Stored calcium modulates inositol phosphate synthesis in cultured smooth muscle cells

Cytosolic Ca²⁺ concentrations ([Ca²⁺](cyt)) and [³H]inositol phosphates ([³H]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 [³H]InsP, but both treatments increased AVP-evoked synthesis of [³H]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, [³H]InsP were reduced. The inhibitory effect of 40 mM K⁺ on AVP-evoked [³H]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 [³H]InsP synthesis or Ca²⁺ transients. These results demonstrate an inverse relationship between SR Ca²⁺ content and evoked [³H]-InsP synthesis. Moreover, they suggest that SR Ca²⁺ may serve as a signal that modulates sarcolemmal [³H]InsP formation.