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.     

Mechanism of inotropic action of xestoquinone, a novel cardiotonic agent isolated from a sea sponge.

Xestoquinone (XQN) isolated from the sea sponge Xestospongia sapra produced dose-dependent cardiotonic effects on guinea pig left and right atria. A direct action of XQN (1-30 microM) on the contractile machinery of cardiac myofilaments was demonstrated in chemically skinned fiber preparations from guinea pig papillary muscles. In atrial preparations, the XQN-induced inotropic effect was markedly inhibited by verapamil or nifedipine, but was not affected by practolol, chlorpheniramine, cimetidine, tetrodotoxin or reserpine. The Ca++ dependence curve for the contractile response of the atria was substantially shifted to the left by XQN (10 microM), and this XQN-induced shift was reversed by verapamil. The time-to-peak tension and relaxation times of the atrial contractions were shortened by XQN, and the action potential duration was markedly prolonged. Whole-cell patch clamp recordings in left atrial strips confirmed that XQN (30 microM) increased the slow inward current. However, there was a temporal dissociation between altered tension development and prolongation of the action potential duration. Cyclic AMP phosphodiesterase activity was inhibited and tissue cyclic AMP content of guinea pig left atria was increased by XQN (0.3-10 microM) in a concentration-dependent manner, but increases in cyclic AMP content did not occur in parallel with increases in contractile response. These observations suggest that an enhancement of intracellular cyclic AMP content and Ca++ influx across the cell membrane contribute to the late phase of XQN-caused cardiotonic responses, whereas the early phase may largely be elicited through direct activation of contractile elements. XQN may provide a novel leading compound for valuable cardiotonic agents.     

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.     

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.     

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)     

5-Hydroxytryptamine antagonist ICS 205-930 blocks cardiac potassium, sodium and calcium currents

Effects of the 5-hydroxytryptamine receptor antagonist ICS 205-930 [(3 alpha-tropanyl)-1H-indole-3-carboxylic acid ester] on cardiac membrane currents were investigated in single isolated ventricular cells using the whole-cell patch clamp method. Ca++ and K+ currents were studied in guinea pig ventricular cells and Na+ currents were studied in ventricular cells from cultured neonatal rat; these cells are more suitable for Na+ current measurements than are ventricular cells from guinea pig. Under current clamp conditions, ICS 205-930 at 3 x 10(-5) M prolonged the action potential plateau and increased its amplitude of guinea pig cell. The effect was reversible. Increasing the concentration to 3 x 10(-4) M shortened the plateau, reduced its amplitude and depolarized the resting membrane potential. The effects between 10(-7) and 10(-3) M were examined under voltage-clamp conditions. ICS 205-930 produced a concentration-dependent suppression of inwardly rectifying K+ currents with an IC50 of 1.95 x 10(-5) M at a test potential of -40 mV. The effects were time-and voltage-dependent and the IC50 increased to 1.16 x 10(-4) M at -100 mV. The time-dependent outward current and the time-dependent outward tail currents upon repolarization to between -10 and -30 mV also were blocked by the drug in a concentration-dependent manner with IC50 of 3.7 x 10(-5) M. Ca++ currents and Na+ currents also were inhibited in the presence of higher concentration of ICS 205-930 (greater than 10(-4) M), although potency was stronger on Na+ currents. The results show that ICS 205-930 exerts mixed class III and class I antiarrhythmic properties in ventricular myocytes.     

o-Isothiocyanate dihydropyridine (oNCS-DHP), a long-acting, reversible inhibitor of the Ca++ channel

The binding characteristics and pharmacological properties of o-isothiocyanate dihydropyridine [oNCS-DHP; 2,6-dimethyl-3,5-dicarbomethoxy-4-(2-isothiocyanatophenyl)-1, 4-dihydropyridine] were investigated in guinea pig heart and ileum. [3H]oNCS-DHP bound to a single population of high-affinity sites (Bmax = 107 fmol/mg of protein and Kd = 0.99 nM) in cardiac membranes, with a specificity characteristic of dihydropyridine receptors. After incubation of membranes with the tracer (0.5 nM), addition of excess nifedipine (1 microM) caused a dissociation of [3H]oNCS-DHP from its binding site. The reversibility of [3H]oNCS-DHP binding was confirmed by the lack of affinity labeling of cardiac membranes as determined by sodium dodecylsulfate-polyacrylamide gel electrophoresis. oNCS-DHP inhibited the inward Ca++ current of isolated guinea pig cardiac myocytes as determined in voltage-clamp experiments. In isolated perfused guinea pig hearts, oNCS-DHP caused a concentration-dependent increase in coronary artery flow and a decrease in left ventricular pressure. The effects of the highest concentration (0.3 microM) were still near maximal after a 1-h washout. Suppression of K+ depolarization-induced contractures of isolated ileal longitudinal muscle strips by oNCS-DHP remained maximal even after 5 h of washout. In all of the three biological test systems investigated, the Ca++ channel activator Bay K 8644 caused a complete and rapid reversal of the inhibitory effects of oNCS-DHP. Thus, it can be concluded that oNCS-DHP does not bind irreversibly to Ca++ channel dihydropyridine receptors in guinea pig heart and ileum. However, the o-isothiocyanatophenyl substituent on the dihydropyridine molecule confers upon the compound a very long duration of Ca++ channel blocking activity.     

Molecular basis for the cardiovascular activities of amrinone and AR-L57

It has been suggested that amrinone and AR-L57 enhance cardiac contractility either by inhibiting phosphodiesterase activity or altering Ca++ homeostasis. Because these novel agents are potentially useful in the management of heart failure, it was of interest to more clearly define their mechanism(s) of action. Amrinone and AR-L57 caused concentration-dependent increases in the contractile states of either perfused guinea-pig hearts or cultured rat cardiomyocytes. To determine whether these actions might result from an increase in sarcolemmal Ca++ movement, the effects of these agents on Ca++ accumulation were studied in a simple system, dog erythrocytes. Both agents promoted erythrocyte Ca++ accumulation in time and concentration-dependent manners, effects that resulted primarily from increased Ca++ entry. However, because these effects were not measurable at inotropic drug concentrations and were apparent only after a 30-min incubation, they did not provide an explanation for the inotropic effects of these agents. Amrinone and AR-L57 inhibited dog heart phosphodiesterase activity (isozyme III) with EC50 values of 23 and 420 microM, respectively; however, only the inotropic responses to amrinone were attenuated by the muscarinic agonist, carbachol, thereby implying a cAMP (cyclic AMP)-dependent mechanism. In cultured ventricular cells, concentrations of amrinone (2 X 10(-4) M) and AR-L57 (3 X 10(-5) M) that caused maximal inotropic responses were associated with the activation of glycogen phosphorylase, but neither drug significantly increased the activation state of cAMP-dependent protein kinase. To further probe the effects of these drugs on intracellular cAMP and Ca++ metabolism, their effects on protein phosphorylation were studied.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Calcium-independent activation of contractile apparatus in smooth muscle by calyculin-A

Calyculin-A (CL-A), a novel marine toxin isolated from Discodermia calyx, caused contraction in the smooth muscle of guinea pig taenia ceci and rat aorta in the presence or absence (with 1 mM ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid) of external Ca++ at concentrations ranging from 1 X 10(-8) to 1 X 10(-6) M. In the presence of external Ca++, the contraction induced by CL-A was accompanied by an increase in the cytosolic free Ca++ concentration [( Ca++]cyt) as measured by the fluorescence indicator fura-2. Verapamil (3 X 10(-6) M) inhibited the increase in [Ca++]cyt, but not tension development caused by CL-A. In the absence of external Ca++, CL-A still caused contraction without changing [Ca++]cyt. Thus, from studies with intact smooth muscle it was demonstrated that, in the absence of external Ca++, CL-A can induce a contraction that was not accompanied by an increase in [Ca++]cyt. In permeabilized taenia, CL-A caused contraction in the absence of Ca++ (with 2 mM ethylene glycol bis(beta-aminoethyl ether)N,N'-tetraacetic acid) at concentrations similar to those required to contract intact tissue. This contraction was inhibited by the nonselective kinase inhibitors such as amiloride (1 X 10(-3) M) and K-252a (2 X 10(-5) M). Low concentrations of Ca++ (approximately 1 X 10(-6) M) augmented the CL-A-induced contraction in the permeabilized taenia. In native actomyosin prepared from chicken gizzard CL-A induced phosphorylation of the 20 kDa myosin light chain (MLC) in the absence of Ca++.(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.     

Release of N-[3H]methylscopolamine from isolated guinea pig atria is controlled by diffusion and rebinding

This study attempts to analyze the inter-relationship between the slow fading of muscarinic antagonist action observed in isolated tissue preparations upon washout and the dissociation kinetics at the receptor level. Isolated guinea pig left atria stimulated electrically at 3 Hz were equilibrated with N-[3H]methylscopolamine ([3H]NMS) and the time course of release of [3H]NMS was studied. In parallel experiments, the fading of the antimuscarinic action of NMS was monitored by repeated determinations of the negative inotropic effect of oxotremorine. In comparison, the dissociation of [3H]NMS from muscarinic receptors was recorded in a membrane suspension of guinea pig right atria. Having been equilibrated at 10(-8) M [3H]NMS, the atria released [3H]NMS extremely slowly, when transferred into a drug-free washout bath: not even half of the initially bound [3H]NMS was lost within 3 hr. With a corresponding time course, 10(-8) M oxotremorine regained its negative inotropic action. In contrast, when 10(-4) M unlabeled NMS was present in the washout bath, it took only 5 min for the [3H]NMS binding to fall by 50%. Similarly, in a membrane-suspension of guinea pig right atria, the half-life time of the [3H]NMS receptor complexes amounted to about 5 min. It is concluded that the dissociation of NMS from its receptor sites proceeds with a half-life time of 5 min in intact atria as well as in the cardiac membrane suspension.(ABSTRACT TRUNCATED AT 250 WORDS)     

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)     

Cardiac histamine receptors: differences between left and right atria and right ventricle.

Histamine can stiumulate the heart by directly interacting with cardiac histamine receptors. In the present study we have investigated the cardiac effects of histamine, 4-methylhistamine (a specific H2-receptor agonist) and 2-pyridylethylamine PEA, a specific H1-receptor agonist] on spontaneously beating right atria and electrically driven left atria and right ventricle strips of the guinea pig. Left atria were driven at 1 Hz and right ventricle strips at 2.5 Hz and at twice the threshold voltage. Histamine and PEA produced a dose-dependent positive inotropic effect on the left atria. The dose-response curves were shifted to the right in a parallel fashion by promethazine (3 x 10(-6)M)+... Burimamide did not affect either dose-response curve. Histamine and 4-methylhistamine had a positive chromnotropic effect on right atria and a positive inotropic effect on right ventricle strips.     

Vasodilatory action of HA1004 [N-(2-guanidinoethyl)-5-isoquinolinesulfonamide], a novel calcium antagonist with no effect on cardiac function

A novel compound, N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA1004) was found to be a potent relaxant of blood vessels. Rabbit aortic strips contracted by 18 mM KCl relaxed in the presence of HA1004, with an ED50 value of 2.2 X 10(-6) M. The isolated guinea-pig atria on right ventricular papillary muscles did not respond to HA1004, even at a concentration of 3 X 10(-4) M. HA1004, like verapamil, produced a competitive inhibition of the Ca++-induced contraction of the depolarized rabbit aorta. The pA2 value of HA1004 for the Ca++-induced contraction was 6.60. Atropine, propranolol, theophylline or indomethacin had no effect on the HA1004-induced relaxation. HA1004 (3 X 10(-6) M) inhibited the contraction produced by Ca++ ionophore, A23187, whereas verapamil or diltiazem had no effect on this contraction, even at a concentration of 3 X 10(-5) M. Moreover, HA1004 produced a competitive inhibition of Ca++-induced contraction of the A23187-treated aorta and inhibited the phenylephrine-induced contraction elicited in ca++-free solution, thereby suggesting that this drug affects intracellular rather than extracellular Ca++. The present findings indicate that HA1004 is a novel calcium antagonistic vasodilator with no effect on cardiac function and is apparently of a different class of calcium antagonist from verapamil.     

MCI-154, a novel cardiotonic agent, reverses the acidic pH-induced decrease in responses of cardiac myofilaments to Ca++: comparison with sulmazole and pimobendan

In the present study, we have examined the effects of decreasing pH from 7.0 to 6.6 on the tension developed by direct activation of the myofilaments in chemically skinned fibers from guinea pig papillary muscles. We then compared the effects of the novel inotropic agents MCI-154, pimobendan and sulmazole, which have direct action on cardiac myofilaments, on the acidic pH-induced changes in responses of the contractile system to Ca++. The reduction of pH from 7.0 to 6.8 shifted the pCa (-log[Ca++] M)-tension relation curve to the right with no change in maximum tension. However, the reduction of pH from 7.0 to 6.6 shifted the pCa tension relation curve to the right and also depressed maximum force development. These effects were reversible by returning to neutral pH (pH 7.0), but were not overcome by increasing the free [Ca++] (decreasing pCa from 4.4 to 4.0). The amplitude of pMg-ATP (-log[MgATP]M)-tension curve in the absence of free Ca++ (Ca++ less than 1 nM, bell-shaped curve) was shifted downward by reducing pH from 7.0 to 6.6. MCI-154 (1-100 microM) reversed the acidic pH-induced decrease of tension development which was activated by pCa 5.8 in a concentration-dependent manner. Moreover, the acidosis induced reductions of maximum tension (pCa, 4.4) and pMgATP 6.0-activated tension (Ca++ less than 1 nM) were also reversed by MCI-154 (1-100 microM) in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Gentamicin blockade of slow Ca++ channels in atrial myocardium of guinea pigs.

Cardiac dysfunction is occasionally detected in patients undergoing treatment with amino-glycoside antibiotics, however, the mechanism responsible for the negative inotropic effect of these agents has not been identified. In the present investigation electrically driven left atria of guinea pigs were used to study the effects of gentamicin on calcium ion (Ca++)-dependent contractile events in heart muscle isolated from in vivo influences. When atria were first inactivated by excess potassium ion (K+; 22mM) and contractions were then restored by isoproterenol (an experimental model that accentuates the contractile dependence of myocardial fibers on influx of Ca++ through specific "slow channels" of the sarcolemma), the cardiac depressant activity of gentamicin (0.1 mM) was profoundly augmented. Conversely, the negative inotropic effect of tetrodotoxin (23.5 micron) was abolished by the same experimental conditions. Also, gentamicin (1 mM) and La+++ (0.5 mM) markedly decreased the positive inotropic response to increased frequency of stimulation; whereas, D600 (1.05 micron) converted the positive frequency-force relationship to a negative relationship. Present data indicate a direct cardiac depressant action of gentamicin, and suggest that this antibiotic adversely affects either the transport system responsible for Ca++ movement through slow channels of the sarcolemma, the availability of Ca++ for translocation to these sites, or both.     

Intramural nerve-mediated inotropic responses of left atria of rats and guinea pigs: demonstration of alpha adrenergic and nonadrenergic noncholinergic responses in guinea pigs

The effects of transmural nerve stimulation (TNS) on contractile responses of rat and guinea pig atria were analyzed pharmacologically. Isolated left atria were electrically driven through AgAgCl field electrodes and TNS was performed by brief introduction of defined stimulation patterns through the same electrodes. Step elevations in stimulating voltage induced biphasic inotropic responses in the left atria of both species: an initial negative component which was usually overwhelmed by a subsequent positive one. The transient negative inotropic response was induced by parasympathetic cholinergic nerve excitation, inasmuch as it was abolished by atropine. In the left atrium of the rat, the TNS-induced positive inotropic response was due exclusively to adrenergic nerve excitation through activation of beta-1 adrenoceptors. In contrast, analysis of the time course of responses in guinea pig left atria after nerve stimulation at 10 Hz revealed a positive inotropic response consisting of two phases; rapid and delayed phases were superimposed upon each other. The rapid phase was reduced by atenolol, a beta-1 antagonist, and attenuated further by prazosin, an alpha-1 antagonist. In the presence of both atenolol and prazosin, TNS of guinea pig left atria still induced a positive inotropic response but it had a slow onset and decay. This is termed the delayed phase response. TNS induced a similar delayed inotropic response in atria from surgically sympathectomized or reserpine-pretreated guinea pigs, from which catecholamine-fluorescence nerves and responses to tyramine were absent. These results demonstrate that TNS excitated adrenergic, cholinergic and nonadrenergic noncholinergic nerves in guinea pig left atria.(ABSTRACT TRUNCATED AT 250 WORDS)     

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)