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

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Abstract
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.