Ca²⁺ stores in smooth muscle cells: Ca²⁺ buffering and coupling to AVP-evoked inositol phosphate synthesis

Cytosolic Ca²⁺ concentrations ([Ca²⁺](cyt)) and [³H]inositol phosphates ([³H]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 (ΔCa²⁺). Neither treatment affected basal [³H]InsP, but both treatments increased AVP-evoked synthesis of [³H]InsP. Likewise, basal [³H]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 ΔCa²⁺. By contrast, ryanodine induced slow increases in [Ca²⁺](cyt) that stabilized within 30 min; subsequent AVP-induced ΔCa²⁺ 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 [³H]InsP synthesis and may represent an important component in the tonic regulation of resting [Ca²⁺](cyt) and vasoconstrictor- and hormone-evoked SR Ca²⁺ release.