AJP - Heart and Circulatory Physiology, Vol 250, Issue 2 313-H319, Copyright © 1986 by American Physiological Society
Na+-H+ exchange is present in sarcolemmal vesicles from dog superior mesenteric artery
A. M. Kahn, H. Shelat and J. C. Allen
The Na+ concentration inside vascular smooth muscle cells is an important
regulator of vascular smooth muscle function, but the mechanisms that
mediate Na+ influx are not known. We studied Na+ transport in a newly
described vesicle preparation preferentially enriched in sarcolemma and
obtained by Mg2+ aggregation and differential centrifugation of homogenized
dog superior mesenteric artery. In the presence of an outwardly directed
proton gradient (pHout = 7.5, pHin = 5.0), 1 mM 22Na+ uptake was stimulated
over twofold relative to the absence of a pH gradient (pHin = 7.5 or 5.0).
pH gradient-stimulated Na+ uptake was inhibited by 1 mM amiloride. 22Na
efflux was stimulated by an inwardly directed proton gradient (pHin = 7.5,
pHout = 5.7 vs. 7.5). The rate of proton efflux from acid-loaded vesicles
was measured by acridine orange fluorescence and was stimulated by 100 mM
Naout but not by Nain = Naout = 100 mM. H+ gradient-stimulated Na+
transport and Na+ gradient-stimulated H+ transport were not due to
electrical coupling between the two cations. The pH gradient-stimulated
component of Na+ transport in the final vesicles, an intermediate fraction,
and microsomes were proportional to the respective enzyme marker activities
for sarcolemma but not for sarcoplasmic reticulum or mitochondrial
membranes. We conclude that Mg2+ aggregation and differential
centrifugation of homogenized vascular smooth muscle yield a vesicle
preparation preferentially enriched in sarcolemma. Furthermore, the
sarcolemma of vascular smooth muscle contains an amiloride-sensitive Na+-H+
proton countertransport system.
