CA2+ STORES IN SMOOTH-MUSCLE CELLS - CA2+ BUFFERING AND COUPLING TO AVP-EVOKED INOSITOL PHOSPHATE SYNTHESIS

Cytosolic Ca2+ concentrations ([Ca2+](cyt)) and [H-3]inositol phosphates ([H-3]InsP) were correlated while decreasing the Ca2+ content of sarcoplasmic reticulum (SR) stores in cultured A7r5 cells at rest and after activation with 8-arginine vasopressin (AVP). Decreasing Ca2+ influx by reducing extracellular Ca2+ or by treatment with verapamil had no effect on resting [Ca2+](cyt) but significantly inhibited the AVP-evoked Ca2+ transients (Delta Ca2+). Neither treatment affected basal [H-3]InsP, but both treatments increased AVP-evoked synthesis of [H-3]InsP. Likewise, basal [H-3]InsP were unaffected by brief (10-30 s) exposures to thapsigargin (TG), while AVP-induced [3H]InsP synthesis was significantly augmented. Similar treatment with TG rapidly increased resting [Ca2+](cyt) and decreased SR Ca2+ by 9-25% as manifested by decreased Delta Ca2+. By contrast, ryanodine induced slow increases in [Ca2+](cyt) that stabilized within 30 min; subsequent AVP-induced Delta Ca2+ were attenuated by 50%. Ryanodine had no effect on either basal or stimulated [3H]InsP levels. Agents that elevate adenosine 3',5'-cyclic monophosphate (cAMP) such as caffeine, 8-bromo-cAMP, and forskolin inhibited AVP-evoked [3H]InsP formation. These observations provide further characterization of a communication pathway between the AVP-sensitive Ca2+ stores in the SR and the plasmalemmal enzyme system involved in the synthesis of inositol 1,4,5-trisphosphate. This pathway is manifested by an inverse relationship between the Ca2+ content of an AVP-sensitive, ryanodine-insensitive SR Ca2+ store and evoked [H-3]InsP synthesis and may represent an important component in the tonic regulation of resting [Ca2+](cyt) and vasoconstrictor- and hormone-evoked SR Ca2+ release.