REGIONAL DIFFERENCES IN REST DECAY AND RECOVERIES OF CONTRACTION AND THE CALCIUM TRANSIENT IN RABBIT VENTRICULAR MUSCLE

The rates of rest decay (for rest periods of between 0.5 min and 10 mi n) and recovery from the rested state (following 10 min of rest) of ce ll shortening and the amplitude of the intracellular calcium transient were compared in epicardial and endocardial ventricular myocytes isol ated from rabbit hearts. The object of these experiments was to determ ine whether reported transmural differences in action potential durati on, myosin type expression and metabolic enzyme content are able to in fluence the control of contraction. Cells isolated from these two regi ons of the ventricular wall displayed almost identical twitch shorteni ng and calcium transient characteristics during steady-state electrica l stimulation at 0.5 Hz. Despite this, rest decay of cell shortening w as faster and recovery from the rested state slower in endocardial cel ls than in epicardial cells. Neither of these differences could be exp lained in terms of changes of calcium transient amplitude or time cour se. We tried to mimic the effect of prolonged rest by application of c affeine to empty the sarcoplasmic reticulum of calcium. The regional d ifferences in recovery of contraction from the rested state were not r eproduced in the recovery of contraction after caffeine application, s uggesting that the effect is produced by something other than refillin g of the sarcoplasmic reticulum. It is suggested that changes in facto rs that affect myofilament calcium sensitivity produce the regional di fferences in rest decay and post-rest recovery of contraction.