Ouabain effects on intracellular potassium activity and contractile force in cat papillary muscle.

The relationship between the positive inotropic and toxic effects of cardiac glycosides and their effects on intracellular ionic composition is incompletely defined. We measured intracellular potassium activity (alpha ik), extracellular potassium activity (alpha ek), resting potential, action potential duration, and contractile force at 32 degrees C in paired papillary muscles from feline right ventricles exposed to ouabain. Muscles used for electrophysiological measurements were quiescent except for isolated stimulation to confirm impalement and record action potential duration. Muscles used for contractile force measurements were quiescent except for 4-min periods when force was measured at a cycle length of 1,400 ms. Muscle length was adjusted to achieve 50% of maximal tension at this cycle length before each experiment. In four experiments, alpha ik and contractile force were measured in the same muscle. Alpha iK was measured with single and double-barrel K-sensitive electrodes. At 10 nM ouabain, action potential duration is prolonged. Among the concentrations tested, the threshold for a clear positive inotropic effect is 0.1 microM ouabain. The threshold for decrease in alpha iK, increase in alpha eK, and decrease in membrane potential is 1 microM, at which concentration toxic signs develop, including arrhythmias, aftercontractions, and alteration in the staircase response of contractile force to repetitive stimulation. Ouabain need not change alpha iK to effect positive inotropy in ventricular muscle, a relationship different from that reported between [K]i (intracellular potassium concentration) and positive inotropy. Higher ouabain concentrations, which others have shown to clearly inhibit active Na and K transport, are shown to upset intracellular potassium activity homeostasis and to consistently produce toxicity.     

The pharmacology of batrachotoxin. VII. Structure-activity relationships and the effects of pH.

The effects of the depolarizing agent, batrachotoxin (BTX), and of various analogs were studied on rat phrenic nerve-diaphragm muscle preparations at 37 degrees C. The structural modifications of BTX included: 1) replacement of the 20alpha-pyrrole-3-carboxylate moiety; 2) alterations of substituents on the pyrrole moiety; 3) clevage of the 3alpha, 9alpha-hemiketal linkage; and 4) quaternization of the tertiary nitrogen of BTX. All of the compounds except batrachotoxinin A (BTX-A), which lacks the 20alpha-substituent, depolarized the postsynaptic membrane, transiently increased the frequency of spontaneous transmitter release to 400 to 600 sec- minus 1 and finally produced blockade of the directly and indirectly elicited muscle twitches. Of the compounds tested, only BTX-A potentiated the muscle twitches. The concentration which elicits a 50% depolarization of the muscle membrane in 1 hour was determined for all the compounds except for BTX-A and for dihydrobatrachotoxin which lacks the 3alpha, 9alpha-hemiketal linkage; these two analogs never depolarized the postsynaptic membrane by more than 10 to 15%. BTX, the 20alpha-2, 4, 5-trimethylpyrrole-3-carboxylate of BTX-A and the 20alpha-ester of BTX-A with 2-ethyl-4-methylpyrrole-3-carboxylic acid (homobatrachotoxin) were the three most potent toxins with doses of 4.5, 12 and 18 times 10- minus 9 M eliciting a 50% membrane depolarization in 1 hour. The quaternary derivative of BTX, the 20alpha-4, 5-dimethylpyrrole-3-carboxylate of BTX-A and 20alpha-2,4-dimethyl-5-acetylpyrrole-3-carboxylate of BTX-A were 24-, 65- and 110-fold less potent than BTX as depolarizing agents, whereas the 20alpha-p-bromobenzoate of BTX-A was 220-fold less potent. Each of these derivatives had the ability to increase sodium permeability since the increase in spontaneous miniature end-plate potential frequency and membrane depolarization were reversed by tetrodotoxin or by reducing the external sodium concentration. BTX was found to be more effective at alkaline pH (pH 9.0), at which it exists almost entirely in the un-ionized form, than at physiological or acidic pH(6.0). The results indicate that the analogs of BTX act by a mechanism similar to that of the parent compound, but that their potency differs and certain compounds may have a more selective action on either the pre- or postsynaptic membrane. For maximal depolarizing activity, a substituted pyrrole moiety is necessary at the 20alpha-position of BTX-A and 3alpha, 9alpha-hemiketal linkage must remain intact providing rigidity for the pentacyclic steroid nucleus.     

Inotropic and lusitropic effects of chlorpromazine on rat left ventricular papillary muscle

The in vitro effects of chlorpromazine on rat cardiac papillary muscle were tested at 10(-6), 10(-5) and 10(-4) M. Mechanical parameters were determined from the contraction and relaxation phases under isotonic and isometric conditions in order to assess contraction, relaxation, contraction-relaxation coupling and load sensitivity of relaxation. The peak power output Emax was determined from the force-velocity relationship. At 10(-6) M, a slight positive inotropic effect was observed, probably related to modifications in cross-bridges kinetics. Negative inotropic effects were observed with 10(-5) and 10(-4) M chlorpromazine. At 10(-5) M, shortening of the isometric relaxation and decrease in R2 = (+dF.dt-1max)/(-dF.dt-1max) suggest that chlorpromazine also diminishes myofilament Ca++ sensitivity. Emax was increased at 10(-6) M (19 +/- 5%, P less than .05), but decreased at 10(-5) M (-28 +/- 10%, P less than .05) and 10(-4) M (-82 +/- 2%, P less than .05). Modifications in the force-velocity relationship at 10(-4) M indicated that lowering myocardial performance by chlorpromazine was associated with a low muscle efficiency from a thermoenergetic point of view. At all concentrations, chlorpromazine impaired the isotonic relaxation and load sensitivity of relaxation. At 10(-4) M, muscle contracture and slowed isometric relaxation were probably due to "calcium overload." These results showed that chlorpromazine finely modulates intrinsic cardiac energetics and mechanics by acting on the sarcoplasmic reticulum, myofilament Ca++ sensitivity and cross-bridges kinetics, according to the level of load and chlorpromazine concentration used.     

Ryanodine: its alterations of cat papillary muscle contractile state and responsiveness to inotropic interventions and a suggested mechanism of action.

Cat right ventricular papillary muscles responded biphasically to cumulative additions of ryanodine. A progressive and pronounced negative inotropic effect was observed with low to intermediate ryanodine concentrations (5 nM-1 muM) while a rebound or reversal of these initial changes back toward pre-drug values was obtained as the ryanodine concentration was further increased to 100 muM. Active force development (DF), the rate of force development (dF/dt), as well as the rate of relaxation all exhibited these bidirectional changes. In contrast, time to peak force underwent only a progressive prolongation over the entire concentration range tested. This response pattern was observed with both normal and K+-depolarized (isoproterenol- or dibutyryl cAMP-restored) preparations. The response to a single addition of 100 muM ryanodine, in the presence of 2.5 mM Ca++ mimicked both the qualitative and quantitative aspects of the cumulative concentration response curve. In the presence of 5.0 mM Ca++ the high concentration of ryanodine no longer caused depression but instead caused only a slowly developing, monophasic increase in DF. Ryanodine also changed the response of ventricular muscle to other inotropic interventions. Ryanodine (1 muM; 2.5 mM Ca++) abolished the normal increase in dF/dt following either paired electrical stimulation (PES) or 50 mOsM mannitol, but not that in response to a doubling of the stimulation rate (0.2--0.4 Hz). After ryanodine exposure, the potentiation of developed force by PES was shifted from the first (regular) to the second (premature) contraction, producing a summation-like waveform. Prior addition of the calcium channel antagonist D600 (1 muM) did not alter ryanodine-induced changes in PES. Caffeine (1 mM) produced alterations in the responses to PES and hyperosmolarity which were similar to those observed with ryanodine. In the presence of high concentrations of both ryanodine (100 muM) and calcium (5 mM) both the transient and steady-state responses to a doubling of the stimulation rate (0.2--0.4 Hz) were markedly depressed, whereas the decrease in DF or dF/dt normally accompanying a reduction in the rate of stimulation was attenuated. The data obtained in the present study are consistent with a functional inhibition of sarcoplasmic reticular calcium release by ryanodine.     

Cardiac tamponade masking clinical presentation and hemodynamic effects of papillary muscle rupture after acute myocardial infarction.

A 67-year-old woman sustained an acute lateral-wall myocardial infarction and was treated with thrombolytic therapy. Postinfarction hypotension developed 3 days later. Clinical findings at that time were consistent with cardiac tamponade, and an echocardiographic study revealed a moderate-sized pericardial effusion. She underwent urgent pericardiocentesis with transient improvement in hemodynamics, followed by deterioration associated with the development of acute pulmonary edema. Follow-up transesophageal echocardiographic imaging revealed papillary muscle rupture with severe mitral regurgitation. The patient underwent urgent surgical intervention consisting of coronary artery bypass grafting and mitral valve replacement. The presence of cardiac tamponade in this patient masked the clinical manifestations of papillary muscle rupture through the hemodynamic effect of tamponade physiology on mitral regurgitation.     

Effects of hydroxocobalamin on rat cardiac papillary muscle

Hydroxocobalamin is a rapid and powerful antidote in acute cyanide poisoning. The effects of hydroxocobalamin (0.1, 0.3, and 1 mM) on intrinsic myocardial contractility were studied on isolated rat cardiac papillary muscles (n=10). Whatever the concentration, hydroxocobalamin did not modify the active isometric force and a slight increase in maximum unloaded shortening velocity was noted at 1 mM. Only 0.3 mM significantly impaired contraction-relaxation coupling under low load, suggesting a slight decrease in sarcoplasmic reticulum function. No changes in contraction relaxation coupling under heavy load were noted, suggesting the lack of modification of myofilament calcium sensitivity. These results suggest that hydroxocobalamin does not induce noticeable changes in intrinsic myocardial contractility. An indirect mechanism might be involved in the previously reported decrease in cardiac function at supratherapeutic concentrations of hydroxocobalamin.     

Comparative ultrastructure of the tip of ventricular papillary muscle.

Electron microscopic studies of the tips of left ventricular papillary muscles from seven human, two monkey, three sheep, and two chicken hearts were done to elucidate the fine structure of myotendinous junctions. The human specimens were from normal hearts obtained 3 to 9 hours postmortem from persons aged 7 months to 30 years (mean, 13.3 years). We found no significant ultrastructural differences between the human hearts and those of monkey, sheep, and chicken. Myocardial fibers were elongated and thinner (tapered) in the tips of papillary muscles. In addition to usual working myocardial cells, the distal end of narrowing muscle fibers also contained small pale cardiocytes containing fewer myofibrils and smaller mitochondria. These cells were similar to P cells or transitional cells in the conduction system. Nerve axons and Schwann cells were commonly seen in the interstitium, usually in association with capillaries. Fibroblasts and axon varicosities were occasionally seen extremely close to the cardiocytes. These specialized myocardial cells associated with rich neural tissue in the papillary muscle tip possibly function as foci of local automaticity. This histologic organization may also represent neurosensory function responding to and monitoring local pressure changes, efferent adrenergic or cholinergic neural activity, or both.