Is there a transient rise in sub-sarcolemmal Na and activation of Na/K pump current following activation of I(Na) in ventricular myocardium?
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)).


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.
Silverman
Benjamin d Z
Bd
Centre for Cardiovascular Biology and Medicine, The Rayne Institute, St. Thomas' Hospital, King's College London, SE1 7EH, London, UK.
Warley
Alice
A
Miller
Jonathan I A
JI
James
Andrew F
AF
Shattock
Michael J
MJ
D000818:Animals; D004577:Electron Probe Microanalysis; D006168:Guinea Pigs; D006352:Heart Ventricles / Q000166:cytology / Q000378:metabolism; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D032383:Myocytes, Cardiac / Q000378:metabolism; D010210:Papillary Muscles / Q000648:ultrastructure; D018408:Patch-Clamp Techniques; D011817:Rabbits; D012508:Sarcolemma / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D015222:Sodium Channels / Q000502:physiology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D015222:Sodium Channels; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase