Rest decay of calcium transients and contractility in feline ventricular myocytes.
To define the respective contributions of sarcoplasmic reticulum Ca2+ release and action potential-dependent Ca2+ influx to the activator Ca2+, rest decay of the cytosolic free Ca2+ (Ca2+i) transient and contractility was measured in feline ventricular myocytes loaded with the Ca2(+)-sensitive fluorescent dye indo-1. Myocytes were stimulated to a steady state and then rested for periods ranging from 2 to 120 s. Rest decay was assayed with both resumption of stimulation and rapid extracellular applications of caffeine. The protocol was then repeated after exposure to ryanodine. Ryanodine changed the Ca2+i transients during steady-state stimulation from large, rapidly rising transients starting near the resting Ca2+i level to small, slowly rising Ca2+i transients superimposed on an elevated diastolic Ca2+i. Under control conditions the stimulation- and caffeine-induced Ca2+i transients exhibited slow monotonic rest decay with similar time constants of decay (180 and 202 s, respectively, from a representative cell). After ryanodine, the rest decay of stimulation-induced Ca2+i transients was seemingly no different. However, no Ca2+i transient could be elicited by caffeine after 15 s of rest. These results suggest that 1) under normal conditions the primary source of activator Ca2+ is the sarcoplasmic reticulum, and 2) the rest decay of contractility seen in feline ventricular myocytes results from time-dependent loss of Ca2+ from this organelle.
DuBell
W H
WH
Department of Physiology, Temple University School of Medicine, Philadelphia, Pennsylvania 19140.
Houser
S R
SR
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002415:Cats; D004558:Electric Stimulation; D006352:Heart Ventricles; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000166:cytology; D012146:Rest; D012433:Ryanodine / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism
D012433:Ryanodine; D002110:Caffeine; D002118:Calcium