The effects of levosimendan on the left ventricular function and protein phosphorylation in post-isschemic guinea pig hearts

The widely accepted theories for the decreased function in the stunned myocardium relate to Ca²⁺ desensitization and free radical-mediated tissue damage of the myofilaments. The aim of the present study was to examine whether the depressed contractile function and Ca²⁺ responsiveness of the stunned myocardium may be restored by a new Ca²⁺ sensitizer (levosimendan), which has been shown to improve the Ca²⁺ response of the myofilaments. The effects of levosimendan on the left ventricular function and the in vivo protein phosphorylation were examined in both the non-ischemic and the stunned myocardium. Myocardial stunning was induced in Langendorff-perfused guinea pig hearts by suspending the circulation for 8 min, followed by a 20-min reperfusion period. Perfusion of post-ischemic guinea pig hearts with levosimendan (0.03–0.48 μM, 6 min) was associated with dose- and time-dependent increases in both dP/dt_max (contractility) and dP/dt_min (speed of relaxation). When the effectiveness of levosimendan was compared in non-ischemic and post-ischemic hearts, no significant differences were noted in the relative stimulatory effects on contractility and relaxation, at any given time point (time-response curve) or concentration (dose-response curve). Perfusion of the guinea pig hearts with a high (0.3 μM) levosimendan concentration did not reveal any qualitative or quantitative difference in the phosphodiesterase inhibitory potential of the compound (elevation of tissue cyclic AMP levels and characteristics of protein phosphorylation) between the non-ischemic and the post-ischemic myocardium. However, when isoproterenol was adminstered to induce maximal in vivo phosphorylation of cardiac phosphorproteins, an attenuation of the ³²P-incorporation into troponin I was noted in the post-ischemic hearts. The decrease in isoproterenol-induced ³²P-incorporation into troponin I was associated with similar alterations in the tissue level of this protein. We conclude that the Ca²⁺ sensitizer levosimendan exerts dose- and time-dependent positive inotropic and lusitropic effects on the postischemic myocardium, lending support to the hypothesis tha Ca²⁺ desensitization of the myofibrils is involved in myocardial stunning. Received: 20 July 1998, Returned for 1. revision: 27 August 1998, 1. Revision received: 6 January 1999, Returned for 2. revision: 5 February 1999, 2. Revision received: 25 February 1999, Accepted: 3 March 1999     

Phospholamban and troponin I are substrates for protein kinase C in vitro but not in intact beating guinea pig hearts

The incorporation of [32P]inorganic phosphate into membranous, myofibrillar, and cytosolic proteins was studied in Langendorff-perfused guinea pig hearts treated with phorbol 12-myristate 13-acetate (PMA) or 1,2-dioctanoylglycerol (D8G), which are potent activators of protein kinase C. Control hearts were perfused with an inactive phorbol ester (4 alpha-phorbol 12,13-didecanoate), which does not cause activation of protein kinase C. To ensure the blockade of different receptor systems, the perfusions were carried out in the presence of prazosin, propranolol, and atropine. Perfusion of hearts with either PMA (4 microM) or D8G (200 microM) was associated with a negative effect on left ventricular inotropy and relaxation. Examination of the 32P incorporation into various fractions revealed that there were no increases in the degree of phosphorylation of phospholamban in sarcoplasmic reticulum, and troponin I and C protein in the myofibrils, although these proteins were found to be substrates for protein kinase C in vitro. However, in the same hearts, there were significant changes in the 32P incorporation into a 28-kDa cytosolic-protein. Examination of the activity levels of protein kinase C in hearts perfused with PMA indicated a redistribution of this activity from the cytosolic to the membrane fraction, suggesting the activation of the enzyme in vivo. These findings indicate that cardiac regulatory phosphoproteins, which may be phosphorylated by protein kinase C in vitro, are not substrates for protein kinase C in beating hearts perfused with phorbol esters or diacylglycerol analogues.     

Effect of palmitate and phorbol ester on synthesis of phosphatidylcholine in isolated guinea pig gastric glands

To better understand the mechanism of phosphatidylcholine synthesis in the stomach, [3H] choline incorporation into phosphatidylcholine in response to agents which have been shown to induce phosphatidylcholine biosynthesis in other tissues was examined using isolated guinea pig gastric glands. Palmitate and 12-O-tetradecanoyl phorbol 13-acetate (TPA) which has been shown to activate protein kinase C directly, stimulated [3H] choline incorporation into phosphatidylcholine in gastric glands, by 189 +/- 12.9%, and 129 +/- 10.4% of control, respectively (n = 4, p less than 0.01, p less than 0.05). On the other hand, dibutyryl cyclic AMP had no effect on the incorporation. When the glands were pulsed with [3H] choline followed by incubation in the presence of palmitate and TPA for 180 min to see the effects of the agents on the limiting step of the phosphatidylcholine synthesis, phosphatidyl-[3H] choline was increased to 167 +/- 7.5% and 142 +/- 7.5% of control respectively (n = 4, p less than 0.01, p less than 0.05). In parallel to the increase in phosphatidylcholine synthesis, phosphoryl-[3H] choline in the glands incubated with palmitate and TPA was decreased as compared with control. These results suggest that palmitate or TPA may stimulate phosphatidylcholine biosynthesis through the activation of cytidylyltransferase in the stomach.     

Inotropic responses to isoproterenol and phosphodiesterase inhibitors in intact guinea pig hearts: comparison of cyclic AMP levels and phosphorylation of sarcoplasmic reticulum and myofibrillar proteins

The influence of selective (milrinone: 10, 50, 100 microM) and nonselective phosphodiesterase (isobutylmethylxanthine: 0.1, 10, 100 microM) inhibitors and beta-adrenergic stimulation (isoproterenol: 0.01, 0.1 microM) on phospholamban and myofibrillar protein phosphorylation was studied in guinea pig hearts perfused with [32P]orthophosphate. Changes in protein phosphorylation were compared to alterations in tissue cyclic AMP (cAMP) levels and positive inotropic effects induced by these agents. Isoproterenol (0.01 microM), milrinone (50 microM), and isobutylmethylxanthine (100 microM) all produced similar, twofold increases in dP/dt and -dP/dt but only stimulation with isobutylmethylxanthine and isoproterenol was associated with significant increases in phospholamban phosphorylation. At these equipotent doses, the effects of isobutylmethylxanthine were associated with higher increases (3.1-fold) in cAMP than those observed with isoproterenol (twofold). Milrinone (50 microM) produced a 2.5-fold increase in cAMP levels but failed to change phospholamban phosphorylation. Higher doses of milrinone (100 microM) resulted in relatively high (4.1-fold) cAMP levels, and this was associated with increased (1.5-fold) phosphorylation of phospholamban. Phosphorylation of troponin I was significantly increased at 0.01 microM and 0.1 microM isoproterenol, while phosphorylation of C protein was observed only at 0.1 microM isoproterenol. Isobutylmethylxanthine and milrinone did not significantly increase phosphorylation of either troponin I or C protein at any of the doses studied. These findings indicate that cardiotonic agents acting via the cAMP pathway may produce similar inotropic responses at different levels of cAMP and phosphorylation of sarcoplasmic reticulum and myofibrillar proteins.     

Role of alpha 1-adrenoceptor activity in progression of cardiac hypertrophy in guinea pig hearts with pressure overload

To elucidate the role of alpha 1- and beta-adrenergic activities in pressure overload hypertrophy, changes of alpha 1- and beta-adrenoceptors were measured by radioligand binding assay, and the preventive effects of alpha 1- and beta-adrenoceptor blockade on cardiac hypertrophy were assessed in guinea pigs after aortic banding. Five days after banding, dry weight of left ventricle had not increased, though wet weight increased due to marked intercellular oedema. In this period, the maximum binding capacity of [3H] prazosin increased to 31.1 (SEM 2.2) fmol.mg-1 from (sham operation) 17.0(2.1) fmol.mg protein-1, p less than 0.01, whereas the maximum binding capacity of [3H]dihydroalprenolol did not increase: 143(16) fmol.mg-1 (banded) v 153(13) fmol.mg-1 (sham). Three weeks after aortic banding, the maximum binding capacity of both ligands increased to 45.6(5.5) fmol.mg-1 and 232(21) fmol.mg-1, respectively, accompanied by a significant increase in left ventricular dry weight, from 0.46(0.02) mg.g-1 (sham) to 0.62(0.08) mg.g-1 (banded), p less than 0.01. Continuous subcutaneous administration of the alpha 1-blocker bunazosin (0.1 mg.kg-1.d-1) significantly attenuated the increase in left ventricular dry weight whereas the beta-blocker propranolol (5 mg.kg-1.d-1) did not: 0.55(0.03) v 0.66(0.04) mg.g-1 respectively, after 3 weeks. These results show that pressure overload elicited an increase in myocardial alpha 1-adrenoceptors before the onset of cardiac hypertrophy, and that an alpha 1-blocker could prevent the development of hypertrophy in the pressure overloaded heart.     

Effects of adenosine on electrical activity of isolated guinea pig hearts

Summary Negative chronotropic and dromotropic effects of adenosine seem to be responsible for its antiarrhythmic action on supraventricular tachyarrhythmias. To further characterize the effects of adenosine on supraventricular arrhythmias heart rate, conduction, refractoriness, the time to steady-state of AV-nodal conduction slowing and of sinus rate reduction were evaluated.Changes of heart rate, conduction intervals and effective refractory periods were determined by the use of a high-resolution ECG recording technique in isolated guinea pig hearts perfused by the method of Langendorff.Adenosine in concentrations of 3 and 10 M reduced sinus rate and prolonged AV-nodal conduction significantly, while intraventricular and His bundle conduction were not altered. The maximal effect of adenosine on the sinus node and AV nodal conduction occurred after 636±109 and 111±35 (mean±SE) beats, respectively.During programmed stimulation at a cycle length of 250 ms, adenosine reduced atrial ERP in a dose-dependent manner. At cycle lengths of 170 and 200 ms, adenosine increased the atrial ERP at 3 M, and then progressively shortened the ERP at higher doses. At all adenosine concentrations used, the usual rate-dependent adaption in ERP was suppressed.These observations explain the efficacy of adenosine against supraventricular tachyarrhythmias where the AV-node forms a part of a reentrant circuit. Adenosine shortened the atrial ERP, but at high pacing rates also led to a relative prolongation of the atrial ERP as the rate-dependent adaption was suppressed. These opposite effects of adenosine may explain earlier contradictory findings of its action on atrial arrhythmias.     

Steroid-binding proteins in guinea pig milk and plasma

The levels of two steroid-binding proteins, progesterone-binding globulin (PBG) (1) and corticosteroid-binding globulin (CBG), present in the plasma of pregnant guinea pigs were determined just before and after parturition. Both PBG (3.37 +/- 2.1 microM) and CBG (10.1 +/- 1.7 microM) were present in high levels just before parturition and were found to decrease with a half-life of 2 days after delivery. PBG and CBG were also found in the milk whey of lactating guinea pigs but at much lower levels (26.5 +/- 12 and 375 +/- 18 nM, respectively, on day 1 post partum). The whey content of each protein declined with a half-life of 3 days. The whey steroid-binding proteins were indistinguishable from their plasma counterparts on the basis of ion exchange and gel filtration chromatographies, sucrose gradient centrifugation, susceptibility to heat denaturation, and hormone binding specificity.     

链霉素对牵张所致豚鼠心肌电生理改变的抑制作用

目的 :观察通过短时结扎在体豚鼠升主动脉致其左心室后负荷增加时是否会引起机械电反馈 ,并观察这种效应是否能够被链霉素抑制。据此来评价牵张激活的离子通道 (stretch activatedchannels ,SACs)是否参与了机械电反馈活动。方法 :采用单相动作电位记录技术 ,记录在体豚鼠左心室前中壁的单相动作电位的过程中结扎其升主动脉 5s ,间隔 5min重复 1次 ,在重复之前由颈静脉注射链霉素 (2 0mg/kg) ,测量 5 0 %和 90 %单相动作电位复极时程 (APD50 和APD90 )并计算结扎升主动脉诱发心律失常的发生率。结果 :应用链霉素前 ,短时结扎升主动脉使左心室后负荷由 (30± 8)mmHg(1mmHg =0 .133kPa)增加到 (70± 17)mmHg(P <0 .0 5 ) ,伴随后负荷的增加 ,APD5 0由 (78± 13)ms缩短为 (6 0± 14 )ms(P <0 .0 5 ) ,APD90 由 (119± 18)ms缩短为 (110± 19)ms(P<0 .0 5 ) ,并且可以观察到 6 7%的心脏发生牵张诱发的心律失常 ;应用链霉素后 ,短时结扎升主动脉可引起左心室后负荷相似的变化 ,但APD50 和APD90 未发生明显改变 ,且只有 17%的心脏发生心律失常。结论 :增加左心室后负荷可引起在体豚鼠心肌APD50 和APD90 缩短 ,并且可诱发心律失常。链霉素对此均有抑制作用 ,提示SACs可能参与了该过程。     

Stereoselective electrophysiological effects of propafenone in Langendorff perfused guinea pig hearts

Summary The present study was focused on the stereoselective electrophysiological effects of (R)-and (S)-propafenone·HCl evaluated in isolated Langendorff perfused guinea pig hearts. Conduction intervals were measured using an ECG-recording method of high resolution. Refractory periods of the different parts of the myocardium were determined by stimulation with premature stimuli, as well as by stimulation with increasing pacing rate (rate-dependent/refractory periods). Drug concentrations of 0.1, 1 and 3 M were tested. Both compounds induced a dose-dependent increase in AV-nodal, His-bundle, and intraventricular conduction time which reached significance (p<0.01) following 3 M of either compound. Sinus rate was also dose-dependently and significantly reduced.(R)- and (S)-propafenone·HCl induced a marked prolongation of the rate-dependent refractory period of sino-atrial (by 140±22%, p<0.01 and by 141±14%, p<0.01, respectively) and AV-nodal (by 34±22%, p<0.01 and by 42±15%, p<0.01, respectively) conduction and of the atrial (by 182±21%, p<0.01 and by 195±15%, p<0.01, respectively) and ventricular (by 93±16%, p<0.01 and by 88±16%, p<0.01, respectively) myocardium. The effective refractory periods evaluated by stimulation with premature stimuli were also significantly prolonged under the influence of (R)- and (S)-propafenone·HCl, except the ventricular myocardial refractoriness by (R)-propafenone·HCl (increase to 114±23%, n.s.). Both compounds showed a strong rate-dependence of their effects and, thus, the refractory periods evaluated by stimulation with increasing pacing rate were significantly more prolonged than the refractory periods evaluated by stimulation with premature stimuli. The main difference between the effects of (R)- and (S)-propafenone·HCl on the cardiac electrical activity is the lack of effect of (R)-propafenone·HCl on the ventricular myocardial refratoriness evaluated by stimulation with premature stimuli.Supported by the Austrian Research Foundation grant P7141, P8014, P8184     

α1肾上腺素受体对豚鼠心肌细胞延迟整流钾电流的调控

人心肌上主要存在α1肾上腺素受体 ,生理状况下 ,α1受体激动不引起心肌的异常电活动 ,但在心肌缺血时 ,由于局部儿茶酚胺聚集 ,受体密度增加 ,使其作用增强 ,并因为心肌对α1受体作用反应的不一致性 ,可能引起致命的心律失常。因此 ,α1受体在心肌缺血及再灌注时诱发的心律失常发生机制中起重要作用。α1受体的这些作用与其影响心肌细胞电生理有关。延迟整流钾电流 (Ｉｋ)作为心肌细胞膜上重要的离子流主要参与动作电位的复极过程 ,不仅受细胞膜电位变化的控制 ,同时也受到细胞膜上α1受体及其细胞内信息传导系统的调控。但有关α1对心肌细…     

Effects of alpha and beta adrenergic blockade on coronary arterial microvessels in the beating canine heart.

OBJECTIVE: The aim was to clarify the effects of alpha and beta adrenergic blockade on coronary arterial microvessels and to assess the role of alpha and beta adrenergic tone in normally beating hearts. METHODS: 47 anaesthetised open chest dogs were studied. The diameters of epicardial arterial microvessels were measured in beating hearts using an incident light fluorescence microscope equipped with a floating objective. Drugs were infused into the left anterior descending coronary artery keeping the heart rate and aortic pressure at control levels. To examine the effect of alpha adrenergic blockade, phentolamine (100 micrograms.kg-1) was given in the absence or presence of beta adrenergic blockade (propranolol 50 micrograms.kg-1). To examine the effect of beta adrenergic blockade, propranolol (50 micrograms.kg-1) or three doses of ICI 118,551 (a selective beta 2 antagonist, 0.1, 0.5, and 1.0 microgram.kg-1.min-1) was given. RESULTS: Coronary arterial microvessels were divided into three groups according to the control diameters (D) of small (D less than 100 microns), medium (100 less than or equal to D less than 200 microns) and large (D greater than or equal to 200 microns) groups. In the absence of beta adrenergic blockade, phentolamine significantly dilated all vessel groups: small +19.6 (SEM 5.6)%, medium +5.8(2.3)%, large +5.3(0.9)%. In the presence of beta adrenergic blockade, the vasodilator effect of phentolamine was completely abolished. Propranolol constricted all vessel groups: small -3.6(1.1)%, medium -4.8(1.0)%, large -3.5(1.0)%. ICI 118,551 significantly constricted the large vessel group [-2.5(0.6)%] at the mid dose, and the medium and large vessel groups [medium -3.1(0.8)%, large -3.5(1.3)%] at the highest dose. CONCLUSIONS: These data indicate that (1) the vasodilator effect of phentolamine is induced by beta adrenergic stimulation; (2) resting alpha adrenergic tone of coronary arterial microvessels is minimal in normally beating hearts, and (3) resting beta adrenergic tone may play a physiological role in coronary arterial microvessels, and beta 2 adrenergic tone predominates in arterial microvessels greater than 100 microns in diameter.     

In vitro acetylcholine biosynthesis in normal and failing guinea pig hearts.

Choline acetyltransferase activity, which is rate limiting in acetylcholine biosynthesis, was measured in the four heart chambers of guinea pigs subjected to (1) sham surgery, (2) constriction of the ascending aorta, (3) constriction of the descending thoracic aorta, and (4) constriction of the pulmonary artery. After 30 days when hypertrophy and heart failure were fully established, choline acetyltransferase was quantified in vitro by a radiochemical assay. In the sham-operated group, enzyme activity expressed in terms of unit weight of cardiac tissue was greatest in the right atrium and the right ventricle and lower in th left atrium and the left ventricle (3.62 plus or minus 0.30, 2.96 plus or minus 0.52, 1.64 plus or minus 0.15, and 1.67 plus or minus 0.22 nmoles/min g-1, respectively). Enzyme activity was reduced (P less than 0.05) in the right atria and the right ventricles of guinea pigs with constriction of the pulmonary artery (1.68 plus or minus 0.37 and 1.31 plus or minus 0.29 nmoles/min g-1, respectively). Enzyme activity also tended to be reduced in the left atria and the left ventricles of guinea pigs with constriction of the aorta. These changes represented a relative dilution of enzyme activity per unit weight but not an absolute depletion, since choline acetyltransferase activity per ventricle was not reduced. The absence of significant changes in the total amount of the neuronal enzyme, choline acetyltransferase, per ventricle contrasted with the observed increases in the myocardial enzyme, carnitine acetyltransferase. These results confirm the presence of significant parasympathetic innervation of the ventricles as well as the atria but do not demonstrate alterations in parasympathetic neurotransmitter biosynthesis in hypertrphied and failing myocardium. The absence of absolute reductions in choline acetyltransferase activity in hypertrophied and failing ventricle contrasts strikingly with the previously reported reductions in tyrosine hydroxylase, which is rate limiting in sympathetic neurotransmitter biosynthesis.     

Effect of drug treatment and aerosoled antigen sensitization on cyclic AMP and beta adrenergic receptors of guinea pig lung

The interaction between the number of beta adrenergic binding sites and the ability of a beta adrenergic agonist, isoproterenol, to increase cyclic AMP content of guinea pig lung slices was studied. A complex relationship was found. Chronic sensitization of the guinea pig to an aerosol of ovalbumin resulted in lung slices which were hyporesponsive to isoproterenol in vitro, yet possessed an unchanged number of beta adrenergic binding sites. Chronic exposure of guinea pigs to aerosoled isoproterenol or acute treatment with hydrocortisone did not change the number of beta adrenergic binding sites or the responsiveness of the tissue to isoproterenol in vitro. However, chronic hydrocortisone treatment increased the number of binding sites found on the lung slices by 44%, yet there was no change in the responsiveness of the tissue to isoproterenol in vitro. These data suggest that drugs and disease may change the relationship between the various components of the beta adrenergic binding-adenylate cyclase complex of lung.     

Tumor-promoting phorbol esters inhibit cardiac functions and induce redistribution of protein kinase C in perfused beating rat heart

Activation of protein kinase C has been implicated in the regulation of a variety of cellular reactions. Although we, and others, have found protein kinase C and its substrate proteins to be present in both membrane and cytosolic fractions in the heart, the physiologic role of this kinase in the regulation of cardiac functions remains unknown. In the present study, we found that in isolated perfused rat heart, administration of phorbol esters 4 beta-phorbol 12,13-dibutyrate(PDBu) and 12-0-tetradecanoylphorbol 13-acetate(TPA), which are specific activators of protein kinase C, produced marked dose-dependent negative changes in inotropy and chronotropy. A dose-dependent decrease in coronary flow was also observed. The diacylglycerol analogues, 1,2-oleoylacetyl-glycerol and 1,2-dioctanoylglycerol, produced similar effects as the active phorbol esters on these isolated perfused hearts. An inactive analogue of phorbol ester, 4 alpha-phorbol, failed to produce any effect. Protein kinase C activity in both membrane and cytosolic fractions prepared from rat heart could be activated by TPA and PDBu at the same concentration range as used in the experiments with perfused hearts. Following perfusion of the hearts with PDBu, a rapid translocation of protein kinase C from cytosolic to membrane fractions was also observed. Our findings provide the first direct evidence that protein kinase C may play a potentially important role in the regulation of cardiac functions.     

Autoradiographic mapping of calcitonin gene-related peptide receptors in human and guinea pig hearts

Calcitonin gene-related peptide (CGRP) is a 37-amino acid peptide that is a potent coronary vasodilator. Although CGRP is found in high concentrations around coronary arteries, its precise function in the control of coronary vasomotor tone remains unclear. We studied the distribution of specific receptors for CGRP in guinea pig and human hearts and found that the highest concentration of specific receptors for CGRP was in the major coronary arteries, which is consistent with the hypothesis that CGRP is implicated in control of coronary vasomotor tone. Areas of coronary artery with atheroma contained significantly decreased (158 +/- 35 grains/1,000 microns 2 tissue, n = 3) binding sites compared with binding sites in normal arteries (266 +/- 10 grains/1,000 microns 2 tissue, n = 11; p less than 0.001, t test). The decrease in receptors for CGRP around atheroma may predispose these vessels to coronary spasm.     

Role of adenosine on ventricular overdrive suppression in isolated guinea pig hearts and Purkinje fibers

The present study was undertaken to demonstrate and characterize potentiation of ventricular overdrive suppression by adenosine. To substantiate that adenosine has an enhanced effect on overdrive suppression, it would be necessary to demonstrate that adenosine increases pause duration independent of slowing spontaneous pre-drive rate. In isolated perfused guinea pig hearts with surgically induced complete atrioventricular block, the effect of adenosine (2-20 microM) on pause duration was compared to two alternative means of slowing the pre-drive rate, i.e., hypothermia (28.0 degrees C to 34.0 degrees C) and cesium chloride (0.3-1.0 mM). The slope value of the linear regression line describing the relationship between pre-drive cycle length and pause duration for adenosine (15.8) was significantly greater than control (1.7), hypothermia (1.7), and cesium chloride (5.4). The competitive adenosine antagonist, aminophylline (60 microM), when infused at the initiation of overdrive during adenosine administration, significantly reduced the effect of adenosine on pause duration by 72.9 +/- 4.2% (mean +/- SEM). The reduction in pause duration by aminophylline was specific for adenosine and did not occur under control conditions or during cesium chloride administration. During hypoxia, aminophylline and adenosine deaminase, when infused at the initiation of overdrive, caused 72.3 +/- 5.6 and 63.3 +/- 6.1% reductions in pause duration, respectively. Endogenous adenosine levels rose significantly with hypoxia (1,687 +/- 202 vs. 36 +/- 4 pmol/min per g during normoxia) and increased significantly further during hypoxic overdrive (3,004 +/- 323 pmol/min per g). In isolated guinea pig Purkinje fibers (n = 4), adenosine (20 microM) increased pause duration by 73.6 +/- 9.9% while only minimally affecting the pre-drive cycle length (7.6 +/- 3.8%). These fibers, when stimulated at 1.5 Hz, also displayed an adenosine-induced reduction in action potential duration at 90% repolarization (16 +/- 2 msec). In addition, we demonstrated that adenosine had an enhanced effect on pause duration in the presence of ouabain (1 microM)-induced attenuation of overdrive suppression. Thus, in isolated Purkinje fibers, it is unlikely that the potentiating effect of adenosine on pause duration, which is independent of its chronotropic effect, is mediated via an enhancement of sodium potassium adenosine triphosphatase pump activity. The effect of adenosine is likely to be secondary to a direct action on outward potassium conductance.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of adenosine on cardiac performance and phosphate compounds in microembolised guinea pig hearts.

OBJECTIVE: The excessive release of adenosine has been reported to cause hyperaemia and to attenuate ischaemic injury in microembolised hearts. The direct effects of exogenous adenosine on cardiac performance were investigated in microembolised guinea pig hearts with the simultaneous measurement of phosphate compounds. METHODS: 21 male guinea pig hearts were perfused according to the Langendorff technique and microembolism was induced by injecting microspheres. Hearts were then treated as follows: group A, adenosine 20 microM and theophylline 60 microM; group B, theophylline 60 microM; group C, saline (control). Cardiac performance was monitored and phosphate compounds were measured by 31P magnetic resonance spectroscopy. RESULTS: Group A showed a significant increase in the left ventricular developed pressure and coronary flow, and a modest restoration of ATP content. Pi/(Pi+Pcr) remained virtually constant. There were no significant changes in left ventricular developed pressure or coronary flow in groups B and C. There was a gradual decrease in tissue ATP content and slight increase in Pi/(Pi+Pcr). CONCLUSIONS: Adenosine improved cardiac performance without adverse effect on energy metabolism in microembolised hearts. Adenosine also restored the ATP stores in microembolised hearts. (Pi = inorganic phosphate; Pcr = phosphocreatine.)