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 (pH(i)) impact on free cytosolic calcium (Ca(i)2+) 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 Ca2+. The rise in Ca2+ was markedly diminished when sarcoplasmic reticulum (SR) Ca2+ stores had been depleted by prior exposure to arginine vasopressin (AVP) in Ca2+-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 Ca(i)2+. This Ca2+ increase was most apparent when pH(i) 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 Ca2+ 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 Ca(i)2+ by promoting its release from a store which is AVP and TMB-8 sensitive, most likely the SR. Cell acidification increases Ca(i)2+ from an intracellular store(s) that is neither AVP nor TMB-8 sensitive. The increase in Ca2+ produced by cell acidification may be explained on the basis of cell buffering such that, as cytosolic H+ increases, it displaces Ca(i)2+ from internal buffers with similar affinities for Ca2+ and H+.