Pancreastatin increases free cytosolic Ca2+ in rat hepatocytes, involving both pertussis-toxin-sensitive and -insensitive mechanisms

Abstract

Freshly isolated rat hepatocytes, loaded with the Ca2+ probe Fluo-3, responded to homologous pancreastatin with a sudden increase in free cytosolic Ca2+ ([Ca2+]i) as well as glucose release. Addition of rat pancreastatin (0.1 microM) to hepatocytes resulted in an increase in [Ca2+]i from 150 nM to 700 nM, which declined back to nearly basal values within 2-3 min. Half-maximal and maximal effects were observed at 0.3 and 100 nM pancreastatin respectively. The increase in [Ca2+]i induced by vasopressin and noradrenaline was very similar in extent (from 150 to 800 nM) to that produced by pancreastatin. Neither the alpha 1-adrenergic blocker prazosin nor the vasopressin antagonist V1 modified the increase in [Ca2+]i induced by pancreastatin. Pig pancreastatin and its 33-49 C-terminal fragment produced about 65 and 75% of the effect of homologous pancreastatin respectively. Glucose production correlated with changes in [Ca2+]i in the same order of potency: vasopressin > rat pancreastatin > pig 33-49 pancreastatin > pig 1-49 pancreastatin. The effect of pancreastatin on [Ca2+]i was decreased by 50% when Ca2+ was omitted from the medium, and totally abolished when hepatocytes were depleted of internal Ca2+ stores by preincubation without Ca2+ and with 2 mM EGTA. When hepatocytes were preincubated for 5 min with PMA, the effects of ATP and noradrenaline were prevented, and those of vasopressin and pancreastatin remained unchanged. The pretreatment of hepatocytes with pertussis toxin diminished the response to pancreastatin and vasopressin. These results suggest that pancreastatin is a new Ca(2+)-mobilizing glycogenolytic hormone acting through a specific receptor which may involve both pertussis-toxin-sensitive and -insensitive GTP-binding regulatory proteins.