Guinea-pig Atrial Muscle Research Articles

The differential effects of a selective kappa-opioid receptor agonist, U50488, in guinea pig heart tissues.

The differential effects of a selective kappa- (κ-) opioid receptor agonist, U50488, were elucidated by monitoring the contraction of isolated guinea pig atrial and ventricular muscles. In electrically driven left atria, U50488 in nanomolar concentration range decreased the contractile force. Norbinaltorphimine (norBNI), a selective κ-receptor antagonist, and pertussis toxin (PTX) abolished the negative inotropic effect of U50488. In contrast, the inhibitory effect was not affected by the pretreatment of atropine or propranolol. Even though U50488 exerted a negative inotropic effect in the left atrium, it did not affect the contractile force of the right atrium and ventricles paced at 2 Hz. Similarly, the beating rate of the spontaneously beating right atrium was also unaffected by U50488. These results indicate that the activation of κ-opioid receptors can only produce negative inotropic effect in left atria via activation of PTX-sensitive G protein in guinea pigs. The absence of negative inotropic effects in right atria and ventricles suggests that there may be a greater distribution of functional κ-opioid receptors in guinea pig left atria than in right atria and ventricles, and the distribution of the receptors may be species-specific.

Green Tea Catechins Increase the Force of Contraction in Isolated Guinea Pig Atrial Muscle Preparations by Increasing the Amplitude of Intracellular Ca<sup>2+</sup> Concentration

It has been reported that green tea catechins enhance the force of contraction of isolated heart muscle preparations. However, it remains controversial whether or not the increase in force of contraction is related to an increase in the intracellular Ca(2+) concentration ([Ca(2+)]i). In this study, the relationship was investigated using a left atrial muscle preparation isolated from guinea pig heart. In the left atrial muscle preparations without fura-2/AM loading, neither EGC (epigallocatechin) nor EC (epicatechin) influenced the force of contraction, but EGCG (epigallocatechin gallate) and ECG (epicatechin gallate) increased the force of contraction in a dose-dependent manner. The ED(50) value of EGCG was significantly higher than that of ECG. In the atrial muscle preparations loaded with fura-2/AM, EGCG and ECG increased the amplitude of [Ca(2+)]i(peak [Ca(2+)]i minus diastolic [Ca(2+)]i) which is associated with the increase in force of contraction. Simple regression analysis between the degree of increase in the force of contraction and the increase in the amplitude of [Ca(2+)]i revealed a positive correlation in EGCG, ECG and CaCl(2). In addition, the slopes of the regression lines of EGCG and ECG were comparable with those of CaCl(2). It was suggested that atrial muscle preparations had a higher affinity for ECG than EGCG, and that the increase in the force of contraction by EGCG and ECG was closely related to the increase in the amplitude of [Ca(2+)]i.

Binding studies on two functional cardioselective antimuscarinic compounds.

Vecuronium and himbacine are antimuscarinic compounds which in functional studies exhibited a ca 6- and 10-fold higher potency at cardiac muscarinic receptors than at ileal muscarinic receptors. However in binding studies both compounds failed to differentiate between [3H](-)-QNB binding sites in guinea-pig atrial and ileal muscle homogenates. In the latter experiments, the dissociation constants of vecuronium in atria and ileum and that of himbacine in ileum were lower than the values determined functionally. The basis for the lack of cardioselectivity in binding studies is not known. These compounds add to the list of functional cardioselective muscarinic receptor antagonists that failed to display selectivity in binding studies with [3H](-)-QNB.

Anticholinergic effect of 2-aminopyridine and its sulfonylcarbamide derivatives on electromechanical activity in guinea pig atrium

The aim of the study was to investigate the effect of class IA antiarrhythmic drug procainamide and its new derivatives PA20, PA23, PA28 and PA53 on carbachol-induced action potential duration and contraction force in guinea pig atrial muscles. Experiments were carried out using standard method of registration of myocardium electromechanical activity. Under control conditions (perfusion of atrial strips with Tyrode solution), mean action potential duration measured at 90% (AP90) and 50% (AP50) of repolarization were 92.5+/-4.5 ms and 44.4+/-2.9 ms (n=18), respectively, and contraction force was of 2.7+/-0.5 mN (n=12). Carbachol (10(-6) M), an agonist of muscarinic acetylcholine receptor and activator of K(Ach) channels, markedly decreased AP90 to 32.4+/-2.4%, AP50 - to 25.4+/-2.2% (n=18) (p<0.001) and contraction force - to 24.2+/-5.8% (n=16) (p<0.05), vs. control. Procainamide and its new sulfonylcarbamide derivatives PA20, PA23, PA28 (10(-5)-3x10(-3) M), and PA53 (10(-5)-10(-3) M) reversed the carbachol-induced action potential duration shortening at different extent: procainamide derivative PA20 (N-cyclohexylsulfonylcarbamide fragment is linked up to benzene ring) had the most potent anticholinergic effect on action potential duration and contraction force of guinea pig atrial muscles. N-ethylsulfonylcarbamide fragment in PA23 or permanent positive charge of aliphatic nitrogen in PA28 had a weaker anticholinergic effect (similar to procainamide action) on action potential duration and contraction force than PA20. The weakest anticholinergic effect was induced by procainamide derivative PA53 with isosteric form of sulfonylcarbamide.

Wavelength dependence of the optically recorded action potentials in guinea pig atrial muscles

The optically recorded action potentials of the frog atrial muscles which lack transverse tubules showed different features from those reported by Heiny and Vergara (Heiny JA, Vergara J. Optical signals from surface and T system membranes in skeletal muscle fibers. J Gen Physiol 1982;80:203–230) in skeletal muscles (Fujishiro N, Kawata H. The wavelength dependence of optically recorded action potentials in the atrial muscles of the bullfrog ( Rana catesbeiana). Comp Biochem Physiol 1996;114A:153–157). We examined whether or not the differences were consistent in other atrial muscles which lack transverse tubules with guinea pig atrial muscles. Two dyes (merocyanine rhodanine and merocyanine oxazolone) were used, and the dependence of the maximum rising phase of the optical signals on the wavelength of the incident beam was analyzed. No dependence was observed between them, and this finding was consistent with the structure of the membrane system of the guinea pig atrial muscles. The optical signals recorded at 718 nm of the incident beam from the guinea pig atrial muscles which stained with merocyanine oxazolone showed a more prominent second rising phase after the initial rapid rising phase of the optical signal than that recorded in the frog atrial muscles. This phase was not observed in the optical signals recorded at other wavelengths. The features of the optically recorded action potentials in guinea pig atrial muscles were consistent with those recorded in frog atrial muscles. Nifedipine did not affect the second rising phase.

Effects of magnesium and glucose, insulin, potassium (GIK) solution on the action potential parameters of guinea-pig atrial muscle.

The present paper aims to explore the effects of [Mg2+]o and glucose, insulin and K+ (GIK) on action potential parameters in guinea-pig atrial muscle. Specimens of atrial appendages were taken from guinea-pig hearts. Action potentials were recorded in isolated atrial trabeculae. Resting potential (RP) and action potential duration at 90% repolarization (APD90) were measured with conventional microelectrode techniques. [Mg2+]o at 6 and 12 mmol L-1 depolarized the RP and prolonged the APD90, whereas 4 mmol L-1 had no effect at all. Glucose alone or in combination with insulin had no effect on action potential parameters. GIK solution with supernormal [K+]o at 6 mmol L-1 depolarized the RP and decreased the APD90. Intervention with [Mg2+]o at 4 mmol L-1 in combination with GIK solution with supernormal [K+]o of 6 mmol L-1, reversibly depolarized the RP, whereas the APD90 was not significantly changed. [Mg2+]o at 12 mmol L-1 in combination with GIK solution with a physiological [K+]o of 4 mmol L-1 prolonged the APD90 whereas the RP was unaffected. [Mg2+]o at 6 and 12 mmol L-1 slightly depolarized the RP and prolonged the APD90. The action potential of normally polarized atrial muscle was not sensitive to supernormal levels of glucose alone or in combination with insulin. The effects of [Mg2+]o in combination with the GIK solutions on action potential parameters seemed to be attributable to the supernormal [Mg2+]o and [K+]o alone, while these seemed to have opposite effects on APD90.

Mechanism of the positive inotropic action of cocaine in the guinea pig atrium

We studied the mechanism of the positive inotropic action of cocaine in isolated guinea pig atria superfused with Tyrode's solution at 31 °C while attached to a force transducer to measure peak tension developed, maximum velocity of development of tension, and time to peak tension. Cocaine 2.9 μM enhanced peak tension developed and velocity of development of tension, and prolonged time to peak tension. The increase in peak tension developed produced by cocaine was not affected by propranolol. On the other hand, the cocaine-induced increase in velocity of development of tension was reduced, but not abolished. In the presence of propranolol and phentolamine combined, the cocaine-induced prolongation of time to peak tension was abolished and the increases of both peak tension developed and velocity of development of tension were significantly smaller than those observed in the absence of the two adrenergic blockers. For all practical purposes, nifedipine completely abolished the increase in peak tension developed induced by cocaine. It is concluded that the positive inotropic effect of cocaine in the guinea pig atrial muscle is predominantly the result of adrenergic-dependent, both α- and β- receptor mediated, as well as adrenergic-independent increases in calcium influx through the L-type calcium channels in the sarcolemma.

Diverse inotropic effects of 5-hydroxytryptamine in heart muscles of various mammalian species

The inotropic effects of 5-hydroxytryptamine (5-HT) on mammalian heart muscles were investigated. 5-HT (10(-8)-10(-3)M) produced increases in the contractile tension of atrial and ventricular muscles isolated from guinea pigs, Japanese monkeys, and humans, but not in rat heart preparations. The maximum percent increase of contraction was largest in guinea pig ventricular muscles (142.0 percent), followed by monkey atrium (86.3 percent), human atrium (71.7 percent), guinea pig atrium (48.7 percent), and monkey ventricle (30.1 percent). The sensitivity to 5-HT, measured as the negative logarithm of the half-maximal inotropic molar contractions of 5-HT, i.e., -logEC(50), was highest in the human atrium (6.65 +/- 0.20), followed by guinea pig atrium (5.53 +/- 0.36), monkey ventricle (4.83 +/- 0.28), guinea pig ventricle (4.56 +/- 0.11), and monkey atrium (4.46 +/- 0.16). The inotropic effects of 5-HT seen in the atrial and ventricular muscles of guinea pigs were abolished in the presence of the beta-receptor blocker, pindolol (8 mu M), while these effects in human atrial muscles and monkey atrial and ventricular muscles were abolished only in the presence of both pindolol (8 mu M) and of prazosin (1 mu M), an alpha(1)-receptor blocker. 5-HT increased the V(max) of the slow response recorded from guinea pig ventricular muscles exposed to high K+ (27 mM) media, whereas this agent did not alter the calcium current of isolated guinea pig ventricular myocytes devoid of sympathetic nerve terminals. In reserpinized guinea pig hearts, 5-HT exerted no inotropic effect on ventricular muscle, yet it had an inotropic effect in the atrial muscle, although the latter effect was considerably depressed, compared to that seen in non-reserpinized atrial muscles. We conclude that the positive inotropic effects of 5-HT observed in the ventricular muscle of the guinea pig and in the atrial and ventricular muscles of the Japanese monkey can be attributed to the release of noradrenaline from sympathetic nerve terminals (indirect effect). In contrast, in human atrial muscles, the positive inotropic effect of 5-HT was apparently the result of stimulation of a specific membrane receptor for 5-HT (direct effect). In guinea pig atrial muscles, both direct and indirect effects of 5-HT were involved in the positive inotropism. An explanation for the lack of sensitivity of rat atrial and ventricular muscles to 5-HT awaits further studies.

Cellular electrophysiological effects of MS‐551, a new class III antiarrhythmic agent

AbstractThe cellular electrophysiological effects of MS‐551, a pure class III antiarrhythmic agent lacking beta‐blocking activity, were compared with those of d‐sotalol and dofetilide in canine Purkinje fibers and ventricular muscles and in guinea pig atrial muscles using the standard microelectrode technique. MS‐551 prolonged the action potential duration (APD) of these cardiac tissues to the same extent in a concentration‐dependent manner (10−6−10−4 M). MS‐551 did not affect other action potential parameters of these tissues except for the Vmax, which was depressed approximately 10% at 10−4 M MS‐551. The extents of prolongation after 10−5 M of MS‐551 were 36, 27, and 35% for canine Purkinje and ventricular muscle and guinea pig atrial muscle, respectively. d‐Stalol at a concentration 10 times higher than MS‐551 produced a similar prolongation of the APD of canine cardiac tissues but only a slight prolongation of the APD of guinea pig atrial muscles. Dofetilide at a concentration 30 times lower than MS‐551 produced a prolongation of the APD of canine Purkinje fibers but a relatively slight prolongation of the APD of both canine ventricular muscles and guinea pig atrial muscles. In canine Purkinje fibers, MS‐551 did not prevent the APD shortening induced by high extracellular potassium (8 and 12 mM), but the APD under these conditions was lengthened compared with controls. Under simulated ischemic conditions (pO2 25–30 mmHg, pH 6.5, extracellular K 8 mM and no glucose) for 30 min, the APD was abruptly shortened during the first 5 min, then gradually shortened further up until 30 min. MS‐551 did not prevent the abrupt APD shortening, but slowed the later gradual shortening. The APD prolonging action of MS‐551 was enhanced at low frequency. These data indicate that (1) MS‐551 is a pure class III antiarrhythmic agent with electrophysiological characteristics basically similar to d‐sotalol and dofetilide; (2) MS‐551 has a relatively greater potency in guinea pig atrial muscle than d‐sotalol and dofetilide; (3) the APD prolonging action of MS‐551 is preserved under high K+ or simulated ischemic conditions; and (4) MS‐551 exhibits a reverse use‐dependence in prolonging APD. © 1995 Wiley‐Liss, Inc.

Intracellular sodium activity and its regulation in guinea-pig atrial myocardium.

1. Intracellular Na+ activity (aNai) and membrane resting potential were studied in quiescent guinea-pig atrial and papillary muscles by means of Na(+)-sensitive and conventional microelectrodes. The effects of the cardioactive steroid dihydroouabain (DHO) on aiNa, force of contraction and sarcolemmal Na+, K(+)-ATPase activity were also investigated. 2. In thirty atria and twenty-two papillary muscles, aNai amounted to 8.0 +/- 0.2 and 4.7 +/- 0.3 mM, respectively (mean +/- S.E.M.). When both tissues were from the same animal, with the same ion-sensitive microelectrode mean aNai values of 7.9 +/- 0.2 and 5.1 +/- 0.5 mM (P < 0.01) were obtained from eight atrial and eight papillary muscles, respectively. 3. Membrane resting potentials (Em) were significantly (P < 0.001) more negative in the papillary muscles (-83.5 +/- 0.7 mV; n = 8) than in the atrium (-78.1 +/- 0.5 mV; n = 8). Deviation of Em from EK (determined by K(+)-sensitive microelectrodes) was 3.0 +/- 0.2 mV in ventricular (P < 0.05) and 6.1 +/- 0.3 mV in atrial preparations (P < 0.05). 4. Inhibition of the Na+ pump by DHO increased aNai of the atrium within 10 min by 0.6 +/- 0.1 (n = 7), 1.3 +/- 0.1 (n = 5) and 3.2 +/- 0.2 mM (n = 5) at 5, 10 and 30 microM, respectively. In the papillary muscle, 10 microM DHO was without effect while aNai rose by 1.0 +/- 0.1 (n = 5) and 2.9 +/- 0.2 mM (n = 6) at 30 and 120 microM DHO. 5. Consistent with the aNai measurements, the potency of DHO to increase force of the isometric contraction was three times higher in atrium than in papillary muscle (stimulation frequency 0.2 Hz). 6. Hydrolytic activity of sarcolemmal Na+,K(+)-ATPase isolated from atria amounted to only one third of that detected in ventricles (0.07 +/- 0.01, n = 6, versus 0.2 +/- 0.01 mumol phosphate released min-1 (g tissue)-1, n = 5). The inhibitory potencies of DHO on sarcolemmal Na+,K(+)-ATPase preparations were found to be identical in the enzymes from either tissue. 7. It is concluded that a lower Na+ pump density is responsible for the higher aNai and for the lower resting membrane potential in atrial as compared to ventricular cells. The regulation of cellular Na+ homeostasis in atrial muscle appears to be closer to the limits of its capacity than in ventricle, explaining the higher sensitivity of the atrium to interventions which impede Na+ pump activity.

Inhibitory Effects of KW-3902, a Selective Adenosine A1-Receptor Antagonist, on the Adenosine-Induced Shortening of Action Potential Duration in Guinea Pig Atrial Muscles

We investigated the effects of KW-3902 (8-(noradamantan-3-yl)-1,3- dipropylxanthine), a newly-synthesized selective adenosine A1-receptor antagonist, on the shortening of action potential duration (APD) in guinea pig atria exposed to adenosine. The APD shortening by adenosine was inhibited by KW-3902 at higher than 10(-8) M, but not by 10(-5) M of KF17837, an adenosine A2-receptor antagonist. These results support the notion that the APD shortening by adenosine in atria is mediated via adenosine A1-receptors. The potency of KW-3902 in antagonizing the APD-shortening were similar to those in antagonizing the negative inotropic and chronotropic action of adenosine in the isolated right atria, suggesting that these responses to adenosine are mediated via the receptors of the same type.

Protection of human, rat, and guinea-pig atrial muscle by mioflazine, lidoflazine, and verapamil against the destructive effects of high concentrations of Ca2+.

In right atrial trabeculae from humans and in left atria from rat and guinea-pig hearts, the protective effects of mioflazine, lidoflazine, and verapamil against the accumulation of cellular calcium were investigated. Two consecutive, cumulative increases in the extracellular calcium concentration, [Ca2+]o (1-25 mmol/l), were induced, in between which the muscles were exposed for at least 30 minutes to solvent or drug. When using solvent in the 30-minute interval, the force of contraction was much lower during the second Ca2+ challenge, while the aftercontractions and the increase in passive tension at high [Ca2+]o tended to be larger. These signs of functional impairment were prevented by exposure to mioflazine or lidoflazine (3 mumol/l each) but not to verapamil (3 mumol/l). Muscles were fixed with glutaraldehyde at the end of the second Ca2+ challenge for morphological and cytochemical examination. After solvent treatment, more than half of the cells were severely damaged, showing cellular edema, contraction-band necrosis, mitochondrial swelling, and nuclear pyknosis; the sarcolemma was devoid of calcium deposits, damaged mitochondria contained either large deposits of calcium or flocculent densities, and in some cells, the cytoplasm was filled with calcium deposits. Following exposure to mioflazine and lidoflazine, but not to verapamil, the number of intact cells after the second Ca2+ challenge was not different from time-matched controls (80-90%). Furthermore, the shifts in cellular calcium distribution were prevented with mioflazine and lidoflazine, whereas verapamil was less effective. There were no species differences with respect to either morphological or contractile changes. In conclusion, exposure of atria to high [Ca2+]o induced similar ultrastructural and cytochemical changes as seen after ischemia-reperfusion induced damage. Indeed, under the mentioned conditions the sarcolemma lost its capacity to exclude Ca2+ after a challenge with high [Ca2+]o and allowed excessive Ca2+ entry. The pathway for this extra Ca2+ remains to be elucidated. L-type calcium channels are probably not involved, since verapamil cannot prevent the Ca2+ overload.

Electrophysiological properties of a new antiarrhythmic agent, bisaramil on guinea-pig, rabbit and canine cardiac preparations

Electrophysiological effects of bisaramil, a novel antiarrhythmic agent, were examined using the conventional microelectrode technique applied to cardiac multicellular preparations from guinea-pigs, rabbits and dogs and the whole-cell patch-clamp technique applied to guinea-pig ventricular myocytes. Bisaramil at 10(-6) M or higher concentrations produced a dose-dependent decrease in the maximum rate of rise (Vmax) of action potentials of guinea-pig papillary muscles without changes in resting membrane potentials. In the presence of bisaramil, trains of stimuli at rates greater than 0.1 Hz led to the use-dependent block of Vmax, which was enhanced at higher frequencies. At a concentration of 3 x 10(-6) M, the degree of use-dependent block was about 35% at 3.3 Hz, of which degree was comparable to those of 10(-4) M disopyramide and lidocaine. The development of Vmax block by bisaramil was expressed by a single exponential function in the same manner as flecainide, whereas the time courses of the block development by disopyramide and lidocaine were described by two exponentials. Recovery time constants from Vmax block were 44.1 +/- 3.4 s and 20.3 +/- 2.3 s for bisaramil and flecainide, respectively. Bisaramil at 10(-6) and 3 x 10(6) M did not change the action potential duration of guinea-pig papillary muscles and rabbit atrial muscles with a significant reduction of Vmax. No change in action potential duration can be explained by depression of both the Ca2+ and the delayed outward K+ currents by bisaramil. On the other hand, 10(-6) M bisaramil shortened action potential duration of canine Purkinje fibers at 50% and 90% of repolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional and structural impairment in human, rat and guinea-pig atrial muscle in response to in vitro calcium overload: a cytochemical study on cellular calcium distribution

Critical accumulation of cellular calcium in ischaemic myocardium is involved in irreversible cell damage. In human right atrial trabeculae and in rat and guinea-pig left atria, we investigated whether direct calcium overload by increasing the extracellular calcium concentration, [Ca 2+] o, leads to similar impairment of function and ultrastructure as observed after ischaemia. The force of contraction was measured during two consecutive cumulative increases in [Ca 2+] o (1–25 mmol/l) separated by 30 min of incubation at low [Ca 2+] o. Compared to the first Ca 2+ challenge, the positive inotropic effect of increasing [Ca 2+] o was depressed during the second one and the after-contractions, and the increase in resting tension developing with high [Ca 2+] o tended to be larger. The ultrastructure of the tissue fixed immediately after excision was well preserved. When fixed after the second Ca 2+ challenge, half of the cells were severely damaged with various signs of cellular Ca 2+ overload similar to those observed after ischaemic damage: the sarcolemma lost its Ca 2+-binding properties, sarcomeres showed contraction band necrosis, the mitochondria had disrupted cristae and contained either large clusters of Ca 2+ precipitate or amorphous densities (Jennings granules). In many cells, calcium precipitates were present in the cytoplasm. The morphological and functional changes were similar in the three species studied. Our results suggest that the deterioration of atrial myocardium after challenge with high [Ca 2+] o or after severe ischaemia may be traced back to a common mechanism, i.e. the sarcolemma loses its competence as a permeable barrier for Ca 2+ and therefore facilitates excessive Ca 2+ entry. However, for the direct demonstration of calcium precipitates as a sign of cytosolic Ca 2+ overload, high [Ca 2+] o are required with are not normally present in the myocardium.

Effects of (S)-nafenodone, a new antidepressant, in isolated guinea-pig atrial and ventricular muscle fibres

The electromechanical effects of a new antidepressant, (S)-nafenodone, were studied in isolated guinea-pig atrial and ventricular muscle fibres. In spontaneously beating right atria, (S)-nafenodone decreased the rate and amplitude of contractions and lengthened the sinus node recovery time. In electrically driven atria, the negative inotropic effect of (S)-nafenodone was less marked than that of imipramine and desipramine but similar to that of lofepramine. (S)-Nafenodone had no effect on the resting membrane potential but decreased the amplitude and maximum upstroke velocity (V max) both in atrial and ventricular muscle fibres; this effect was less marked than that produced by imipramine and desipramine. In atrial fibres, but not in ventricular fibres, (S)-nafenodone lengthened the action potential duration, but in both tissues it increased the duration of the effective refractory period out of proportion to the change in action potential duration. Moreover, it shifted the concentration-response curve for Ca 2+ downwards and decreased the amplitude of the slow atrial contractions induced by histamine as well as the amplitude and V max of the slow action potentials induced by isoprenaline in papillary muscles. It is concluded that (S)-nafenodone exerted fewer cardiodepressant effects than imipramine and desipramine in isolated guinea-pig atrial and ventricular muscle fibres less.

Positive Inotropic and Chronotropic Effects of 8-Substituted Derivatives of Cyclic AMP and Activation of Protein Kinase A

Inotropic and chronotropic effects of 8-substituted derivatives of cyclic AMP (8-SH, 8-SCH2C6H5, 8-N3, 8-SCH3, 8-Br, 8-N(CH3)2, 8-OCH3) were studied using guinea pig atrial and ventricular muscle preparations and correlated with the activation of the protein kinase A derived from the bovine myocardium. All the compounds produced positive inotropic and chronotropic effects. A good correlation was found between the chronotropic effect and the activation of the enzyme, while such a good correlation was not found between the enzyme activation and the positive inotropic effect. However, after treatment of the preparation with theophylline, the positive inotropic effects of some derivatives were potentiated to such a degree that the positive inotropic effects became well-correlated to the activation of the protein kinase. To elucidate the mechanism of the potentiation by theophylline, the effects of 8-phenyltheophylline and 3-isobutyl-1-methylxanthine on the positive inotropic effects of 8-Br and 8-OCH3 cyclic AMPs were studied. While 3-isobutyl-1-methylxanthine potentiated the effects of both compounds, 8-phenyltheophylline potentiated the effect of only 8-OCH3 cyclic AMP and only in the atria. These results suggest that the positive inotropic and chronotropic effects of 8-substituted cyclic AMP essentially due to the activation of the protein kinase A, with the hydrolysis of the compounds by phosphodiesterase and (in the atria) activation of adenosine R-receptor subserving the negative inotropic effect intervening.

Electrophysiological and mechanical effects of calcitonin gene‐related peptide on guinea‐pig atria

1. The effects of calcitonin gene-related peptide (CGRP) on mechanical and electrophysiological responses were studied in the guinea-pig atrial muscle preparations and in single cells. 2. CGRP (greater than 10(-9) M) enhanced the twitch contraction in a concentration-dependent manner in electrically driven left atria and increased heart rate in spontaneously beating right atria. The positive inotropic and chronotropic effects of CGRP were not inhibited by propranolol but were attenuated by reduction of the calcium concentration in the bathing medium. 3. In single left atrial cells, CGRP slightly hyperpolarized the resting potential but did not affect the other action potential parameters significantly. 4. Under whole-cell voltage-clamp conditions, CGRP increased the calcium inward current. The peptide also increased the steady inward current elicited by hyperpolarization and the late outward current by depolarization. 5. These results suggest that CGRP may produce the positive inotropic and presumably chronotropic effects by increasing calcium inward current. CGRP also increases the potassium permeability. Such effects on ionic currents may not produce any apparent change in the action potential conformation, due to their opposite directional actions and relatively weak potencies.

Endothelin has no positive inotropic effect in guinea‐pig atria or papillary muscle

Endothelin is a recently discovered, highly potent vasoconstrictor peptide. In isolated atria from rat and guinea-pig, endothelin has been reported to elicit a positive inotropic effect. The purpose of the present study was to compare the effects of endothelin on electromechanical coupling in guinea-pig atrial and ventricular muscle. In isolated, electrically driven specimens of atria and papillary muscle, action potentials and isometric contractions were recorded in the basal state and 30 min following non-cumulative exposure to endothelin (100 nM). In the atria, endothelin reduced action potential overshoot and relaxation velocity, and increased resting tension. In the papillary muscle the peptide slightly shortened the duration of the action potential. Endothelin did not affect peak tension, either in the atria or in the papillary muscle. These data contrast with earlier reports on a positive inotropic effect of endothelin in guinea-pig atria.

Somatostatin decreases the calcium inward current in guinea‐pig atria

1. The effects of somatostatin on mechanical and electrophysiological responses were studied in guinea-pig atrial muscle preparations and single cells. 2. Somatostatin (greater than or equal to 10(-8) M) decreased the twitch contraction in a concentration-dependent manner in electrically driven left atria and spontaneously beating right atria. However, the beating rate was not affected. 3. The negative inotropic effect of somatostatin was transient. Desensitization to this agent developed slowly during continuous exposure to the peptide. 4. In single atrial cells, somatostatin significantly shortened the action potential duration, but the resting potential and action potential amplitude were not affected. 5. Under whole cell voltage-clamp conditions, somatostatin decreased the calcium inward current without affecting the sodium and potassium currents. 6. These results suggest that somatostatin selectively acts on the calcium channel of guinea-pig atrial cells to reduce the calcium inward current, which in turn gives rise to the negative inotropic effect.

Displacement of [ 3H]oxotremorine-M binding by muscarinic antagonists in guinea-pig atrial and ileal membrane homogenates

Displacement of [ 3H]oxotremorine-M ([ 3H]oxo-M) binding by muscarinic antagonists that are functionally non-selective (atropine), ileoselective (4-DAMP) or cardioselective (gallamine, pancuronium, vecuronium, himbacine) was investigated in guinea-pig atrial and ileal longitudinal muscle membranes. [ 3H]Oxo-M bound to a single population of high affinity sites in atrial (K spD = 11.40 nM) and ileal (K spD = 6.15 nm) membranes. Atropine displaced [ 3H]oxo-M binding sites in a competitive manner, showing similar affinities in the two tissues. 4-DAMP showed two binding sites in ileum but not in atria. The dissociation constant at the high affinity site in ileum was ca 5-fold lower than the value observed in atria, indicating ileoselectivity. Vecuronium also displaced [ 3H]oxo-M binding in a competitive manner and exhibited similar affinities in both tissues. Gallamine, pancuronium and himbacine displayed two binding sites in each of the two tissues with the majority of sites (ca. 60–80%) showing high affinity. Overall the cardioselective antagonists do not exhibit any consistent correlation between the affinities found in functional experiments and those determined in binding experiments.