STORED CALCIUM MODULATES INOSITOL PHOSPHATE SYNTHESIS IN CULTURED SMOOTH-MUSCLE CELLS

Cytosolic Ca2+ concentrations ([Ca2+]cyt) and [H-3]inositol phosphates ([H-3]InsP) were correlated while varying the Ca2+ content of the sarcoplasmic reticulum (SR) in cultured A7r5 cells at rest and during activation with [Arg8]-vasopressin (AVP). Thapsigargin (TG) raised and superfusion with 0 Ca2+ lowered [Ca2+]cyt, but both treatments decreased SR Ca2+ and AVP-evoked Ca2+ transients. Neither TG nor 0 Ca2+ affected basal [H-3]InsP, but both treatments increased AVP-evoked synthesis of [H-3]InsP. Exposure for several minutes to 40 mM K+ solution, BAY K 8644, or low-Na+ solution all elevated [Ca2+]cyt and, thereby, increased SR Ca2+, as manifested by augmented AVP-evoked Ca2+ transients. In all three cases, AVP-evoked, but not basal, [H-3]InsP were reduced. The inhibitory effect of 40 mM K+ on AVP-evoked [H-3]InsP synthesis was blocked when SR Ca2+ uptake was prevented by TG. Brief (30-s) exposures to 40 mM K+, which elevated [Ca2+]cyt but not SR Ca2+ loading, did not modify AVP-evoked [H-3]InsP synthesis or Ca2+ transients. These results demonstrate an inverse relationship between SR Ca2+ content and evoked [H-3]InsP synthesis. Moreover, they suggest that SR Ca2+ may serve as a signal that modulates sarcolemmal [H-3]InsP formation.