Regional differences in the regulation of intracellular sodium and in action potential configuration in rabbit left ventricle

In this study we report measurements of the intracellular sodium concentration ([Na+](i)) in cardiac myocytes isolated from sub-endocardial and sub-epicardial regions of the rabbit left ventricle. These measurements show that [Na+](i) is significantly higher in sub-epicardial than in sub-endocardial myocytes both at rest and during steady-state stimulation 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 recovery [Na+](i) rose significantly faster in sub-endocardial cells. The effect that this difference in sodium regulation may have on the rate of recovery of the calcium transient post-rest is discussed. The steady-state differences in [Na+](i) do not appear to have the effect on contraction we would expect i.e. steady-state contraction is larger in sub-endocardial cells where [Na+](i) is lower. Changes in [Na+](i) brought about by altering the frequency of stimulation are associated with the expected changes of contraction. Action potentials were found to be significantly longer in sub-endocardial cells and following rest action potential duration was shortened although regional differences were maintained. No regional differences in this effect of rest or in recovery of the action potential post-rest were found.