Effects of high extracellular calcium concentrations on phosphoinositide turnover and inositol phosphate metabolism in dispersed bovine parathyroid cells.

Abstract

We previously showed that high extracellular calcium (Ca2+) concentrations raise the levels of inositol phosphates in bovine parathyroid cells, presumably via the G protein-coupled, "receptor-like" mechanism through which Ca2+ is thought to regulate these cells. To date, however, there are limited data showing Ca(2+)-evoked hydrolysis of phosphoinositides with attendant increases in the levels of the biologically active 1,4,5 isomer of inositol trisphosphate (IP3) that would be predicted to arise from such a receptor-mediated process. In the present studies we used HPLC and TLC, respectively, to quantify the high Ca(2+)-induced changes in various inositol phosphates, including the isomers of IP3, and phosphoinositides in bovine parathyroid cells prelabeled with [3H]inositol. In the absence of lithium, high Ca2+ dose dependently elevated the levels of inositol-1,4,5-trisphosphate [I(1,4,5)P3], with a maximal, 4- to 5-fold increase within 5 s; the levels of inositol 1,3,4-trisphosphate [I(1,3,4)P3] first rose significantly at 5-10 s and remained 5- to 10-fold elevated for at least 30 minutes. These changes were accompanied by reciprocal 29-36% decreases in PIP2 (within 5-10 s, the earliest time points examined), PIP (within 60 s), and PI (within 60 s). These results document that, as in other cells responding to more classic "Ca(2+)-mobilizing" hormones, the high Ca(2+)-evoked increases in inositol phosphates in bovine parathyroid cells arise from the hydrolysis of phosphoinositides, leading to the rapid accumulation of the active isomer of IP3. The latter presumably underlies the concomitant spike in the cytosolic calcium concentration (Ca(i)) in parathyroid cells.