acetylstrophanthidin and cobaltous-chloride

Transient changes of extracellular free calcium in rabbit ventricular muscle under nonsteady state conditions were measured with double-barreled calcium microelectrodes. Resumption of stimulation after a rest interval produces a cumulative decrease of extracellular free calcium often by more than 10% (with bulk extracellular free calcium = 0.2 mM). The extracellular free calcium returns to the bulk value as a new steady state is achieved. The changes of extracellular free calcium recorded presumably represent net calcium uptake and loss by cardiac muscle cells. These cumulative extracellular free calcium depletions are blocked by 0.5 mM cobalt and 1 microM nifedipine and are increased to 167 +/- 11% of control by the calcium agonist Bay k 8644 (1 microM) and to 620 +/- 150% of control by increasing stimulus frequency from 0.2-2 Hz. Caffeine (10 mM) inhibits the cumulative extracellular free calcium depletions, probably by rendering the sarcoplasmic reticulum unable to accumulate calcium. It is proposed that the extracellular free calcium depletions recorded represent, in large part, calcium which has entered the cells and has been taken up by the sarcoplasmic reticulum (which had become depleted of calcium during the rest interval). Nifedipine and cobalt inhibit these cumulative depletions presumably by preventing the calcium entry which could subsequently be accumulated by the sarcoplasmic reticulum. The net cellular calcium uptake produced by such a post-rest stimulation protocol can also be inhibited by 1-3 microM acetylstrophanthidin and reduction of extracellular sodium to 70 mM. Acetylstrophanthidin and low extracellular sodium would be expected to shift the sodium-calcium exchange in favor of increased calcium uptake, which may, in turn, prevent the loss of sarcoplasmic reticulum calcium during the rest interval. This would limit the amount of calcium which the sarcoplasmic reticulum could take up with subsequent activation. In contrast to the results with caffeine, ryanodine (1 microM) increases the magnitude and rate of calcium uptake after a rest interval, indicative of a fundamental difference in the actions of caffeine and ryanodine. When stimulation is stopped in the presence of ryanodine, extracellular free calcium increases much faster than in control. This suggests that ryanodine may enhance calcium uptake by the sarcoplasmic reticulum during repetitive stimulation and may enhance calcium efflux from the sarcoplasmic reticulum during quies

Topics: 3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; Animals; Caffeine; Calcium; Cobalt; Electric Stimulation; Extracellular Space; Heart; In Vitro Techniques; Microelectrodes; Myocardium; Nifedipine; Rabbits; Ryanodine; Strophanthidin

To determine whether Na+-Ca2+ exchange is a physiologically significant Ca2+ efflux mechanism in rabbit ventricle, we investigated the effects exerted on postrest contractions by interventions that alter the transmembrane distribution of Na+ or Ca2+ so as to retard Ca2+ efflux via this system. Contractions elicited after rest periods of 0.25-10 min in duration were studied. The following interventions increased postrest contractions much more than those elicited by rhythmic stimulation: 1) Na+ pump inhibition by cardiac glycosides or by a reduction in extracellular K+, 2) reduction of extracellular Na+ (maintaining a constant [Ca2+]-to-[Na+]2 ratio), and 3) elevation of extracellular Ca2+. In contrast, isoproterenol, norepinephrine, and histamine produced comparable increases in both rhythmically stimulated and postrest contractions, suggesting that the postrest contractile potentiation was not just the result of a general increase in inotropic state. Ryanodine, which appears to antagonize sarcoplasmic reticulum (SR) Ca2+ release in cardiac muscle, markedly reduced the amplitude of the postrest contractions, but only modestly decreased rhythmically stimulated responses. Results suggest 1) that Ca2+ released from SR is involved in postrest response, 2) that Na+-Ca2+ exchange serves as a Ca2+ efflux pathway in normally polarized resting rabbit ventricle, and 3) that this activity in part determines the amount of Ca2+ available for release from SR.

Topics: Animals; Calcium; Cobalt; Heart; In Vitro Techniques; Male; Myocardial Contraction; Myocardium; Periodicity; Rabbits; Ryanodine; Sarcoplasmic Reticulum; Sodium; Strophanthidin