AJP - Renal Physiology, Vol 260, Issue 3 402-F409, Copyright © 1991 by American Physiological Society
Role of intracellular calcium in volume regulation by rabbit medullary thick ascending limb cells
C. Montrose-Rafizadeh and W. B. Guggino
Physiology Department, School of Medicine, Johns Hopkins University, Baltimore, Maryland 21205.
Previous studies demonstrated that Ca2(+)-activated K+ channels in luminal
membrane of rabbit medullary thick ascending limb cells (MTAL) are
activated on exposure of the cells to hyposmotic solutions [J. Taniguchi
and W. B. Guggino. Am. J. Physiol. 257 (Renal Fluid Electrolyte Physiol.
26): F347-F352, 1989]. In this study, we investigated the mechanism of
activation of Ca2(+)-activated K+ channels in MTAL cells exposed to
hyposmotic solutions. MTAL cells swell in hyposmotic medium and regulate
volume back toward the starting volume. This regulatory volume decrease
(RVD) is inhibited at high medium K+ concentrations or by presence of
quinine or Ba2+ in extracellular medium, suggesting involvement of K+
channels. Measurements of intracellular Ca2+ concentrations with fura-2
show that intracellular Ca2+ rises in hyposmotic solutions and that this
rise does not occur in absence of extracellular Ca2+. Nifedipine and
verapamil also inhibit rise in intracellular Ca2+. Decreasing intracellular
Ca2+ by removal of external Ca2+ in presence of EDTA or by chelation of
intracellular Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic
acid (BAPTA) inhibits RVD. We conclude that hypotonic solutions activate K+
efflux probably via K+ channels and Ca2+ influx via a nifedipine- and
verapamil-sensitive pathway. Lowering intracellular Ca2+ removes the
ability of MTAL cells to regulate volume in hyposmotic solutions.
