Excitation-contraction coupling in cardiac Purkinje fibers. Effects of cardiotonic steroids on the intracellular [Ca2+] transient, membrane potential, and contraction.

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Abstract
The [Ca2+]-activated photoprotein aequorin was used to measure [Ca2+] in canine cardiac Purkinje fibers during the positive inotropic and toxic effects of ouabain, strophanthidin and acetylstrophanthidin. The positive inotropic effect of these substances was associated with increases in the 2 components of the aequorin signal, L2 and L2. On the average, strophanthidin at 10-7 M produced steady, reversible increases in L1, L2 and peak twitch tension of 20, 91 and 240%, respectively. This corresponds to increases in the upper-limit spatial average [Ca2+] from 1.9 .times. 10-6 M to 2.1 .times. 10-6 M at L1 and from 1.4 .times. 10-6 M to 1.8 .times. 10-6 M at L2. Elevation of diastolic luminescence above the control level was not detected. At higher concentrations (5 .times. 10-7 M), strophanthidin produced aftercontractions, diastolic depolarization and trasient depolarizations, all of which were associated with temporally similar changes in [Ca2+]. During these events, diastolic [Ca2+] rose from the normal level of .apprx. 3 .times. 10-7 M up to 1-2 .times. 10-6 M. The negative inotropic effect of 5 .times. 10-7 M strophanthidin was not associated with a corresponding decrease in the [Ca2+] transient but was associated with a change in the relationship between [Ca2+] and tension. Assuming a Na+-lag mechanism of cardiotonic steroid action: at low concentrations of drug, increased Ca2+ uptake by the sarcoplasmic reticulum prevents a detectable rise in cytoplasmic [Ca2+] during diastole, but this increased Ca2+ uptake results in increased release of Ca2+ during the action potential. At higher drug concentrations, observable [Ca2+] changes during diastole activate tension and membrane conductance changes.