMP-dependent positive inotropes (isoprenaline or forskolin) or the cAMP-independent inotrope Bay K 8644. Cumulative concentration-response curves were obtained in naive or prestimulated preparations for L-PIA or the potassium channel activator, cromakalim, for comparison. L-PIA and cromakalim exerted negative inotropy in naive atrial tissues, whereas only cromakalim was active in naive papillary muscles. In atria prestimulated with isoprenaline (31 nM) or forskolin (1.4 microM), the negative inotropy of L-PIA was enhanced compared with naive tissues. In contrast, prestimulation with Bay K 8644 (1 microM) exerted a significant functional antagonism of the response to L-PIA. In the case of cromakalim, prestimulation with isoprenaline exerted a functional antagonistic effect. In papillary muscles, an indirect negative inotropic effect of L-PIA was only seen in tissues prestimulated with the cAMP-dependent inotropes isoprenaline (31 nM) or forskolin (2.4 microM), and not in naive tissues or those prestimulated by Bay K 8644 (333 nM). As with atria, prestimulation with isoprenaline exerted a functional antagonistic effect on the response to cromakalim. These results suggest that the P1-receptor agonist, L-PIA, exerts its indirect negative inotropic effects in left atria by two mechanisms.2+ with cAMP-dependent positive inotropes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Systemic NO synthase inhibition blunts the chronotropic, but not the inotropic response to isoprenaline in man.

There is evidence that nitric oxide (NO) is involved in the chronotropic, the inotropic, and the vasodilator response to beta-adrenoceptor agonists. In the present study we hypothesized that inhibition of NO synthase may modulate the systemic vascular and cardiac effects of isoprenaline, a beta-adrenoceptor agonist, in healthy subjects. Subjects received stepwise increasing doses of isoprenaline (0.1-0.8 microg/min) in the absence or presence of systemic NO-synthase inhibition using two intravenous doses of N-monomethyl-L-arginine (L-NMMA; dosage 1, 3.0 mg/kg over 5 min, followed by 30 microg/kg/min over 75 min; dosage 2, 6.0 mg/kg over 5 min, followed by 60 microg/kg/min over 75 min) or peripheral vasoconstriction using exogenous endothelin-1 (ET-1; 5.0 ng/kg/min for 80 min). The chronotropic (RR interval) and the inotropic (QS2c) responses were assessed by noninvasive measurement of systolic time intervals. L-NMMA alone did not influence QS2c, but did increase the RR interval (P < 0.001) and the mean arterial blood pressure (P = 0.003). L-NMMA did not attenuate the blood pressure and the QS2c responses to isoprenaline, but significantly and dose-dependently blunted the heart rate response to beta-adrenoceptor stimulation (P = 0.029). ET-1 decreased the RR interval (P < 0.001) and increased the mean arterial blood pressure (P = 0.028). Our results indicate that beta-adrenoceptor mediated effects on the heart rate are much more susceptible to NOS inhibition than inotropic responses. This indicates that NO has an important role in heart rate control during beta-adrenoceptor stimulation.     

Interactions between prostaglandins E2 and F2alpha and isoprenalin in the dog heart in situ]

The effects of PGE2 and PGF2alpha on the isoprenaline induced elevation of myocardial contractility were studied on hearts of anesthetized dogs. The steady state effects were determined after single or simultaneous infusion of the substances. Two indices of contractility were used for the quantification of inotropic effects. PGE2 but not PGF2alpha showed positivinotropic effects. PGE2 significantly inhibited the inotropic responses induced by isoprenaline (p less than 0,01); whereas PGF2alpha was without any effect in this direction. The results are discussed with respect of a postjunctional action of PGE2.     

Species differences in the positive inotropic response to DPI 201-106, a novel cardiotonic agent

The positive inotropic activity of the novel cardiotonic DPI 201-106 was investigated in rat and guinea pig isolated hearts. For comparative purposes, the adenylate cyclase stimulant forskolin and the sodium channel agonist veratridine were also evaluated in both species. DPI 201-106 and veratridine produced greater inotropic effects in rat hearts than in guinea pig hearts, whereas forskolin produced comparable effects. In both species the inotropic response to DPI 201-106 and veratridine, but not forskolin, was reversed by the sodium channel antagonist tetrodotoxin. These results confirm that the positive inotropic effect of DPI 201-106 is due to stimulation of the sodium channel and demonstrate for the first time that species differences exist in the inotropic response to this novel cardiotonic drug.     

Relative resistance of functional beta2-adrenoceptor-mediated smooth muscle responses to in vitro desensitization.

The effects of in vitro incubation of rat isolated left atria, pulmonary artery rings, and aortic rings with isoprenaline (10(-6) M for 6 h) were examined to compare the degree of desensitization of beta1- and beta2-adrenoceptor-mediated functional responses. The experimental protocols were carefully controlled to exclude influence from persistence of agonist in the tissues after the prolonged exposures, time-dependent changes in tissue sensitivity, and the methods of plotting the data. Concentration-response curves for isoprenaline were constructed before incubation with isoprenaline and, after washout during 1 h, a second curve was obtained. Two protocols were employed: firstly, the preincubation curve was constructed to ensure that a maximum response was obtained (>10(-6) M) and, secondly, the preincubation curve was constructed to a maximum isoprenaline concentration of 10(-6) M. Preincubation curves were corrected for time-dependent changes in sensitivity from sham-incubation control experiments. There was significant desensitization of the beta1-adrenoceptor-mediated positive inotropic responses of the left atria, using both protocols, seen as rightward shifts (dose ratios: 4.48 +/- 1.12 and 8.39 +/- 2.3) of the concentration-response curves and depression of the maximum responses (77.0 +/- 3.2 and 60.8 +/- 5.5%). In contrast, the beta2-adrenoceptor-mediated relaxations of the noradrenaline-constricted pulmonary artery and aorta did not display a significant loss of sensitivity. When the relaxation responses were plotted as a percentage of the noradrenaline-induced tone, there was no significant rightward shift of the concentration-response curves in the pulmonary artery (dose ratios: 2.82 +/- 1.33 and 2.24 +/- 0.62) or aorta (dose ratios: 1.43 +/- 0.62 and 1.31 +/- 0.27) and thus no desensitization.     

Cardiac adrenoceptors at low temperature: what is the experimental evidence for the adrenoceptor interconversion hypothesis?

Isolated heart preparations of frog and rat were used to test the validity of the adrenoceptor interconversion hypothesis. This hypothesis claims that low temperature converts the inotropic beta-adrenoceptors in isolated frog and rat heart to alpha-adrenoceptors. The present results do not support the adrenoceptor interconversion hypothesis. In the isolated frog ventricle, lowering the temperature from 24 C to 14 C did not significantly alter the inotropic potency of the sympathomimetic drugs isoprenaline, epinephrine, and phenylephrine and did not reduce the potency of the beta-adrenoceptor blocking drug propranolol as an epinephrine antagonist. In the isolated rat left atrium, lowering the temperature from 31 C to 17-19 C did not significantly change the inotropic potency of isoprenaline, norepinephrine and phenylephrine, did not diminish the potency of propranolol, and did not increase the potency of the alpha-adrenoceptor blocking drug phentolamine.--Benfey, B. G. Cardiac adrenoceptors at low temperature; what is the experimental evidence for the adrenoceptor interconversion hypothesis?     

Selective antagonism of peptidoleukotriene responses does not reduce myocardial damage or neutrophil accumulation following coronary artery occlusion with reperfusion

This study was designed to assess the effect of a peptidoleukotriene receptor antagonist, SK&F 104353, for limiting myocardial damage and neutrophil accumulation in rats subjected to myocardial reperfusion injury (MI/R). In conscious rats, SK&F 10,4353 (25 mg/kg, i.v.) antagonized LTD4-induced vasopressor responses by 90% and 60% at 1 and 4 hr, respectively, indicating effective blockade of peptido-leukotriene responses. In another group of animals subjected to 30 min of coronary artery occlusion with reperfusion for 24 hr, myocardial injury and neutrophil infiltration were determined by measuring creatine phosphokinase (CPK) specific activity and myeloperoxidase (MPO) activity, respectively, in the left ventricular free wall (LVFW). Myocardial CPK levels were 8.1 +/- 0.2 U/mg protein in Sham-MI/R vehicle-treated animals, and were significantly decreased to 6.4 +/- 0.6 U/mg protein in MI/R-vehicle animals. Myocardial MPO values were 1.5 +/- 0.5 U/g LVFW in Sham-MI/R vehicle-treated animals, and significantly increased to 4.3 +/- 0.6 U/g LVFW in MI/R-vehicle animals. Administration of SK&F 10,4353 (25 mg/kg, i.v.) 1 min prior to coronary occlusion and 3.5 hr post reperfusion had no effect on the loss of myocardial CPK specific activity or the increase in MPO levels (p greater than 0.05, compared to the MI/R-vehicle group). Thus, at a dose that antagonized LTD4-induced vasopressor responses, SK&F 104353 did not attenuate either the extent of myocardial injury or inflammatory cell accumulation associated with myocardial ischemia/reperfusion. These results suggest that peptidoleukotrienes do not contribute to the progression of myocardial ischemic/reperfusion injury.     

Characterization of the hemodynamic activities of fenoldopam and its enantiomers in the dog

Fenoldopam (SK&F 82526) is a potent and selective dopamine DA-1 agonist with demonstrated renal vasodilator and antihypertensive activities in experimental animals and humans. Fenoldopam is a racemic mixture of two enantiomers, SK&F R-82526 and SK&F S-82526. The R-enantiomer is uniformly reported to be more potent than the racemate; in contrast, there is controversy regarding potency of the S-enantiomer. In these studies, the renal and systemic hemodynamic activities of fenoldopam and its enantiomers are characterized in anesthetized, phenoxybenzamine-treated dogs. The results show that the renal and systemic vasodilator activities of fenoldopam are properties of the R-enantiomer; the S-enantiomer is essentially inactive. The renal and systemic vasodilator properties of SK&F R-82526 are antagonized in a competitive fashion by the DA-1 antagonist, SK&F R-83566, but not the DA-2 antagonist, domperidone. Ganglionic blockade did not attenuate renal vasodilation associated with SK&F R-82526. Thus, the mechanism of SK&F R-82526-associated vasodilation, like that previously established for fenoldopam, is via stimulation of postganglionic DA-1 receptors.     

Reversal of mu-opioid-mediated respiratory depression by alpha 2-adrenoceptor antagonism.

The present study was performed in order to evaluate the effects of the selective alpha 2-adrenoceptor antagonist 6-chloro-2,3,4,5-tetrahydro-3-methyl-1H-3-benzazepine (SK&F 86466) on dermorphin-induced analgesia, respiratory depression and inhibition of locomotor activity in the conscious rat. Intracerebroventricular (icv) administration of dermorphin (3 nmol/rat) decreased respiration rate and relative ventilatory minute volume maximally by 38% and 50% of baseline respectively. SK&F 86466 dose-dependently reversed the dermorphin-induced depression of ventilatory parameters, while SK&F 86466 exerted no effect on dermorphin-induced analgesia or depression of locomotor activity due to catalepsia. It appears, therefore, that alpha 2-adrenoceptors selectively interact with mu 2-opioid-receptor mediated effects, such as respiratory depression, but are not involved in the modulation of mu 1-opioid-related effects, such as supraspinal analgesia and depression of locomotor activity.     

Regulation of agonist and antagonist binding to striatal D-1 dopamine receptors: studies using the selective D-1 antagonist [3H]SK&F R-83566

The potent and D-1 versus D-2 selective dopamine receptor antagonist, SK&F R-83566, was radiolabelled with tritium and was used as a radioligand for examination of D-1 receptors in rat striatum. Binding of the radioligand was stereoselective, saturable and reversible. In homogenates of rat striatum, nonspecific binding of the radioligand was less than 5% of total binding, the KD was 1.1 +/- 0.2 nM and the Bmax was 1130 +/- 70 fmoles/mg protein. Results of competition binding analyses yielded a pharmacological profile that was characteristic of dopamine D-1 receptor interaction. Competition studies of dopamine agonists against the potent antagonist radioligand indicated multiple affinities of agonist binding to the D-1 receptor. Displacement was best fit to a two-site model of ligand binding and high and low affinities were subject to regulation by guanine, but not adenine, nucleotides. Antagonist binding was not complex and was unaffected by guanine nucleotides. The role of monovalent cations in regulating D-1 receptor binding was evaluated by comparing effects of Na+, Li+, and K+ on binding of the antagonist [3H]SK&F R-83566 and the agonist [3H]fenoldopam (SK&F 82526). Whereas agonist binding was reduced in a concentration dependent fashion by monovalent cations with a ranking of potency Li+ greater than Na+ greater than K+, antagonist binding was enhanced by the cation Na+ but little affected by Li+ or K+. This effect of relatively low concentrations of Na+ to decrease agonist binding and increase antagonist binding suggests similarities between the D-1 receptor which is positively-coupled to adenylate cyclase and other receptors, e.g. alpha 2 adrenergic receptors, which are negatively-coupled to adenylate cyclase.     

Selective antagonism of peptidoleukotriene response does not reduce myocardial damage opr neutrophil accumulation following coronary artery occlusion with reperfusion

This study was designed to assess the effect of a peptidol eukotriene receptor antagonist. SK&F 104353, for limiting myocardial damage and neutrophil accumulation in rats subjected to myocardial reperfusion injury (MI/R). In conscious rats, SK&F 104353 (25 mg/kg, i.v.) antagonized LTD₄-induced vasopressor responses by 90% and 60% at 1 and 4 hr, respectively, indicating effective blockade of peptidol eukotriene responses. In another group of animals subjected to 30 min of coronary artery occlusion with reperfusion for 24 hr, myocardial injury and neutrophil infiltration were determined by measuring creatine phosphokinase (CPK) specific activity and myeloperoxidase (MPO) activity, respectively, in the left ventricular free wall (LVFW). Myocardial CPK levels were 8.1 ± 0.2 U/mg protein in Sham-MI/R vehicle-treated animals, and were significantly decreased to 6.4 ± 0.6 U/mg protein in MI/R-vehicle animals. Myocardial MPO values were 1.5 ± 0.5 U/g LVFW in Sham-MI/R vehicle-treated animals, and significantly increased to 4.3 ± 0.6 U/g LVFW in MI/R-vehicle animals. Administration of SK&F 105353 (25 mg/kg, i.v.) 1 min prior to coronary occlusion and 3.5 hr post reperfusion and no effect on the loss of myocardial CPK specific activity or the increase in MPO levels (p > 0.05, compared to the MI/R-vehicle group). Thus, at a dose that antagonized LTD₄-induced vasopressor responses, SK&F 104353 did not attenuate either the extent of myocardial injury or inflammatory cell accumulation associated with myocardial ischemia/ reperfusion. These results suggest that peptidoleukotrienes do not contribute to the progression of myocardial ischemic/reperfusion injury.     

The effect of experimental diabetes on the twenty-four-hour pattern of the vasodilator responses to acetylcholine and isoprenaline in the rat aorta

The aim of this study was to investigate whether time-dependent variations in the relaxant effect of acetylcholine, an endothelium-dependent vasorelaxant via muscarinic receptors, and isoprenaline, a nonselective β-adrenoceptor agonist in rat aorta, are influenced by streptozotocin (STZ)-induced experimental diabetes. Adult male rats were divided randomly into two groups: control and STZ-induced (STZ, 55 mg/kg, intraperitoneal) diabetes. The animals were synchronized to a 12:12 h light-dark cycle (lights on 08:00 h) and sacrificed at six different times of day (1, 5, 9, 13, 17, and 21 hours after lights on; HALO) eight weeks after STZ injection. The in vitro responsiveness of thoracic aorta rings obtained from control and diabetic rats to acetylcholine (10^-9-10^-5 M) and isoprenaline (10^-10-10^-3 M) was determined in six different times. EC₅₀ (the concentration inducing half of the maximum response) values and maximum responses were calculated from cumulative concentration-response curves of the agonists and were analyzed with respect to time and STZ treatment. Treatment, time, and interactions between treatment and time were tested by two-way analysis of variance (ANOVA). To analyze differences due to biological time, one-way ANOVA was used. STZ treatment did not significantly change EC₅₀ values or maximum responses for both agonists. There were statistically significant time-dependent variations in the EC₅₀ values for isoprenaline and maximum responses for both acetylcholine and isoprenaline in control groups by one-way ANOVA, but significant time-dependent variations disappeared in the aortas isolated from STZ-induced diabetic rats. The vasodilator responses to acetylcholine and isoprenaline failed to show any significant interaction (treatment×time of study) between STZ treatment and time of sacrifice in both EC₅₀ values and maximum responses by two-way ANOVA. These results indicate there is a basic temporal pattern in the responses to acetylcholine and isoprenaline in rat aorta which continues in diabetes. It is shown for the first time that experimental diabetes does not change the 24 h pattern of responses to acetylcholine and isoprenaline, and that time-dependent variations in the responses to these agonists disappear in diabetic animals. Although further studies are required to define the underlying mechanism(s) of these findings, results suggest that experimental diabetes can modify the time-dependent vasorelaxant responses of rat aorta. This may help to understand the circadian rhythms in cardiovascular physiology and pathology or in drug effects in diabetes.