Xestoquinone, a novel cardiotonic agent activates actomyosin ATPase to enhance contractility of skinned cardiac or skeletal muscle fibers

Xestoquinone (XQN), a novel cardiotonic principle from the sea sponge Xestospongia sapra, enhanced Ca+(+)-induced tension development of chemically skinned fibers from guinea pig cardiac muscle, even at both free Ca++ concentrations as low as -log molar free Ca++ (pCa) 9 to 8. In skinned fibers from guinea pig skeletal muscle, XQN (10 microM) also increased developed tension with a similar Ca++ dependence to that for cardiac fibers. In contrast to the unique Ca+(+)-dependence of XQN effects, the reference drug sulmazole enhanced Ca+(+)-induced tension development of skinned cardiac fibers at pCa 6.6 but did not affect it at pCa 8. In natural actomyosin from canine cardiac muscle, as well as in that from rabbit skeletal muscle, XQN (1-30 microM) enhanced the rate and extent of superprecipitation. Moreover, XQN produced a concentration-dependent increase in the myofibrillar ATPase activity of canine cardiac muscle, even at very low free Ca++ concentrations below the normal threshold for ATPase activation (pCa 9-8). The natural actomyosin ATPase activity of chicken smooth muscle was not influenced by XQN (up to 30 microM). In cardiac myofibrils, no significant difference was observed between the bound 45Ca+(+)-pCa relationship curves in the presence and absence of XQN (10 microM). Furthermore, XQN (30 microM) did not cause or potentiate Ca+(+)-induced Ca++ release from cardiac sarcoplasmic reticulum vesicles. These observations suggest that XQN directly activates actomyosin ATPase activity of cardiac and skeletal myofibrils, thus producing an enhanced superprecipitation activity as well as an increase in skinned fiber contractility.(ABSTRACT TRUNCATED AT 250 WORDS)     

The influence of ketamine on inotropic and chronotropic responsiveness of heart muscle.

The influence of ketamine on the inotropic and chronotropic responsiveness of heart muscle was examined in spontaneously beating right atrial preparations and in electrically driven left atrial preparations of guinea pigs. Ketamine (2.63 X 10(-5) to 4.2 X 10(-4) M) decreased heart rate of right atria and decreased contractile tension and its maximum rate of increase in both right and left atrial preparations (right atria greater than left atria). Ketamine did not prevent the heart rate increase produced by norepinephrine (NE; 1 X 10(-8) to 1 X 10(-4) M) in right atria; however, the maximum heart rate was consistently lower in ketamine-treated than in control muscles even after exposure to NE. Although contractile tension was decreased by ketamine, the maximum inotropic response to NE was consistently greater in ketamine-treated atria than in control atria. An inhibitor of the slow Ca++ current in heart muscle, D600, depressed the contractile effects of NE but did not prevent the positive inotropic interaction of ketamine and NE. Ketamine similarly enhanced the inotropic responses to norepinephrine (1 X 10(-6) M), epinephrine (1 X 10(-6) M), isoproterenol (1 X 10(-7) M) and dibutyryl cyclic adenosine 3':5'-monophosphate (AMP; 4 X 10(-3) M) in left atria electrically paced at a constant frequency of contraction of 1 Hz; however, ketamine inhibited the positive inotropic response to increased frequency of stimulation (0.1-3.0 Hz) and to ouabain (3 X 10(-7) M). These findings demonstrate that ketamine can exert a selective positive inotropic influence in heart muscle independent of heart rate or direct or reflexogenic autonomic nervous system changes, and suggest that this activity could in some way be associated with an alteration of the intracellular disposition of cyclic AMP.     

Cardiotonic action of [8]-gingerol, an activator of the Ca++-pumping adenosine triphosphatase of sarcoplasmic reticulum, in guinea pig atrial muscle

[8]-Gingerol (gingerol), a component of ginger, produced a concentration-dependent positive inotropic effect on guinea pig isolated left atria at concentrations of 1 X 10(-6) to 3 X 10(-5) M. Gingerol also exhibited positive inotropic and chronotropic effects on guinea pig right atria. The gingerol-induced inotropic effect was abolished by ryanodine, but was little affected by propranolol, chlorpheniramine, cimetidine, tetrodotoxin, diltiazem or reserpine. The time to peak tension and relaxation time within a single contraction were shortened by gingerol (1 X 10(-5) M) as well as isoproterenol, whereas they were prolonged by BAY K 8644. In guinea pig isolated atrial cells, gingerol (3 X 10(-6) M) caused an increase in the degree and the rate of longitudinal contractions. In guinea pig left atria, gingerol (1 X 10(-6) to 3 X 10(-5) M) gave little influence on the action potential, although it increased the contractile force of the atria. The whole-cell patch-clamp experiments showed that the slow inward current was little affected by gingerol (1 X 10(-6) to 3 X 10(-5) M) in voltage-clamped guinea pig cardiac myocytes. The measurement of extravesicular Ca++ concentration using a Ca++ electrode indicated that gingerol (3 X 10(-6) to 3 X 10(-5) M) accelerated the Ca++ uptake of fragmented sarcoplasmic reticulum (SR) prepared from canine cardiac muscle in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiotoxic effects of maitotoxin, a principal toxin of seafood poisoning, on guinea pig and rat cardiac muscle

Maitotoxin (MTX), a principal toxin of seafood poisoning, produced powerful cardiotoxic effects on guinea pig isolated left atria at concentrations of 5 X 10(-9) to 3 X 10(-8) g/ml. The MTX-induced increase in resting tension of atria was abolished by Co++, D600 or Ca++-free solution. The tissue Ca content and 45Ca uptake of guinea pig atria were increased by MTX (5 X 10(-9) to 3 X 10(-8) g/ml), and these increases were inhibited markedly by Co++. In isolated rat cardiac myocytes, irreversible contracture was produced by MTX (10(-8) g/ml), and this effect of MTX was suppressed by verapamil or Ca++-free solution. The intracellular free Ca++ concentration of isolated rat myocytes was increased greatly by MTX (10(-9) to 3 X 10(-8) g/ml). Furthermore, the myocardial cells sampled from guinea pig left atria were characterized ultrastructurally by severely overcontracted sarcomeres, swollen mitochondria, peripheral aggregation of nuclear chromatin and loss of granular glycogen, and these morphological changes were abolished by omitting Ca++ from the medium. These results suggest that MTX increases the Ca++ influx through the cardiac muscle membrane to create the Ca-overloaded state and thus caused the cardiotoxic effects.     

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.     

Extracellular magnesium and cardiotonic steroid toxicity in isolated myocardial preparations

This study examined effects of extracellular magnesium (Mg++0) on the positive inotropic and toxic actions of cardiotonic steroids in cardiac muscle isolated from guinea pig heart. Increasing concentrations of Mg++0 produced a negative inotropic effect in electrically paced, left atrial muscle and decreased the sensitivity to arrhythmogenic actions of digoxin without affecting the maximum developed tension observed before dysrhythmic activity. Other signs of toxicity such as contracture were less sensitive to the antagonistic effects of Mg++0. Estimates of fractional occupancy suggested that the increased tolerance to digoxin-induced arrhythmias was mediated by an altered responsiveness to given levels of receptor binding. Experiments in partially purified membrane preparations demonstrated that elevations in Mg++ increased affinity for [3H]ouabain without affecting binding site density. Na+,K+-adenosine triphosphatase activity in these membrane preparations was also enhanced by Mg++; however, increases in buffer Mg++ concentration had no effect on the Na+-pump in intact tissue. In summary, these results indicate that elevations in Mg++0 act directly on myocardium to diminish the sensitivity to cardiotonic steroid-induced arrhythmias. Furthermore, data suggest that this antagonistic action of Mg++0 is not mediated by alterations in receptor binding or Na+-pump reserve capacity.     

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)     

Increase in Ca++ sensitivity of the contractile system by MCI-154, a novel cardiotonic agent, in chemically skinned fibers from the guinea pig papillary muscles

The effects of MCI-154, a novel cardiotonic agent, on the contractile protein system and the sarcoplasmic reticulum (SR) were investigated by using thin bundles of chemically skinned fibers from the guinea pig papillary muscles. In the skinned muscle fibers treated with 50 micrograms/ml of saponin, MCI-154 shifted the -log[Ca++]M-tension relation curve to the left and upward in the concentration-dependent manner (10(-7) to 10(-4) M). This was confirmed also in the skinned muscle fibers treated with 250 micrograms/ml of saponin which destroyed not only the surface membrane but also the function of SR. Sulmazole (10(-4) M) shifted the -log[Ca++]M-tension relation curve to the left but the effect was about 100 times less potent than that of MCI-154. Unlike MCI-154, sulmazole had little effect on the maximum tension development induced by -log[Ca++]M 4.4. Milrinone did not affect the Ca++-induced tension development in the skinned cardiac fibers. Higher concentration of MCI-154 (10(-4) M) also increased amplitude of -log[Mg-ATP]M-tension-curve in the absence of free Ca++ ion (bell-shaped curve) to the upward. Initial rate and plateau phase of Ca++ uptake by the SR in the skinned fibers treated with 50 micrograms/ml of saponin was increased slightly by MCI-154 at the concentrations of 10(-6) and 10(-4) M. MCI-154 had no effect on the Ca++-induced Ca++ release mechanism in the SR. These results suggest that an increase in Ca++ sensitivity of the contractile protein system is responsible for, at least in part, the mechanism of the positive inotropic action of MCI-154.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Pharmacological characterization of a new Ca(2+) sensitizer

The benzimidazole molecule was modified to synthesize a Ca(2+) sensitizer devoid of additional effects associated with Ca(2+) overload. Newly synthesized compounds, termed 1, 2, 3, 4, and 5, were evaluated in spontaneously beating and electrically driven atria from reserpine-treated guinea pigs. Compound 3 resulted as the most effective positive inotropic agent, and experiments were performed to study its mechanism of action. In spontaneously beating atria, the inotropic effect of 3 was concentration-dependent (3.0 microM-0.3 mM). Compound 3 was more potent and more active than the structurally related Ca(2+) sensitizers sulmazole and caffeine, but unlike them it did not increase the heart rate. In electrically driven atria, the inotropic activity of 3 was well preserved and it was not inhibited by propranolol, prazosin, ranitidine, pyrilamine, carbachol, adenosine deaminase, or ruthenium red. At high concentrations (0.1-1.0 mM) 3 inhibited phosphodiesterase-III, whereas it did not affect Na(+)/K(+)-ATPase, sarcolemmal Ca(2+)-ATPase, Na(+)/Ca(2+) exchange carrier, or sarcoplasmic reticulum Ca(2+) pump activities of guinea pig heart. In skinned fibers obtained from guinea pig papillary muscle and skeletal soleus muscle, compound 3 (0.1 mM, 1 mM) shifted the pCa/tension relation curve to the left, with no effect on maximal tension and no signs of toxicity. Compound 3 did not influence the basal or raised tone of guinea pig isolated aorta rings, whose cells do not contain the contractile protein troponin. The present results indicate that the inotropic effect of compound 3 seems to be primarily sustained by sensitization of the contractile proteins to Ca(2+).     

Vascular and cardiac effects of a new dihydropyridine derivative, YC-170: a comparison with Bay K 8644

The pharmacological effects of YC-170, a new dihydropyridine derivative, were studied in the rabbit aortic strips and guinea pig cardiac preparations and compared with those of Bay K 8644. In the rabbit aortic strips, YC-170 produced contraction in normal physiological saline solution ([K+]0 = 5.9 mM) in a concentration-dependent manner. Increasing the [K+]0 of the medium to 15 mM enhanced the contractile response. The maximum contraction produced by YC-170 at [K+]0 of 15 mM was comparable to that by Bay K 8644. However, YC-170 induced relaxation when the strip was contracted by 60 mM K+. In guinea pig left atrium, YC-170 produced a positive inotropic effect in a concentration-dependent manner, but its extent was far less than that of Bay K 8644. Like Bay K 8644, however, YC-170 increased the time to peak tension and relaxation time of the isometric tension, and prolonged the action potential duration. YC-170 failed to produce a positive inotropic action in the papillary muscle in which Bay K 8644 was a potent positive inotropic agent. In spontaneously beating right atria, YC-170 caused a negative chronotropic effect, whereas Bay K 8644 a positive one. The positive inotropic and vasoconstrictor effects of YC-170 were antagonized competitively by a Ca++ antagonist nicardipine. When the left atria were depolarized with high-K+ medium, the positive inotropic effect of YC-170 was attenuated progressively with increasing [K+]0 and at 13.2 mM K+ a negative inotropic effect was induced by YC-170.(ABSTRACT TRUNCATED AT 250 WORDS)     

SR 33557, a novel calcium entry blocker. I. In vitro isolated tissue studies.

The effects of SR 33557 on isolated cardiovascular preparations were compared to those of nifedipine, verapamil and diltiazem. In rat aortic strips, SR 33557, like nifedipine, verapamil and diltiazem, caused a significant and simultaneous inhibition of potassium-induced 45Ca++ influx and contractile responses (nifedipine greater than SR 33557 greater than verapamil greater than diltiazem). SR 33557 also antagonized Ca(++)-induced contractions in K(+)-depolarized aorta preparations (pA2:9.08 +/- 0.03) and is the first calcium channel antagonist, structurally not related to 1,4-dihydropyridines, to inhibit competitively contractions induced by BAY K8644. In spike-generating vascular smooth muscle (rat portal vein), contractures evoked by noradrenaline (4 microM) or KCl (100 mM) were reduced by all four antagonists, the pharmacological potency being nifedipine greater than SR 33557 greater than verapamil greater than diltiazem. Unlike SR 33557, nifedipine, verapamil and diltiazem showed a parallel enhancement of frequency of spontaneous contractions in rat portal vein in spite of a concentration-related reduction in amplitude. By using rabbit atrial preparations, spontaneous right atrial rate and electrically stimulated (120/min) basal contractions of left atria were used as indices of chronotropy and inotropy. The potency series for negative chronotropic effects was nifedipine greater than SR 33557 greater than verapamil greater than diltiazem. For negative inotropic effects the potency order was verapamil greater than nifedipine greater than SR 33557 greater than diltiazem, respectively. Thus, SR 33557 should depress heart rate to a greater extent than ventricular contractility. These results suggest that SR 33557 is a potent calcium entry blocker that (unlike verapamil and diltiazem) is particularly selective for vascular smooth muscle and devoid of any potent negative inotropic actions.     

Digitalis-induced mechanical toxicity: protection by slow Ca++ channel blockers

In the in vitro perfusion of the isolated heart, toxic doses of cardiac glycosides produce an inotropic response which is followed by a decline in contractile force and an increase in the resting tension. Several reports in the literature indicate that the subsequent decline in contractile force may be related to cardiac cellular Ca++ overload. The purpose of the present study was to determine if the slow Ca++ channel blockers such as verapamil and nifedipine, which block Ca++ influx through voltage-dependent gated channels, can reduce or prevent the digitalis-induced decline in contractile force (mechanical toxicity). Langendorff preparations of isolated perfused guinea pig heart were used for the present study. The data obtained demonstrate that 1 to 2 microM ouabain in the perfusion medium produced mechanical toxicity in the hearts after an initial inotropic response. Verapamil or nifedipine, when combined with ouabain in the perfusion medium, increased the magnitude of the inotropic response and delayed or abolished the mechanical toxicity in a dose-dependent manner. No changes in the sarcolemmal Na+,K+-adenosine triphosphatase or ouabain binding were observed in the presence of verapamil or nifedipine. The data suggest that simultaneous use of verapamil or nifedipine may protect against digitalis-induced mechanical toxicity.     

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.     

Cardiovascular properties of LF 2.0254, a new potent vasoselective calcium channel blocker with a slow onset of action

The effects of LF 2.0254, a new 1,4-dihydropyridine derivative, were studied in rabbit aorta stimulated by various contractile agents and in isolated guinea pig atria. LF 2.0254 inhibited in a time-dependent fashion K(+)- and CA2(+)-induced contractions of rabbit aorta with respective IC50 of 2.7 nM and 1.7 nM. An action of LF 2.0254 at the voltage operated calcium channel was consistent with the finding that LF 2.0254 antagonized 45Ca2(+)-uptake induced by depolarisation of smooth muscle, and since contractile responses evoked by (-) norepinephrine and the calcium ionophore A23187 were insensitive to the drug. In isolated paced left atria and spontaneously beating right atria of guinea pig, LF 2.0254 added for 60 min at 1 microM only slightly decreased the contractile force and beating rate. In anesthetized open-thorax dogs, LF 2.0254 (1 to 100 micrograms/kg, iv) dose-dependently lowered systemic blood pressure and increased cardiac output with a slower onset of action than nifedipine. LF 2.0254 and nifedipine decreased total peripheral, coronary, femoral and vertebral resistance. In marked contrast to nifedipine, LF 2.0254 induced only a slight decrease in left ventricular dP/dt. In conscious hypertensive dogs LF 2.0254 (0.1 and 0.3 mg/kg per os) decreased blood pressure and concomitantly increased heart rate and plasma renin activity. It is concluded that LF 2.0254 differ markedly from nifedipine in its more gradual onset of action and greater selectivity for the vasculature with respect to the myocardium.     

On the mechanism of vasodilating action of berberine: possible role of inositol lipid signaling system.

We have studied the effect of the alkaloid berberine on the contraction of guinea pig aortic strips induced by various stimuli. Berberine (25-200 microM) inhibited the response of the strips to norepinephrine and histamine, but did not decrease the high K(+)-elicited contraction. The antagonism of berberine was not competitive because in the presence of the alkaloid, maximum response to agonists could not be obtained. Analysis of the drug's effect on the time course of norepinephrine-induced contraction showed that berberine reduced both the rate and the relative contribution to developed tension of the initial, rapid phase, whereas the slow, later component was less affected. Berberine inhibited the response of aortic strips incubated in 0 mM Ca++ to norepinephrine, but did not reduce caffeine-induced contraction and also inhibited phospholipase C-activated contractile response, which has been ascribed to production of inositol phosphate-3 in smooth muscle cells. In cultured arterial smooth muscle cells (A7r5 line), the alkaloid did not significantly decrease the production of inositol phosphates activated by Arg8-vasopressin. The pattern of berberine action is difficult to reconcile with an involvement of the contractile machinery and suggests that the drug has no effect on the voltage-operated calcium channels. Although an antagonism at the receptors or an increase of cyclic AMP or cyclic GMP cannot be completely excluded, we suggest that at least one component of the berberine inhibitory effect may be due to its action on some step of the chain of events linking receptors to contractile response.     

A polypeptide (AP-A) from sea anemone (Anthopleura xanthogrammica) with potent positive inotropic action.

Anthopleurin-A (AP-A), a polypeptide with MW ca. 5500 (53 amino acids), isolated from the sea anemone, Anthopleura xanthogrammica (Brandt), elicited a potent positive inotropic effect but without an accompanying chronotropic effect on the isolated cardiac muscles of rat, rabbit, guinea pig and cat. Similarly in dogs and cats in situ, i.p. injections of AP-A increased the contractile force without effect on heart rate or blood pressure. The cardiotonic potency for AP-A was equivalent to that of isoproterenol but much greater than that for ouabain or glucagon on the isolated cardiac muscle. AP-A increased the contractile force (cardiac output) and decreased atrial pressure in dog heart during pentobarbital-induced failure. This inotropic effect was not inhibited by propranolol pretreatment. The Ca++ requirement to restore the contractile force was less in AP-A-treated than in ouabain or isoproterenol-treated tissues. After AP-A treatment, the cardiac contractility was more resistant to hypoxia and to low or high temperature stress than ouabain-treated or control preparations. AP-A at 5 10(-9) M increased the duration of the action potential, its mean rate of rise and conduction in the guinea-pig atria and ventricles. At the maximum effective concentration, AP-A did not inhibit Na+, K+-activated adenosine triphosphatase, phosphodiesterase (high Km and low Km) and cyclic 3',5'-adenosine monophosphate content of guinea-pig heart. AP-A (5 X 10(-8) to 5 X 10(-7) M) neither contracted nor relaxed the isolated vascular smooth muscle. The results suggest that AP-A may be useful in the clinical management of cardiac failure and as an experimental tool to study the pharmacology and physiology of cardiac muscle.     

Potent stimulation of myofilament force and adenosine triphosphatase activity of canine cardiac muscle through a direct enhancement of troponin C Ca++ binding by MCI-154, a novel cardiotonic agent

In the present study we have analyzed a likely biochemical mechanism underlying the Ca++-sensitizing action of MCI-154 (6-[4-(4'-pyridyl)aminophenyl)-4,5-dihydro-3(2H)-pyridazinone hydrochloride), a novel cardiotonic agent, on the contractile protein system. MCI-154 (10(-7) to 10(-4) M) enhanced the tension development induced by -log molar-free Ca++ concentration (pCa) 5.8 in chemically skinned fiber from the canine right ventricular muscle in a concentration-dependent manner. At pCa 7.0, MCI-154 (10(-7) to 10(-4) M) markedly increased adenosine triphosphatase (ATPase) activities of canine myofibrils and reconstituted actomyosin. In myofibrils and reconstituted actomyosin, MCI-154 (10(-7) to 10(-4) M) caused a parallel shift of the pCa-ATPase activity relation curve to the left without affecting the maximum activity, suggesting an increase in Ca++ sensitivity. MCI-154 (10(-8) to 10(-4) M) had little effect on actin-activated, Mg++, Ca++ and (K+, EDTA)-ATPase activities of myosin. Ca++ binding to cardiac myofibrils or purified cardiac troponin was increased by 10(-4) M MCI-154. These results suggest that MCI-154 enhances Ca++ binding to cardiac troponin C to elevate the Ca++ sensitivity of myofilaments and thus may cause a positive inotropic action in cardiac muscle. MCI-154 may provide a valuable tool for studying the molecular mechanism by which Ca++ regulates the contractile system.     

Effect of endothelin-1 on guinea pig gallbladder smooth muscle in vitro.

The purpose of this study was to investigate the pharmacological activity of endothelin-1 (ET-1) on guinea pig gallbladder smooth muscle. Guinea pig gallbladder muscle strips were mounted in 10-ml siliconized organ baths containing Krebs' solution. After 1 hr of equilibration, ET-1 was added cumulatively. ET-1 induced slow-developing and long-duration contractile responses. The EC50 was approximately 10 nM. ET-1 was 5 times less potent than cholecystokinin (EC50, 2 nM), but 20 and 40 times more potent than carbachol (EC50, 200 nM) and histamine (EC50, 400 nM), respectively. The concentration-response curve to ET-1 was not affected by tetrodotoxin (0.1 microM) or by the muscarinic antagonist, atropine (10 microM). The neuronal N-type calcium channel blocker, omega-conotoxin (0.1 microM), had no significant effect on the ET-1 concentration-response curve. In contrast, the contractile effect to ET-1 was reduced markedly by removal of extracellular calcium or by the voltage-dependent calcium channel blockers nicardipine and diltiazem. Substitution of strontium (an inhibitor of intracellular calcium release) for Ca++ significantly reduced the response to ET-1. The cyclooxygenase inhibitor indomethacin had no significant effect on the contractile activity of ET-1. These finding suggest that ET-1 is a potent contractile stimulant of guinea pig gallbladder and that it acts directly on the smooth muscle. The activity depends on extracellular Ca++, suggesting involvement of Ca++ influx via the voltage-dependent Ca++ channel and on intracellular calcium.     

Ca++ inhibition of isoproterenol responses in mammalian skeletal muscle

In noncontracting mouse hemidiaphragms incubated in modified Krebs-Ringer--bicarbonate buffer with 10 mM Ca++, isoproterenol-stimulated phosphorylase a formation, conversion of phosphorylase kinase to the activated form, elevation of cyclic AMP-dependent protein kinase activity ratios and increase in cyclic AMP concentrations were reduced 35 to 50% over the responses in buffer with 2.5 mM Ca++. In buffer with 10 mM Ca++, the initial rate of isoproterenol-stimulated cyclic AMP accumulation was 59% of that in buffer with 2.5 mM Ca++. The inhibitory action of Ca++ on cyclic AMP accumulation was antagonized by verapamil, but not by inhibitors of cyclic nucleotide phosphodiesterase activity. In buffer with 2.5 mM Ca++, isoproterenol-stimulated cyclic AMP accumulation was inhibited by A23187 and caffeine, agents that can increase intracellular Ca++ concentrations. In addition to Ca++, high concentrations of Co++, Ni++, Mn++ and, to a lesser extent, Sr++ inhibited the isoproterenol response. The results of these studies indicate that high buffer Ca++ concentrations inhibit the response of the glycogenolytic pathway to isoproterenol by an action on cyclic AMP formation. We propose that the site of the inhibitory action of Ca++ is the divalent metal activator site associated with hormone-stimulated adenylate cyclase activity.