Cytosolic free calcium regulation in response to acute changes in intracellular pH in vascular smooth muscle

This study examined the mechanisms whereby alterations of intracellular pH (pHi) impact on free cytosolic calcium (Cai2+) in cultured rat aortic vascular smooth muscle cells (VSMC) assayed in the presence of HCO3/CO2. Rapid cell alkalinization, effected by the exposure to NH4Cl or removal of CO2 from the superfusate, produced a rapid increase in Cai2+. The rise in Cai2+ was markedly diminished when sarcoplasmic reticulum (SR) Ca2+ stores had been depleted by prior exposure to arginine vasopressin (AVP) in Ca(2+)-free media or when SR release and reuptake of Ca2+ were blocked by the addition of 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8), but was unaffected by the removal of external Ca2+ or inhibition of Ca2+ entry using NiCl2. Cell acidification also resulted in a rapid increase in Cai2+. This Cai2+ increase was most apparent when pHi was very low (< 6.6) and was unaffected by removal of external Ca2+ or NiCl2 addition. Unlike the effect of cell alkalinization, the increase in Cai2+ associated with cell acidification was not prevented by pretreatment with AVP or TMB-8. We conclude that, in cultured VSMC, acute intracellular alkalinization and, to a lesser extent, acidification result in release of Ca2+ from internal stores. Alkalinization increases Cai2+ by promoting its release from a store which is AVP and TMB-8 sensitive, most likely the SR. Cell acidification increases Cai2+ from an intracellular store(s) that is neither AVP nor TMB-8 sensitive. The increase in Cai2+ produced by cell acidification may be explained on the basis of cell buffering such that, as cytosolic H+ increases, it displaces Cai2+ from internal buffers with similar affinities for Ca2+ and H+.