Objective: The primary aim of this study was to investigate whether activation of Na influx via voltage-gated Na channels can elevate sub-sarcolemmal ('fuzzy-space') [Na] and transiently activate Na/K pump current (I(p)).
Methods and results: Initially, Na/K pump activity was characterised in whole-cell voltage-clamped single guinea-pig ventricular myocytes. I(p) was activated by intracellular Na with a K(m) of 15.5 mM and a Hill coefficient of 1.7. Extracellular K activated I(p) with a K(m) of 1.6 mM. In these experiments, a finite ouabain-sensitive I(p) was measured when the pipette [Na] was zero. This suggests that there is an accumulation of Na in a sub-sarcolemmal space that is not in equilibrium with the bulk cytosol (which is assumed to be efficiently dialysed by the low-resistance patch-pipettes used). Such a sub-sarcolemmal Na gradient was observed in separate experiments in intact rabbit papillary muscles using electron probe X-ray microanalysis. In these studies, a fuzzy-space of limited Na diffusion was observed 100-200 nm below the sarcolemmal membrane. This sub-sarcolemmal Na gradient was similar whether muscles were frozen at peak-systole or end-diastole suggesting that the fuzzy-space Na does not change over the course of the contractile cycle. This was further investigated in isolated guinea pig myocytes where evidence for a transient activation of I(p) was sought immediately after the activation of voltage-gated Na channels. A single clamp step from -80 to 0 mV activated Na influx but, in the 10-2000 ms immediately following the initial Na influx no evidence for a transient activation of I(p) was observed. Similarly, no activation of I(p) could be detected immediately following a train of 20 rapid (5-Hz) pulses designed to maximise Na influx.
Conclusions: These studies provide evidence for the existence of a maintained sub-sarcolemmal elevation of [Na] in ventricular myocardium; however, this fuzzy-space [Na] did not change immediately after the activation of Na influx via voltage-gated Na channels or throughout the contractile cycle.