REGIONAL DIFFERENCES IN THE REGULATION OF INTRACELLULAR SODIUM AND INACTION-POTENTIAL CONFIGURATION IN RABBIT LEFT-VENTRICLE

In this study we report measurements of the intracellular sodium conce ntration ([Na+](i)) in cardiac myocytes isolated from sub-endocardial and sub-epicardial regions of the rabbit left ventricle. These measure ments show that [Na+](i) is significantly higher in sub-epicardial tha n in sub-endocardial myocytes both at rest and during steady-state sti mulation at 0.5 Hz. During a 10-min rest the rate of fall of [Na+](i) was identical in cells from the two regions but during post-rest recov ery [Na+](i) rose significantly faster in sub-endocardial cells. The e ffect that this difference in sodium regulation may have on the rate o f recovery of the calcium transient post-rest is discussed. The steady -state differences in [Na+](i) do not appear to have the effect on con traction we would expect i.e. steady-state contraction is larger in su b-endocardial cells where [Na+](i) is lower. Changes in [Na+](i) broug ht about by altering the frequency of stimulation are associated with the expected changes of contraction. Action potentials were found to b e significantly longer in sub-endocardial cells and following rest act ion potential duration was shortened although regional differences wer e maintained. No regional differences in this effect of rest or in rec overy of the action potential post-rest were found.