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 min) and recovery from the rested state (following 10 min of rest) of cell shortening and the amplitude of the intracellular calcium transient were compared in epicardial and endocardial ventricular myocytes isolated from rabbit hearts. The object of these experiments was to determine whether reported transmural differences in action potential duration, myosin type expression and metabolic enzyme content are able to influence the control of contraction. Cells isolated from these two regions of the ventricular wall displayed almost identical twitch shortening and calcium transient characteristics during steady-state electrical stimulation at 0.5 Hz. Despite this, rest decay of cell shortening was faster and recovery from the rested state slower in endocardial cells than in epicardial cells. Neither of these differences could be explained in terms of changes of calcium transient amplitude or time course. We tried to mimic the effect of prolonged rest by application of caffeine to empty the sarcoplasmic reticulum of calcium. The regional differences in recovery of contraction from the rested state were not reproduced in the recovery of contraction after caffeine application, suggesting that the effect is produced by something other than refilling of the sarcoplasmic reticulum. It is suggested that changes in factors that affect myofilament calcium sensitivity produce the regional differences in rest decay and post-rest recovery of contraction.