Effect of histamine-sensitizing factor of Bordetella pertussis on pharmacologic response of rat atria

The effects of sensitization with the histamine-sensitizing factor (HSF) of Bordetella pertussis as well as Bordetella vaccines on a pharmacologic response in rat heart preparations were determined. In normal rats the spontaneous beating of atria in vitro through the positive inotropic action produced by the addition of epinephrine was inhibited immediately by addition of acetylcholine, whereas in the B. pertussis vaccine-treated rats the exciting atria were scarcely inhibited by acetylcholine. Neither B. parapertussis nor B. bronchiseptica vaccines induced such an altered atrial response in rats. Of the B. pertussis cell components purified HSF induced the altered response at the minimal dose of 0.1 microgram per rat, and a dose of 1 microgram or more produced the maximal change. This altered atrial-inducing activity of HSF was inactivated by heating at 63 C for 30 min, and was neutralized by anti-HSF rabbit serum. The altered response rose quickly in 1 day after i.v. injection of 1 microgram of HSF, reached a plateau in 3 to 5 days, which lasted at least 14 days, and disappeared completely 56 days later. HSF failed to produce directly any functional damage to the beating atria in vitro, and to induce the altered response of the normal rat atria by incubation with as much as 10 microgram of HSF per bath (50 ml) for 4 hr. A trace stimulation was found in the normal rat atria as well as in perfused frog hearts, if HSF was given directly at a dose of 20 microgram per bath.     

Blood pressure and cardiac tissue responses to prostacyclin (PGI2) in various species

The effect of prostacyclin (PGI2) on blood pressure and heart rate (in vivo) and on isolated heart tissue has been investigated in different species. Isolated cardiac tissue had limited responses to PGI2 tested at 10(-13) to 10(-5) M. Cultured neonatal rat heart cells did not respond to PGI2, neither did intact rat hearts or rabbit cardiac tissue. Guinea pig and rat atria showed limited dose-dependent responses to PGI2 at concentrations greater than 10(7) M. In rat atria, 10(-5) M PGI2 produced a limited elevation of tissue cAMP content. When given by intravenous injection or infusion, PGI2 produced hypotension in anaesthetized primates (three species), rat, rabbit, pig, and dog. As a vasodepressor in all species, PGI2 (on a weight basis) was more active than prostaglandins of the B or E type and, in most species tested, it was approximately five times more active than PGE2. Heart responses in intact animals were often paradoxical in that decreases in heart rate often accompanied blood pressure falls.     

Increased sensitivity to neurotensin in fasted rats

The present study was designed to investigate the influence of fasting on the ability of neurotensin (NT) to produce hypotension in anesthetized rats, to constrict the coronary vessels of perfused rat hearts and to stimulate isolated rat stomach strips and spontaneously beating atria of rats. The hypotensive effects of NT, in contrast to those elicited by bradykinin and isoproterenol, were markedly increased in 24h-fasted and 72h-fasted rats compared to control fed rats. The increase in perfusion pressure provoked by NT in perfused hearts derived from 24h-fasted and 72h-fasted rats was much larger than in hearts derived from control fed animals. On the other hand, the constrictor action of angiotensin in perfused rat hearts was not altered by fasting. The stimulatory effect of NT in stomach strips and atrial preparations derived from fasted rats was not increased when compared to tissues from control fed animals. It is suggested that prolonged period of fasting in rats may contribute to sensitize the cardiovascular system of these animals to the action of NT.     

Influence of microwaves on the beating rate of isolated rat hearts

Previous reports have shown that microwave exposure can decrease the beating rate of isolated rat hearts. These experiments were conducted at room temperature and with the hearts exposed to air. We observed arrhythmia frequently at room temperature, and the variation of heart beat was so large that it makes the results difficult to reproduce. Therefore, we employed a double-circulating system to provide perfusion through the coronary artery and around the outside of the heart to maintain the rat hearts at 37.7 degrees C. No arrhythmias were observed in our experiments, and the hearts were beating for at least 1 h. The effects of 16-Hz modulated 2,450-MHz pulsed microwaves (10 microseconds, 100 pps) on the beating rate of 50 isolated rat hearts were studied. Results showed no statistically significant changes of heart rate in exposed groups at SARs of 2 and 10 W/kg compared with the control group. The effect seen at 200 W/kg was shown to be similar to that resulting from heating the heart.     

Effects of anti-cholinoceptor antibodies on the frog heart]

Research on isolated hearts of Rana temporaria has shown that upon treating them with anti-cholinoreceptor antibodies, there occurs a considerable reduction of inhibition effect on cardiac activity of acetylcholine or carbacholine. A reduction of inhibition effect was noticed after incubation of frog's heart with antibodies against cholinoreceptors obtained from motor-denervated muscles of frogs as well as from muscles of mice Balb/c. Cholinoreceptor protein was obtained and purified by A. Sobel's method. Control tests were made with serum of non-immunized rabbits and rabbits immunized with material obtained from non-denervated muscles of frogs. It was concluded that acetylcholinoreceptor antibodies are capable of provoking atropine-like effect on frog's heart. According to our data, anti-cholinoreceptor antibodies as well as cholinoreceptors are relatively non-specific to species.     

Rat heart is a site of leptin production and action

Leptin, the 16-kDa peptide hormone product of the ob gene, is produced primarily by adipocytes and was initially thought to exert its effects exclusively through actions on the hypothalamus via distinct leptin receptors termed OB-R. However, recent data show that leptin is produced elsewhere and that receptors are present in many other tissues. Using real-time PCR, we determined whether leptin and its receptors are present in the rat heart and demonstrated regional distribution patterns and gender differences as well as the effect of ischemia and reperfusion. Gene expression of leptin and its receptors (OB-Ra, OB-Rb, and OB-Re) was identified in myocytes and whole heart homogenates from all regions of the heart of male and female rats, with the highest abundance in left and right atria of male and female rats, respectively. No differences in regional distribution of OB-R were evident in male rat hearts. In female rats, expression was highest in right atria for all three isoforms and was significantly greater than in male rats. Ischemia and reperfusion significantly downregulated leptin and OB-R expression, although this was more pronounced in male rat hearts. Leptin release in the coronary effluent was also detected using ELISA, although this was generally unaffected by global ischemia and reperfusion. Our results demonstrate for the first time the presence of the leptin system, including the peptide and its receptors, in all regions of the rat heart. In view of emerging evidence for cardiac effects of leptin, it is proposed that the heart is a target for leptin action and that the peptide modulates function through a paracrine- or autocrine-dependent manner.     

Endothelin is a positive inotropic agent in human and rat heart in vitro

We have investigated the response to endothelin of isolated atrial and ventricular trabeculae from failing human hearts obtained at transplant. Results indicate that endothelin exerts a significant positive inotropic effect on human atrial and ventricular tissue, with increases in developed tension of 74.6 +/- 14.1% (+/- SEM) and 9.9 +/- 4.0%, respectively. Further studies on rat cardiac muscle demonstrate that the greater inotropic effect on atrial than ventricular muscle is also exhibited by the rat heart in vitro, with 39.9 +/- 10.7% and 17.1 +/- 5.9% increases in developed tension for atria and papillary muscle, respectively. Studies in rat atria also provide no evidence for an effect of endothelin on the frequency of spontaneous contractions. These results suggest that the potential exists for regulation of cardiac function in humans and rats by endothelial-derived factors such as endothelin, possibly via augmentation of atrial systole.     

In Vitro Induction of Altered Response of Rat Atria to Acetylcholine with Histamine-Sensitizing Factor of Bordetella pertussis

Purified histamine-sensitizing factor (HSF) of Bordetella pertussis induced in vitro an alteration in the pharmacologic response of rat atria to acetylcholine. Spontaneously beating atrial preparations isolated from rats were exposed to HSF at a concentration of 50 ng/ml at 37 C for 30 min to 4 hr and washed with Krebs-Ringer solution, and then tested at 1-hr intervals up to 28 hr during incubation for their responses to epinephrine and acetylcholine. At 13 hr after exposure to HSF, irrespective of the exposure period, the HSF-treated atria, in which positive-inotropic action of epinephrine was manifested, depressed the negative-inotropic response to acetylcholine. The activity of HSF was neutralized by anti-HSF serum only in the first 3 hr of exposure. Only 1.8% of the added ¹²⁵I-labeled HSF bound “specifically” to one pair of atria for the manifestation of the altered response.     

Prenatal cocaine alone and combined with nicotine alters ANG II and IGF-1 induced left atrial contractions in aging male offspring

Prenatal cocaine or nicotine affects inotropic activity in the hearts of rat offspring. However, the long-term consequence of this exposure on the cardiac response to hormonal challenge is unknown. We assessed the inotropic effects of angiotensin II (ANG II) and insulin-like growth factor 1 (IGF-1) in the left atria of 19.0-24.5 month-old male rats exposed on gestation days 8-21 to 1 of 6 treatments: low cocaine (LC) (20 mg/kg) or high cocaine (HC) (40 mg/kg); 20 mg/kg cocaine and high nicotine (5 mg/kg nicotine) (LC/HN); 40 mg/kg cocaine and low nicotine (2.5 mg/kg nicotine) (HC/LN); pair fed: yoked to HC (PF); saline: injection of 0.9% NaCl (SAL). Isometric contractions were assessed by electrical stimulation of isolated left atria superfused with Tyrode solution (control) to which ANG II (10-7 mol/L, 20 min) and IGF-1 (10-8 mol/L, 20 min) in the presence of ANG II were added sequentially. Offspring in all cocaine groups showed a higher peak tension development (PTD) to ANG II than PF controls. This increase in PTD was attenuated by subsequent addition of IGF-1 in all except HC offspring. However, with the HC/LN combination the IGF-1 effect on PTD was again evident. The velocities of contraction and relaxation were positively affected by ANG II only in the combined prenatal drug groups; IGF-1 reduced only contraction velocity. Our data demonstrate that IGF-1 reverses the positive inotropic effect of ANG-II in atrial muscle of aging rats and that gestational exposure to only high doses of cocaine eliminates this protective response. It appears that combined prenatal exposure to cocaine and nicotine does not exacerbate the decline in cardiac function and responsiveness to inotropic drugs seen in the aging heart.     

Rapid purification of mammalian cardiac troponin T and its isoform switching in rat hearts during development

A rapid purification of troponin T from adult hearts of various species has been developed. The purification procedure included 60 degrees C treatment of the high salt extract, ammonium sulfate fractionation, and DEAE-cellulose column chromatography. The troponin T purified from the bovine left ventricle contained two isoforms, which differed in both apparent molecular mass and isoelectric point. Both isoforms were able to bind to F-actin filaments only in the presence of tropomyosin. Monoclonal antibody JLT12 against rabbit skeletal troponin T cross-reacted with both isoforms of bovine cardiac troponin T. There was no detectable difference in the relative amount of these two isoforms among different portions (atria, right and left ventricles) of the bovine heart. The purified protein was used as an antigen to immunize mice, and a mouse antiserum with high titer and specificity to both isoforms was subsequently obtained. This antiserum also cross-reacted with cardiac troponin T from chicken, rabbit, and rat. The antibodies were further used to probe cardiac development in rats by Western blotting and immunoprecipitation. The results clearly showed that there was a switch of troponin T isoforms between hearts from 20-day-old rat embryos and hearts from 14-day-old rats. Immunoprecipitation of the in vitro translation products of poly(A)+ RNA isolated from day 5 rat hearts revealed the presence of two isoforms of troponin T, suggesting that two mRNAs coding for these two isoforms existed in the heart cells. It is of interest to not that some profound changes in the morphology and function of cardiac muscle have also been detected at this time of development. Troponin T isoform switching thus may well represent an important marker for cardiac development and function.     

Regulation of atrial contraction by PKA and PKC during development and regression of eccentric cardiac hypertrophy

ANG II plays a major role in development of cardiac hypertrophy through its AT1 receptor subtype, whereas angiotensin-converting enzyme (ACE) inhibitors are effective in reversing effects of ANG II on the heart. The objective of this study was to investigate the role of PKA and PKC in the contractile response of atrial tissue during development and ACE inhibitor-induced regression of eccentric hypertrophy induced by aortocaval shunt. At 1 wk after surgery, sham and shunt rats were divided into captopril-treated and untreated groups for 2 wk. Then isometric contraction was assessed by electrical stimulation of isolated rat left atrial preparations superfused with Tyrode solution in the presence or absence of specific inhibitors KT-5720 (for PKA) and Ro-32-0432 (for PKC) and high Ca2+. Peak tension developed was greater in shunt than in sham hearts. However, when expressed relative to tissue mass, hypertrophied muscle showed weaker contraction than muscle from sham rats. In sham rats, peak tension developed was more affected by PKC than by PKA inhibition, whereas this differential effect was reduced in the hypertrophied heart. Treatment of shunt rats with captopril regressed left atrial hypertrophy by 67% and restored PKC-PKA differential responsiveness toward sham levels. In the hypertrophied left atria, there was an increase in the velocity of contraction and relaxation that was not evident when expressed in specific relative terms. Treatment with ACE inhibitor increased the specific velocity of contraction, as well as its PKC sensitivity, in shunt rats. We conclude that ACE inhibition during eccentric cardiac hypertrophy produces a negative trophic and a positive inotropic effect, mainly through a PKC-dependent mechanism.     

Enhanced chronotropic and inotropic responses of rat myocardium to cholinergic stimulus with aging.

The ability of the heart to respond to adrenergic stimulation diminishes with aging, and this may be one of the factors contributing to the age-associated decline in cardiac stress responsiveness. On the other hand, little is known about the impact of aging on the responsiveness of the heart to cholinergic stimulation. In this study, we determined the chronotropic and inotropic responses of the isolated, Langendorff-perfused hearts from adult (6-8 months) and aged (28-30 months) rats to cholinergic agonists so as to assess age-related alterations in postsynaptic cholinergic control of heart function. The results showed the following. (i) In isolated perfused spontaneously bearing rat hearts, the negative chronotropic response to acetylcholine (10(-9)-10(-5) M) was up to 4-fold greater in the aged compared with adult hearts; this age-related difference was less marked (2-fold) but not abolished in the presence of a maximally effective concentration (5 microM) of the cholinesterase inhibitor eserine. (ii) The cholinesterase-resistant agonist carbachol (10(-9)-2.5 x 10(-6) M) elicited a 2- to 3-fold greater negative chronotropic response in the aged compared with adult hearts. (iii) In isolated perfused, electrically paced (4 Hz) rat hearts, carbachol (10(-9)-10(-5) M) elicited a concentration-dependent negative inotropic response, which was 2-fold greater in the aged compared with adult heart at all carbachol concentrations. (iv) Acetylcholinesterase activities (micromoles per gram per hour) were 50-60% lower in the aged atria (83 +/- 21) and ventricles (24 +/- 6) than in adult atria (210 +/- 20) and ventricles (47 +/- 7).(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiac metabolism in the Myxinidae: physiological and phylogenetic considerations

Cardiac muscle hearts of Atlantic hagfish continuously function under hypoxic conditions that would lead to cardiac failure in most other vertebrates. Contractile performance of hagfish systemic hearts is resistant to anoxia and respiratory poisons but shows a significant decrement when carbohydrate catabolism is blocked by 0.5 mM iodoacetic acid. Enzyme activity profiles of hagfish ventricle reveal a robust capacity for glycolysis of carbohydrate in comparison to that for general aerobic metabolism and catabolism of alternate metabolic fuels. Isolated working hagfish ventricles preferentially oxidize radiolabeled glucose even when fatty acid fuels are present in the incubation medium. Work output of the isolated ventricular preparation is maintained only in the presence of exogenous glucose. The results indicate that energy metabolism of the hagfish myocardium is predominantly carbohydrate-based and that energy demand of the tissue can be sustained by anaerobic glycolysis during extended periods of extreme hypoxia. Cardiac metabolism of this primitive species is compared with that of hearts from higher vertebrates and an evolutionary hypothesis relating cardiac workload to preferred metabolic fuel is discussed.     

Carbachol-induced Suppression of Contraction Rhythm in Spontaneously Beating Cultured Cardiac Myocytes From Neonatal Rats

Nitric oxide (NO) and reactive oxygen species (ROS) are known to play various functional and pathophysiological roles as an intracellular messenger in the heart. In this study, we investigated whether the increased production of NO and/or ROS was involved in the cholinergic regulation of rhythmic contraction in spontaneously beating cultured cardiac myocytes from neonatal rats. Exposure of cultures to carbachol, an agonist of muscarinic acetylcholine receptors (mAchR), produced a dose-dependent decrease in the beat rate of cultured cardiac myocytes, and such a effect was significantly attenuated by pre-treatment with an NOS inhibitor, as well as an NO scavenger. In addition, exposure to an NO donor (SNAP) also decreased the beat rate dose-dependently. Carbachol- or SNAP-induced suppression of the contraction rhythm was significantly attenuated by co-treatment with 5-hydroxydecanoate (5-HD). In contrast, treatment with diazoxide decreased the beat rate dose-dependently. Carbachol treatment increased the intensity of 2',7'-dichlorodihydrofluorescein fluorescence, suggesting that the production of ROS was enhanced by the treatment. In addition, the carbachol- or diazoxide-induced suppression of contraction rhythm was attenuated by co-treatment with 2-mercaptopropionyl glycine, a scavenger of ROS. The present study has suggested that the mAchR-NO-mitoK ATP -ROS pathway is a factor responsible for carbachol-induced suppression of contraction rhythm in cultured cardiac myocytes.     

Species comparison of adenosine A1 receptors in isolated mammalian atrial and ventricular myocardium

Various species have been used as models to study the effects of adenosine (ADO) on atrial and ventricular myocardium, but few direct tissue comparisons between species have been made. This study further characterizes adenosine A(1) receptor binding, adenylate cyclase activity and direct and indirect A(1) receptor-mediated functional activity in atrial and ventricular tissue from Sprague-Dawley rats and Hartley guinea pigs. Rat right atria (RA) were found to be significantly more sensitive to cyclopentyladenosine (CPA), while guinea pig left atria (LA) were more sensitive to CPA. After the addition of isoproterenol (ISO), the reduction of CPA response in rat RA was significantly greater than in guinea pig; however, after ISO treatment, the guinea pig LA was more sensitive to CPA than the rat. Adenylate cyclase inhibition by CPA was significantly greater in atria and ventricles obtained from guinea pig than rat. In competition binding experiments, guinea pig RA had significantly more high affinity sites than rat, but the K(i)s were not significantly different. There were no significant differences between guinea pig LA and rat LA. Guinea pig ventricular tissue had fewer high affinity sites than rat without any differences in their K(i) values. In antagonist saturation experiments, the density and affinity of A(1) receptors in guinea pig cardiac membranes were significantly greater than in rat. Our results indicate definite species differences as well as tissue differences between rat and guinea pig. These differences must be considered when interpreting studies using rat and guinea pig tissue as models for cardiac function.     

The Role of α7 Nicotinic Acetylcholine Receptor in Modulation of Heart Rate Dynamics in Endotoxemic Rats

Previous reports have indicated that artificial stimulation of the vagus nerve reduces systemic inflammation in experimental models of sepsis. This phenomenon is a part of a broader cholinergic anti-inflammatory pathway which activates the vagus nerve to modulate inflammation through activation of alpha7 nicotinic acetylcholine receptors (α7nACHR). Heart rate variability represents the complex interplay between autonomic nervous system and cardiac pacemaker cells. Reduced heart rate variability and increased cardiac cycle regularity is a hallmark of clinical conditions that are associated with systemic inflammation (e.g. endotoxemia and sepsis). The present study was aimed to assess the role of α7nACHR in modulation of heart rate dynamics during systemic inflammation. Systemic inflammation was induced by injection of endotoxin (lipopolysaccharide) in rats. Electrocardiogram and body temperature were recorded in conscious animals using a telemetric system. Linear and non-linear indices of heart rate variability (e.g. sample entropy and fractal-like temporal structure) were assessed. RT-PCR and immunohistochemistry studies showed that α7nACHR is expressed in rat atrium and is mainly localized at the endothelial layer. Systemic administration of an α7nACHR antagonist (methyllycaconitine) did not show a significant effect on body temperature or heart rate dynamics in naïve rats. However, α7nACHR blockade could further reduce heart rate variability and elicit a febrile response in endotoxemic rats. Pre-treatment of endotoxemic animals with an α7nACHR agonist (PHA-543613) was unable to modulate heart rate dynamics in endotoxemic rats but could prevent the effect of endotoxin on body temperature within 24 h experiment. Neither methyllycaconitine nor PHA-543613 could affect cardiac beating variability of isolated perfused hearts taken from control or endotoxemic rats. Based on our observations we suggest a tonic role for nicotinic acetylcholine receptors in modulation of heart rate dynamics during systemic inflammation.     

Acetylcholine supersensitivity in the rat heart produced by neonatal sympathectomy

Effects of neonatal sympathectomy with antiserum to nerve growth factor or 6-hydroxydopamine on the acetylcholine sensitivity of the rat left atria were investigated. Sensitivities to acetylcholine of atria from immunologically and chemically sympathectomized rats were much higher than that of control at 4 weeks of age. These results suggest possible involvement of the sympathetic nervous system in regulation of cardiac cholinergic sensitivity.     

Cholinesterases and postnatal development of the negative chronotropic effects of acetylcholine in albino rats

The negative chronotropic effects of acetylcholine were studied in the isolated atria of the hearts of albino rats aged 2, 15, 29 and 47 days and adult. In untreated preparations, i.e. with full cholinesterase activity, the strongest effects were observed in newborn animals; with advancing age the reaction grew weaker. If cholinesterase activity was inhibited with physostigmine, the differences between the various age groups were obliterated. It is thus evident that the actual acetylcholine sensitivity of the sinoatrial node tissue does alter during postnatal life, but that growing cholinesterase activity reduces the amount of acetylcholine diffusing from the medium into the acetylcholine receptor zone. The change which takes place in cholinesterase activity in the myocardial tissue during postnatal life is so great that is must be taken into account when considering the development of cholinergic control of cardiac function.     

Effects of magnesium depletion on myocardial high-energy phosphates and contractility

The effect of prolonged magnesium depletion on contractility, phosphorylating activity, and organic phosphates of spontaneously beating isolated rat atria was studied. Rats were fed a Mg-deficient diet for 8 weeks, during which serum Mg fell from 1.85 +/- 0.02 to 0.52 +/- 0.10 mg/dl. Atrial contractile activity was measured for 1 hr and at the end of this period tissue samples were taken for the determination of the phosphorylated intermediates. Mg depletion was associated with (a) reduced intracellular inorganic phosphorus and adenine nucleotides; (b) elevated creatine phosphate; (c) reduction in contractile force (CF) with no change in atrial beat rate (BR). There were no significant differences in the activities of creatine phosphokinase and adenylate kinase in control and Mg-depleted rat atrial homogenates determined in the presence of 5 mM MgCl2. Addition of various concentrations of MgCl2 to the medium resulted in an immediate reduction in both CF and BR of normal and Mg-depleted rat atria. Intraperitoneal administration of MgCl2 to Mg-depleted rats resulted in complete recovery of CF of isolated atria. This improvement in CF occurred without changes in the levels of inorganic phosphate and adenine nucleotides. The reduced intracellular level of high-energy phosphate or inorganic phosphate cannot therefore be responsible for the impaired contractility seen in Mg-depleted heart muscle. On the other hand, the fact that the creatine phosphate levels were higher in magnesium depletion suggests that myofibrillar utilization of creatine phosphate is more impaired than production, analogous to phenomena seen in postanoxic recovery.     

Nucleoside transport in heart: species differences in nitrobenzylthioinosine binding, adenosine accumulation, and drug-induced potentiation of adenosine action

The site-specific binding of the potent and selective nucleoside transport inhibitor, [3H]nitrobenzylthioinosine (NBMPR), to the nucleoside transport system of cardiac membranes of several species was investigated. The affinity of [3H]NBMPR for these sites ranged from 0.03 nM in rat to 0.78 nM in dog. The maximal binding capacity of cardiac membranes for [3H]NBMPR was also species dependent and was greatest in bovine and guinea pig heart (2551 and 1700 fmol/mg protein, respectively) and least in rat (195 fmol/mg protein). The affinities of recognized nucleoside transport inhibitors and benzodiazepines for these transport inhibitory sites in guinea pig and rat heart were estimated by studying the inhibition of the site-specific binding of [3H]NBMPR in competition experiments. These values were compared with their inhibitory effects on the transporter-dependent accumulation of [3H]adenosine in guinea pig and rat cardiac muscle segments and with their ability to potentiate the negative inotropic action of adenosine in electrically driven guinea pig and rat left atria. In guinea pig heart, the recognized nucleoside transport inhibitors and benzodiazepines had an order of affinity (dilazep greater than hydroxynitrobenzylthioguanosine greater than dipyridamole greater than hexobendine much greater than lidoflazine much greater than flunitrazepam greater than diazepam greater than lorazepam greater than flurazepam) for the NBMPR site which was similar to those for the inhibition of [3H]adenosine accumulation and for potentiation of adenosine action. In contrast, in rat heart, where the maximal binding capacity of [3H]NBMPR was lower (eightfold), the nucleoside transporter dependent accumulation of [3H]adenosine was also lower (sixfold) and the negative inotropic action of adenosine was not significantly potentiated.(ABSTRACT TRUNCATED AT 250 WORDS)