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 may also stimulate an accumulation of inositol phosphates. This suggests a possible positive feedback regulation of the primary accumulation of Ins(1,4,5)P3 induced by vasopressin. In order to test such a possibility we studied the vasopressin-induced Ins(1,4,5)P3 accumulation, where intracellular calcium mobilization is artificially suppressed 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 microM vasopressin is inhibited by around 50% when measured 5 s after stimulation. This inhibition is not due to an alteration of the VIa 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 hypothesis: (1) calcium at an intracellular physiological concentration (0.1-1 microM) 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.
Mouillac
B
B
Centre CNRS-INSERM de Pharmacologie-Endocrinologie, Montpellier, France.
Balestre
M N
MN
Guillon
G
G
D001127:Arginine Vasopressin / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002460:Cell Line; D002462:Cell Membrane / Q000201:enzymology; D004533:Egtazic Acid / Q000494:pharmacology; D005246:Feedback; D007295:Inositol Phosphates / Q000378:metabolism; D000072181:Inositol Polyphosphate 5-Phosphatases; D015759:Ionomycin / Q000494:pharmacology; D007700:Kinetics; D009543:Nifedipine / Q000494:pharmacology; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D010738:Type C Phospholipases / Q000378:metabolism*
D007295:Inositol Phosphates; D001127:Arginine Vasopressin; D004533:Egtazic Acid; D015759:Ionomycin; D010744:Phosphoric Monoester Hydrolases; D000072181:Inositol Polyphosphate 5-Phosphatases; D010738:Type C Phospholipases; D009543:Nifedipine; D002118:Calcium