Calcium-dependent depolarizations and contractions in ventricular myocardial fibers: effects of adrenaline and cardiac glycosides.
Intracellular membrane potentials and contractions were recorded in ventricular trabeculae from sheep and calf hearts. Using a sucrose gap and constant current pulses of 500 msec duration, Ca-dependent depolarizations were elicited in Ca-containing Tyrode's solution + tetrodotoxin (TTX) (1-2 X 10(-5) gm/ml; [Na]e 149.2 mM) or in Na-free solution with sucrose as sodium substitute. Adrenaline (10(-8) -5 X 10(-7) gm/ml), which has been shown to enhance Ca inward current, increased (dV/dt)max and contractile force both in Na-free and in Na-containing solutions + TTX. Digitoxigenin (10(-7) -3 X 10(-7) gm/ml), however, did not change (dV/dt)max and contractions in the absence of Na. In the presence of Na (+TTX), digitoxigenin increased contractile force but decreased (dV/dt)max. Under the assumption that the Ca-dependent depolarizations are carried by Ca it is concluded 1) that in contrast to adrenaline, digitoxigenin does not increase the permeability of the myocardial cell membrane to Ca and 2) that the positive inotropic effect of digitoxigenin is not due to an enhancement in Ca inward current during depolarization.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002297:Cardanolides / Q000494:pharmacology*; D002417:Cattle; D004073:Digitoxigenin / Q000494:pharmacology*; D004837:Epinephrine / Q000494:pharmacology*; D006321:Heart / Q000502:physiology; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009200:Myocardial Contraction / Q000187:drug effects*; D009206:Myocardium / Q000378:metabolism*; D012756:Sheep; D012964:Sodium / Q000502:physiology
D002297:Cardanolides; D004073:Digitoxigenin; D012964:Sodium; D002118:Calcium; D004837:Epinephrine