Effects of cytochalasin-B and phloretin on digitalis inotropy

Phloretin and cytochalasin-B are known to inhibit sugar transport across the cell membrane of many tissues. Both of these agents at concentrations of 100 and 20 microM, respectively, blocked the inotropic effects of ouabain and acetylstrophanthidin (AS) in isolated rabbit atria and papillary muscle preparations. Neither of these agents had any effect of its own on contractile force. Addition of phloretin or cytochalasin-B after the inotropic response to ouabain was fully established did not reverse the effect. The potency of cytochalasin-B was greater in atria than in papillary muscles, 1 microM of cytochalasin caused significant inhibition of the inotropic effect of ouabain in atria without significant effect in papillary muscles. Phloretin but not cytochalasin-B decreased the binding of [3H]ouabain to a semipurified sarcolemmal preparation isolated from canine left ventricular muscle. Neither ouabain nor AS had a substantial positive inotropic effect in atria suspended in substrate-free medium. Substitution of pyruvate (5 mM) for glucose did not fully support their inotropic effect in atria. Papillary muscles behaved differently, in that substrate-free as well as pyruvate media almost fully supported the inotropic effects of ouabain and of low concentrations of AS. Higher concentrations (greater than 250 ng/ml) of As produced a negative inotropic response in substrate-free medium. The possibility that an "active" sugar transport system is required for digitalis inotropy is ruled out by the observation that 2-deoxyglucose also prevents the inotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of batrachotoxin on cat papillary muscle.

The effects of batrachotoxin (BTX) upon the contraction and transmembrane potential of cat right ventricular papillary muscles were studied in vitro at 37 degrees C. BTX (2.0 x 10(-9) M) increased isometric contractile force by about 50% from control force, decreased the potential difference across the cell membrane to approximate -50 mV and produced spontaneous contractions of the papillary muscles. Each BTX-induced spontaneous contraction was accompanied by a spontaneous action potential which was generated when an oscillation in membrane potential reached threshold level. Spontaneous activity ovvurred only in muscles which were previously stimulated electrically. The positive inotropic effect of BTX was accompanied by an increase in the rate of force development. Papillary muscles from cats pretreated with reserpine did not differ from normal muscles in their responses to BTX treatment. Tetrodotoxin (2.0 x 10(-7) M) antagonized the effects of BTX, a finding which suggests that the actions of BTX are mediated by a selective increase in membrane permeability to sodium ions. The resultant BTX-elicited increase in the intracellular sodium ion concentration may increase the force of contraction through an augmentation of calcium influx via the sodium-calcium exchange system.     

Ultrasound intensity and contractile characteristics of rat isolated papillary muscle

The effect of graded intensities of continuous wave ultrasound on the contractile performance of isolated papillary muscle of rat was tested. Under isometric conditions rat left ventricular papillary muscles (n = 48) were electrically stimulated to contract at rates of 30, 60, 120 and 240 beats per minute. Muscles were perfused with a Tyrode solution at 30 degrees C under normoxic conditions. Ultrasound at intensities of 0.25, 0.50, 1.0 and 2.0 W/cm2 spatial average temporal average (SATA) at 963 kHz was applied to the muscles while recording muscle contractile characteristics. The analog data were digitized and stored on disk for analysis by computer. This revealed a significant (p less than 0.001) increase in peak developed force (F), peak rate of force development (+dF/dt) and peak rate of myocardial relaxation (-dF/dt) that was linearly related to ultrasound intensity. The muscles were more sensitive to ultrasound at 240 contractions per minute. Resting force was significantly decreased by ultrasound. Although bath temperature increased according to the ultrasound intensity, control studies in papillary muscles (n = 24) on the correlation between contractile parameters and temperature revealed that bulk heating could not account for the positive inotropic action with ultrasound. These data confirm the inotropic effect of continuous wave ultrasound on myocardial tissue and point to the possibility of applying this phenomenon therapeutically.     

Negative inotropic effect of platelet-activating factor: association with a decrease in intracellular sodium activity

Platelet-activating factor (PAF) is an autacoid whose cardiovascular actions include a potent negative inotropic effect. The mechanism of this decrease in myocardial contractility is still at issue, as both a decrease and an increase in trans-sarcolemmal Ca++ influx have been reported. Because changes in intracellular sodium activity (aiNa) are known to influence myocardial contractility, we investigated whether PAF affects aiNa. Thus, we have measured contractile responses to PAF (1 nM-1 microM) in isolated guinea pig right ventricular papillary muscles paced at constant rate, and recorded transmembrane action potential and aiNa with conventional and sodium-selective microelectrodes, respectively. Our findings suggest that PAF does not affect slow inward Ca++ current, because PAF neither affected nor prevented histamine-induced restoration of contractile responses in K+-depolarized papillary muscles. On the other hand, we found the negative inotropic effect of PAF to be associated with a shortening of the action potential duration and with a decrease in aiNa. The specific PAF antagonist compound CV-3988 inhibited all three electro-mechanical responses. Our findings imply that the decrease in contractile force caused by PAF may depend on the reduction in aiNa; as aiNa falls, intracellular Ca++ may be lost via the Na+/Ca++ exchange and contractility decreases. The shortening of the action potential duration by PAF may reflect a decrease in Na+ influx and the consequent reduction in aiNa.     

Pharmacological analysis of the cardiac actions of xamoterol, a beta adrenoceptor antagonist with partial agonistic activity, in guinea pig heart: evidence for involvement of adenylate cyclase system in its cardiac stimulant actions

The pharmacological effects of xamoterol, a beta adrenoceptor antagonist with partial agonistic activity, were examined in guinea pig cardiac preparations and compared with those of isoproterenol to assess possible mechanisms of its cardiac stimulant actions. Xamoterol produced a positive inotropic effect in the papillary muscles and a positive chronotropic effect in the spontaneously beating right atria in a concentration-dependent manner. The maximum inotropic and chronotropic effects of xamoterol were about 33 and 35% of those of isoproterenol, respectively. Although xamoterol failed to produce a consistent increase in contractile force in the left atria, the positive inotropic effect of the agent was observed clearly in preparations obtained from reserpine-pretreated animals. The positive inotropic and chronotropic effects of xamoterol were antagonized by atenolol, but not by ICI 118,551. On the other hand, xamoterol antagonized competitively the positive inotropic and chronotropic responses to isoproterenol. In papillary muscles the increases in contractile force induced by xamoterol and isoproterenol were depressed markedly in the presence of carbachol or adenosine. In all of left atria, right atria and papillary muscles obtained from reserpine-pretreated animals, xamoterol caused a significant elevation in cyclic AMP levels, while inhibiting the isoproterenol-induced increase in cyclic AMP levels. Computer-assisted analysis of concentration-response curves for the inhibition by xamoterol of the binding of [125I]iodocyanopindolol in the membranes from guinea pig ventricles showed the existence of the 5'-guanylylimidodiphosphate sensitive, highly affinity site of beta adrenoceptors for xamoterol, suggesting that xamoterol may induce the formation of a ternary complex with the beta adrenoceptor and a stimulatory guanine nucleotide regulatory protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of mitoxantrone on excitation-contraction coupling in guinea pig ventricular myocytes

The mechanisms of the inotropic effect of mitoxantrone (MTO), a synthetic dihydroxyanthracenedione derivative with antineoplastic activity, was investigated in guinea pig ventricular myocytes using whole-cell patch-clamp methods combined with fura-2 fluorescence and cell-edge tracking techniques. In right ventricular papillary muscles, 30 microM MTO increased isometric force of contraction as well as action potential duration (APD) in a time-dependent manner. The force of contraction was increased approximately 3-fold within 4 h. This positive inotropic effect was accompanied by a prolongation of time to peak force and relaxation time. In current-clamped single myocytes treated with 30 microM MTO for 30 min, an increase of cell shortening by 77% and a prolongation of APD by 19% was observed. Peak amplitude of the intracellular Ca(2+) transients was also increased by 10%. The contribution of APD prolongation to the enhancement of cell shortening induced by MTO was assessed by clamping control myocytes with action potentials of various duration. Prolongation of APD(90) (ADP measured at 90% of repolarization) by 24% led to an increase of cell shortening by 13%. When the cells were clamped by an action potential with constant APD, MTO still caused an increase of cell shortening by 59% within 30 min. No increase of the peak intracellular Ca(2+) transients, however, was observed under this condition. We conclude that both the APD prolongation and a direct interaction with the contractile proteins contributed to the positive inotropic effect of MTO.     

Whether phenylephrine exerts inotropic effects through α- or β-adrenoceptors depends upon the relative receptor populations

Phenylephrine produced concentration-related positive inotropic responses in isolated left atria and papillary muscles of guinea-pigs and rats. In rat tissues, these responses were unaffected by propranolol but antagonized by prazosin and therefore mediated via alpha 1-adrenoceptors. The alpha 1-adrenoceptor agonist methoxamine also exerted positive inotropic effects in these rat tissues. The maximum alpha-adrenoceptor-mediated effect of methoxamine (relative to the isoprenaline maximum) was greater than that of phenylephrine in left atria (in the presence of propranolol), whereas in papillary muscles phenylephrine exerted the greater maximum. In guinea-pig papillary muscles, the response to phenylephrine was unaffected by prazosin but was antagonized by propranolol and therefore caused by stimulation of beta-adrenoceptors. Methoxamine had no effect in guinea-pig papillary muscles. Guinea-pig left atria produced biphasic concentration-response curves for phenylephrine, the lower portion being antagonized by phentolamine and was therefore alpha-adrenoceptor-mediated, while the upper portion was antagonized by propranolol and therefore beta-adrenoceptor-mediated. Methoxamine exerted a small inotropic response, the maximum of which was similar to that of the first component of the phenylephrine response. Phenylephrine was a partial agonist for the cardiac beta-adrenoceptor. The density of rat ventricular alpha-adrenoceptors was 4 times greater than beta-adrenoceptor density, as measured by [3H]-prazosin and [3H]-dihydroalprenolol binding. This explains why the responses of rat papillary muscles were alpha-adrenoceptor-mediated. In contrast, the density of beta-adrenoceptor binding sites in guinea-pig ventricles was 6 times greater than the alpha-adrenoceptor density. This explains why the phenylephrine responses were beta-adrenoceptor-mediated in guinea-pig papillary muscles. In the left atria of guinea-pigs, which displayed both alpha- and beta-adrenoceptor-mediated responses, the densities of alpha- and beta-adrenoceptor binding sites were similar. Thus, phenylephrine exerts positive inotropic effects through alpha- or beta-adrenoceptors depending upon their relative densities.     

TMB-8 as a pharmacologic tool in guinea pig myocardial tissues. I. Effects of TMB-8 on force of contraction and on action potential parameters in atrial and papillary muscles.

The compound 8-)N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8) had been introduced as an intracellular Ca++ antagonist. We have studied the effects of TMB-8 on electrical and mechanical activity of isolated cardiac tissues in order to estimate its spectrum of action in heart muscle. In spontaneously beating right atria of the guinea pig, TMB-8 (1-100 microM) had a negative chronotropic effect. In left atria, TMB-8 (1-100 microM) induced a frequency-dependent biphasic inotropic effect: A transient increase in force of contraction was followed by a sustained decrease; the latter could be antagonized partially by an increase in [Ca++]o. TMB-8 prolonged the time-to-peak force. At high concentrations of TMB-8 (greater than 10 microM), the electrical stimulation threshold was elevated. TMB-8 (20 microM) competitively inhibited the positive inotropic effect of Bay K 8644 and reduced the magnitude of the positive inotropic and/or chronotropic effects of veratridine, (-)-isoproterenol, forskolin, histamine and (-)-phenylephrine. TMB-8 (30 microM) prolonged the action potential duration (APD) [in particular at 90% of repolarization (APD90)] and the refractory period, and decreased the AP amplitude and Vmax. In right ventricular papillary muscles, TMB-8 (30 microM) shortened the APD (APD20 = APD50 greater than APD90) and the refractory period but hardly affected the AP amplitude and Vmax. The resting membrane potential remained unchanged in both tissues. These findings suggest that in addition to interference with the Ca++ release from the sarcoplasmic reticulum, TMB-8 also affects the membrane conductances for cations.     

Positive inotropic effect and phosphoinositide breakdown mediated by arachidonic acid and prostaglandin F2 alpha

We investigated the mechanism of arachidonate- and prostaglandin-induced alteration of cardiac contractile activity in isolated rat left ventricular papillary muscles. Superfusion with 10(-6) to 10(-4) M arachidonic acid resulted in a slow developing positive inotropic effect (PIE) in a concentration-dependent manner. The PIE was abolished by pretreatment with 10(-5) M indomethacin. Prostaglandin (PG) F2 alpha also produced a significant PIE in a concentration-dependent manner, but the EC50 value was approximately 2 orders of magnitude lower and the maximum contractile response was 2-fold higher than those of arachidonate. PGE2 and PGI2 were without an effect on contractile force at concentrations ranging from 10(-9) to 10(-6) M. Both arachidonate and PGF2 alpha provoked slow responses in the partially depolarized muscles in a time course similar to that of development in the PIE. Neither arachidonate nor PGF2 alpha affected tissue levels of cyclic AMP and cyclic GMP, but these molecules increased accumulations of [3H]inositol phosphates (IPs) in a concentration-dependent manner similar to that observed for their PIE. The enhanced accumulation of [3H]IPs induced by arachidonate was abolished by pretreatment with 10(-5) M indomethacin. Although an increase in [3H]IP level was relatively rapid in PGF2 alpha-treated tissues, maximum accumulations of [3H]IPs were identical between arachidonate- and PGF2 alpha-treated tissues. Thus, for comparable increases in [3H]IPs, there was a greater PIE with PGF2 alpha than with arachidonate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Positive inotropic effects in isolated ventricular myocardium from non-failing and terminally failing human hearts

The positive inotropic responses to isoprenaline, dobutamine, histamine, forskolin, isobutyl-methylxanthine (IBMX), dibutyryl-cyclic adenosine monophosphate (db-cAMP), ouabain and calcium were studied in isolated, electrically driven papillary muscle strips from either terminally failing human hearts or non-failing donor myocardium. The positive inotropic effect of calcium has been taken to evaluate the maximal force of contraction of each individual muscle strip ('contractile reserve'). In the non-failing heart, the maximal positive inotropic effect of isoprenaline, dobutamine, IBMX, ouabain and calcium were not significantly different, but were significantly greater than histamine. In terminally failing hearts, the positive inotropic effects of agents stimulating the adenylate cyclase by a receptor-dependent mechanism (isoprenaline, dobutamine and histamine) and the phosphodiesterase inhibitor IBMX are less than in the normal heart. Furthermore, these compounds gave a markedly reduced inotropic effect compared with forskolin, db-cAMP and ouabain, which gave maximal responses similar to calcium in the failing hearts. The data did not differ when the increase of force of contraction was related to the diameter of each preparation. These results indicate that a defect in adenylate cyclase occurs in the failing human heart, presumably located at the regulatory stimulatory subunit (Gs) of the adenylate cyclase since effects through stimulatory receptors were reduced. Responses from activation of the catalytic subunit or through cAMP-dependent protein kinases were less affected. Since the positive inotropic effect of IBMX is also impaired, it is suggested that the basal rate of cAMP production is also reduced in heart failure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition by 5-hydroxytryptamine of the beta adrenoceptor-mediated positive inotropic responses to catecholamines in rabbit papillary muscles: direct interaction with beta adrenoceptors.

The effects of 5-hydroxytryptamine (5-HT) on the positive inotropic responses to catecholamines were investigated in isolated rabbit papillary muscles. 5-HT produced a concentration-dependent positive inotropic effect, an effect which was antagonized by prazosin, but not by propranolol. The positive inotropic effect of 5-HT diminished greatly in muscles from rabbits pretreated with 6-hydroxydopamine. Thus, it is likely that 5-HT causes a release of norepinephrine and increases force of contraction indirectly through alpha-1 adrenoceptors. In the presence of prazosin, 5-HT exerted a concentration-dependent inhibition of the positive inotropic response to isoproterenol. The positive inotropic responses to tyramine and a beta-1 adrenoceptor agonist T-1583 were also inhibited by the addition of 5-HT. The inhibitory effect of 5-HT on the beta adrenoceptor-mediated responses was unaffected by methysergide, ketanserin, ICS 205-930 or atropine. Pretreatment with pertussis toxin did not block the inhibitory effect of 5-HT on the inotropic response to isoproterenol, while abolishing the cholinergic interaction against the isoproterenol response. In contrast to its antagonizing effect on the inotropic response to isoproterenol, 5-HT produced an additive effect on the positive inotropic response to norepinephrine. However, when neuronal amine uptake was blocked by cocaine, the positive intropic response to norepinephrine was suppressed by the addition of 5-HT. 5-HT inhibited (-)-[125I]iodocyanopindolol binding to the membranes from rabbit ventricles with a monophasic displacement curve.(ABSTRACT TRUNCATED AT 250 WORDS)     

Demonstration of an alpha adrenoceptor-mediated inotropic effect of norepinephrine in rabbit papillary muscle

Other investigators have claimed that norepinephrine does not evoke a significant alpha adrenergic inotropic effect in rabbit ventricular myocardium in contrast to some other mammalian species, indicating an important functional limitation of the cardiac alpha adrenoceptors. We therefore characterized the inotropic effects of norepinephrine in isometrically contracting rabbit papillary muscles. We studied both contraction and relaxation by measuring developed tension and its first and second derivatives. Both the influence of propranolol and prazosin on concentration-effect curves of norepinephrine and the qualitative characteristics of the responses revealed that norepinephrine evoked both alpha and beta adrenergic inotropic effects. The alpha adrenergic response to norepinephrine was qualitatively markedly different from the beta adrenergic effect and qualitatively similar to the alpha adrenergic effect of phenylephrine which was also characterized for comparison. Although the alpha adrenergic response to norepinephrine was marked, the beta adrenergic effect was the dominating one when norepinephrine was administered alone. Thus, the beta adrenergic effect had to be extensively blocked to reveal the prazosin-sensitive alpha-1 adrenergic response. It is concluded that also in rabbit papillary muscles, norepinephrine evokes inotropic effects through both alpha and beta adrenoceptors.     

The cardiac pharmacology of tolbutamide.

Recent clinical studies have suggested an association of tolbutamide therapy with an increased incidence of cardiovascular deaths. Due to the paucity of information concerning the acute cardiac actions of tolbutamide, the effects of this agent upon cardiac contractility and automaticity were examined under in vivo and in vitro conditions in rabbit, cat and dog heart muscle preparations. Tolbutamide (10(-6) to 3 x 10(-3) M) produced a biphasic inotropic response with a peak positive inotropic response at 3 X 10(-3) M which was 13.7 +/- 5.1% of the maximal obtainable increase in tension. Similar studies in cat papillary muscle resulted in a response that averaged 19% of the maximal increase in contractile force. In contrast, canine papillary muscles as well as the intact canine heart failed to develop a positive inotropic response to tolbutamide. Responses of rabbit atrial strips to isoproterenol were not potentiated by previous exposure to tolbutamide. Exposure of rabbit atria to theophylline, 2.5 X 10(-4) M, did not potentiate the inotropic effects of tolbutamide. Stidies in spontaneously beating rabbit right atria and cat papillary muscle-Purkinje fiber preparations demonstrated that tolbutamide does not have the potential to augment automaticity in these tissues. In intact dog heart, the intracoronary administration of tolbutamide did not lead to disturbances in cardiac rhythm, providing additional evidence that tolbutamide does not increase ventricular automaticity. It is concluded that tolbutamide possess a species-specific positive inotropic effect in rabbit and cat but not in the dog. The inotropic effect is small when compared to the maximum inotropic response and is observed only in vitro. Tolbutamide lacks the ability to enhance cardiac pacemaker activity. These data do not support the conclusions of previous investigatirs concerning the possible deleterious cardiac effects of tolbutamide.     

Effects of the cyclic GMP lowering agent LY83583 on the interaction of carbachol with forskolin in rabbit isolated cardiac preparations

Cyclic GMP (cGMP) has been proposed to be involved in mediating negative inotropic responses to muscarinic agonists in the presence of cyclic AMP (cAMP)-generating agents in the heart. In order to investigate this hypothesis, the effects of the novel cGMP lowering agent, LY83583, on carbachol-induced increases in cGMP levels and decreases in tension were measured in rabbit isolated left atria and right ventricular papillary muscles, in the presence and absence of the adenylate cyclase activator, forskolin. In vehicle-treated preparations, negative inotropic responses to 3 microM carbachol in the presence of 3 microM forskolin were accompanied by significant increases in cGMP levels. Carbachol had no significant effect on forskolin-induced increases in cAMP levels. LY83583 (10 microM) reduced basal tension and basal cGMP levels, and completely abolished carbachol-induced increases in cGMP both in left atria and in papillary muscles. The LY83583 significantly reduced the magnitude of the negative inotropic responses of papillary muscles to carbachol in the presence of forskolin, but had no effect on these responses in left atria. Although a causal relationship has not been established, these data suggest that cGMP may be involved in negative inotropic responses to muscarinic stimulation in the presence of cAMP-generating agonists in ventricular muscle, but not in atria.     

Negative inotropic effect of leukotrienes: leukotrienes C4 and D4 inhibit calcium-dependent contractile responses in potassium-depolarized guinea-pig myocardium

The effects of the sulfidopeptide leukotrienes (LTs) on the contractile response of electrically paced guinea-pig right ventricular papillary muscles in vitro were studied. LTs caused a concentration-dependent (1 nM-20 microM) negative inotropic effect; the order of relative potency was LTC4 greater than or equal to LTD4 greater than LTE4. A maximal 30% decrease in contractility occurred with 1 microM LTC4. The LT-induced decrease in contractile force was not mediated by cyclooxygenase products of the arachidonic acid cascade, as it was not influenced by indomethacin (14 microM). On the other hand, the slow-reacting substance-antagonist compound FPL 55712 (480 nM) caused a marked shift to the right of the LTC4 concentration-response curve. Because the negative inotropic effect of LTD4 was attenuated by increasing [Ca++]o, we next assessed the negative inotropic effect of LTs under conditions in which myocardial contractility depends solely on the slow inward Ca++ current. As a model, we used the isoproterenol- or histamine-induced restoration of contractile response in papillary muscles rendered inexcitable by 22 mMK+. LTC4 (16-480 nM) and LTD4 (20-600nM) inhibited isoproterenol- and histamine-induced restoration of contractility in a dose-dependent manner; a maximal 90% inhibition occurred with 0.48 microM LTC4. This effect of LTs was reversed by an elevation in [Ca++]o from 1.8 to 5.4 mM and prevented by FPL 55712 (480 nM). In muscles maintained at 5.4 mM [K+]o, LTC4 (160 and 480 nM) and LTD4 (1 microM) shifted the force-frequency curve (0.1-2 Hz) downwards in a parallel fashion; a similar alteration was obtained by lowering [Ca++]o to 1 mM.     

Negative chronotropic and positive inotropic actions of phencyclidine on isolated atrial muscle in guinea pigs and rats

Chronotropic and inotropic actions of phencyclidine were studied in spontaneously beating right atrial muscle and electrically paced left atrial muscle preparations isolated from guinea-pig or rat hearts. In right atrial muscle preparations, phencyclidine (10-100 microM) decreased the frequency of spontaneous beating. Guinea-pig and rat heart preparations had similar sensitivities to this action of phencyclidine. The negative chronotropic effect was not altered by atropine. A high concentration of naloxone failed to affect the chronotropic effect of phencyclidine in guinea-pig muscle, but significantly reduced the effect in rat heart muscle preparations. Phencyclidine (1-100 microM) caused positive inotropic effects in both guinea-pig and rat heart left atrial muscle electrically stimulated at 1.5 Hz; rat heart preparations had a higher sensitivity to the positive inotropic action of phencyclidine. The positive inotropic effect was reduced by verapamil, nifedipine and relatively high concentrations of diltiazem, but was not affected by propranolol, phentolamine, tripelennamine, atropine or ryanodine, indicating that the effect is not mediated by adrenergic, histaminergic or cholinergic systems or does not involve ryanodine-sensitive calcium pools. Inactivation of the fast sodium channels by partial membrane depolarization, and subsequent restoration of the contraction by raising the extracellular Ca++ concentration, did not abolish the positive inotropic action of phencyclidine. These results suggest that the negative chronotropic effect of phencyclidine is not mediated by a stimulation of the muscarinic receptor. The positive inotropic effects of phencyclidine seem to result from an increase in Ca++ influx through the slow channels of the cardiac cell membrane.     

Cardiac effects of adenosine and adenosine analogs in guinea-pig atrial and ventricular preparations: evidence against a role of cyclic AMP and cyclic GMP

The effects of adenosine, the Ri site adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA), the Ra site agonist 5'-N-ethylcarboxamideadenosine (NECA) and the P site agonist 2',5'-dideoxyadenosine (DIDA) on force of contraction, cyclic AMP (cAMP) and cyclic GMP (cGMP) content and on transmembrane action potential were studied in isolated electrically driven left auricles and papillary muscles from guinea pigs. Furthermore, the effects on adenylate cyclase activity in a particulate membrane preparation were investigated. In the auricles, adenosine, PIA and NECA had negative inotropic effects which were accompanied by a shortening of the action potential. Theophylline antagonized these effects which are likely mediated by R site adenosine receptors. DIDA was ineffective. Except for a small positive inotropic effect of adenosine the analogs were ineffective in the papillary muscles. None of the mechanical effects was accompanied by a change in cAMP and cGMP content in the intact preparations. In the broken cell preparation PIA and NECA had no effect on adenylate cyclase activity. Adenosine and DIDA inhibited the enzyme. The latter effects can be classified as P site-mediated effects. In conclusion, distinct mechanical, i.e., negative inotropic effects of adenosine and its analogs in the heart are observed in auricular preparations only. These effects are unlikely to be related to the cAMP and/or cGMP system. Instead, they are probably due to a direct shortening of the action potential which, in turn, is conceivably due to an increase in K+ outward current and a secondary decrease in Ca++ inward current. This effect is apparently mediated by cardiac R site adenosine receptors which are not detectably coupled to adenylate cyclase.     

Frequency modulation of the inotropic action of isoproterenol in mouse heart

In mouse right ventricular strips, field-stimulated to contract isometrically in an oxygenated bicarbonate-buffered physiological salt solution at 22--24 degrees C, the EC50 for the inotropic action of isoproterenol decreased from 37 nM in muscles stimulated at 0.2 Hz to 5 nM in muscles stimulated at 3.3 Hz. At higher rates of contraction, there was also an increased sensitivity to the inotropic actions of norepinephrine and epinephrine but not to those of Ca++ and N6,O2'-dibutyryl cyclic AMP. Increasing the Ca++ concentration further decreased the EC50 for isoproterenol at 3.3 Hz but had no effect on the EC50 at 0.2 Hz. The leftward shift of the contractile response curve at 3.3 Hz was inhibited by verapamil (0.6 microM) and Mn++ (0.25 mM). The stimulation of cyclic AMP accumulation was approximately 6-fold more sensitive to isoproterenol at 3.3 than at 0.2 Hz, but isoproterenol increased contractile force at concentrations two orders of magnitude lower than those that significantly increased cyclic AMP accumulation. Inhibition of cyclic AMP phosphodiesterase activity further increased the sensitivity to the inotropic actions of isoproterenol but did not attenuate the frequency difference. The results indicate that isoproterenol-stimulated Ca++ influx through the slow channel plays an important role in the mechanism of the increased sensitivity to the inotropic action of isoproterenol found at higher frequencies of contraction. Although cyclic AMP accumulation was also frequency dependent, its role in the inotropic action of isoproterenol in mouse heart is not clear.     

Regional differences in the responses of guinea-pig cardiac tissue to carbachol and their potentiation by hypothermia

The responses of guinea-pig isolated cardiac tissues to carbachol were examined. Tension responses of paced left atria and right ventricular papillary muscles, rate responses of spontaneously beating right atria and working hearts and contractility (+dP/dt) of paced and unpaced working hearts were obtained at 38 degrees C. Carbachol induced negative inotropic and chronotropic responses of atria, abolishing tension and rate at the maxima. The spontaneously beating heart also exhibited negative chronotropy. The papillary muscles displayed partial inhibition of tension but, in tissues from reserpine-pretreated animals, negative inotropy was absent. Similarly, no reduction of contractility of paced working hearts was obtained. It was concluded that muscarinic receptors mediating a direct inhibition of ventricular muscle are virtually absent and that the small response obtained in untreated tissue may be due either to inhibition of endogenous catecholamine release via presynaptic receptors or to antagonism of released norepinephrine. Lowering the temperature to 30 or 25 degrees C affected resting tension, rate and contractility and the magnitude of carbachol responses. The concentration-response curves, when plotted as a percentage of the maximum, were displaced to the left by cooling of the atria and papillary muscles. The papillary muscles now exhibited a response after reserpine pretreatment. In working hearts, the concentration-response curves for the fall in spontaneous rate were also shifted to the left, but this was not significant, probably because the temperature could be reduced to only 30 degrees C, below which contractions ceased. Cooling of guinea-pig isolated cardiac preparations therefore induced supersensitivity to the muscarinic effects of carbachol.     

Differential effects of somatostatin on atrial and ventricular contractile responses in guinea pig heart: influence of pretreatment with islet-activating protein

The effects of somatostatin on the contractile response of guinea pig cardiac preparations were investigated and compared with those of carbachol and adenosine. Somatostatin produced a concentration-dependent negative inotropic effect in the left atria, which was accompanied by a decrease in action potential duration. The maximum decrease in contractility which was obtained at 3 x 10(-6) M was around 40% of the predrug control values and far less than those produced by carbachol and adenosine. Somatostatin failed to produce inotropic effect on the papillary muscle and did not influence the spontaneously beating rate of the right atria. In the papillary muscles, however, somatostatin inhibited the positive inotropic effect of isoproterenol in a concentration-dependent manner as did carbachol and adenosine. In addition, somatostatin caused a significant inhibition of the isoproterenol-induced increase in cyclic AMP levels without affecting the basal level of cyclic AMP. In the papillary muscle, the inhibitory effect of somatostatin on the positive inotropic response to isoproterenol was significantly attenuated by pretreatment with islet-activating protein, and was significantly antagonized by the somatostatin antagonist cyclo[7-aminoheptanoyl-Phe-D-Trp-Lys-Thr(Bzl)]. These results suggest that somatostatin receptors in guinea pig ventricular muscles are coupled with adenylate cyclase via islet-activating protein-sensitive GTP-binding protein, whereas the negative inotropic effect of somatostatin in the left atria might be mediated by a subtype of somatostatin receptors which is different from that in the ventricle.