POSITIVE FEEDBACK-REGULATION OF PHOSPHOLIPASE-C BY VASOPRESSIN-INDUCED CALCIUM MOBILIZATION IN WRK1 CELLS

In WRK1 cells vasopressin stimulates Ins(1,4,5)P3 accumulation and mobilizes intracellular calcium. These two phenomena are transient and exhibit similar time-courses. Experiments performed on intact cells or membrane preparations demonstrate that calcium by also stimulate an accumulation of inositol phosphate. This suggests a possible feedback regulation of the primary accumulation of Ins(1,4,5)P3 accumulation, vasopressin. In order to test such a possibility we studied the vasopressin-induce Ins(1,4,5)P3 accumulation, where intracellular calcium mobilization is artificially supressed by incubating the cells with EGTA in the presence of ionomycin. Under these conditions the accumulation of Ins(1,4,5)P3 induced by 1 μM vasopressin is inhibited by around 50% when measured 5 s afte stimulation. This inhibition is not due to an alteration of the V1a vasopressin receptor binding properties, a reduction of the amount of substrate available for the phospholipase C, a stimulation of the Ins(1,4,5)P3 5-phosphatase or an activation of the Ins(1,4,5)P3 kinase. It is more likely the consequence of the suppression of calcium wave generated by Ins(1,4,5)P3 which may in its turn stimulate a phospholipase C. Different arguments favour this hypotheis: (1) calcium at an intracellular physiological concentration (0.1-1 μM) is able to stimulate a phospholipase C; (2) artificially increasing the [Ca2+]i inside the WRK1 cell induces an accumulation of Ins(1,4,5)P3; and (3) the time-course of the inhibition of Ins(1,4,5)P3 accumulation induced by an EGTA/ionomycin treatment correlates well with that of the calcium mobilization. Altogether these results suggest that Ins(1,4,5)P3 accumulation in WRK1 cells may result from two distinct mechanisms: a direct vasopressin receptor-mediated PLC activation which is independent of calcium and a calcium-mediated PLC activation related to the intracellular calcium mobilization. © 1990.