AJP - Cell Physiology, Vol 256, Issue 2 C234-C240, Copyright © 1989 by American Physiological Society

ARTICLES

Regulation of intracellular calcium by cell pH in vascular smooth muscle cells

M. S. Siskind, C. E. McCoy, A. Chobanian and J. H. Schwartz
Thorndike Memorial Laboratory, Boston City Hospital, Massachusetts.

Intracellular calcium (Cai2+) and intracellular pH (pHi) are important regulators of a variety of intracellular processes. Cai2+ is a regulator of muscle contraction, but the role of pHi is unclear. The purpose of this study was to determine the effect of alterations of pHi on Cai2+. A7r5 vascular smooth muscle cells (VSMC) were grown to confluence on glass cover slips. Cai2+ was determined with the fluorescent probe fura-2 and pHi with 2,7-bis-carboxyethyl-5(6)-carboxy-fluorescein (BCECF). Alkalinization of the VSMC by exposure to 20 mM NH4Cl (delta pHi 0.41 +/- 0.07) resulted in a rise in Cai2+ from 99 +/- 8 to 146 +/- 13 nM (n = 5) in the presence of extracellular Ca2+ (Cao2+). In the absence of Cao2+, NH4Cl-induced alkalinization also resulted in a Cai2+ rise (delta Cai2+ = 26 +/- 4 nM, n = 5). Similar changes in Cai2+ were observed when cells were alkalinized by exposure to nigericin in a KCl buffer (pH 7.7). Neither 100 microM verapamil or 100 microM 8,8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate HCl (TMB-8) altered the alkaline-induced changes. After cellular Ca2+ stores were partially depleted by exposure to AVP in a Ca2+-free solution, subsequent cell alkalinization induced no changes in Cai2+. These results demonstrate that alkalinization of VSMCs leads to a rise in cytosolic Ca2+ via release of intracellular Ca2+ stores. The intracellular Ca2+ storage sites appear to be the same as those sites sensitive to AVP. Thus pHi may regulate Cai2+ and thereby play a role in the regulation of vascular smooth muscle tone.

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