Participation of Ca²⁺ and Calmodulin in Rat Pancreatic Enzyme Secretion Induced by Secretin, Forskolin, and Dibutyryl Cyclic AMP

Abstract

The role of Ca²⁺ and calmodulin in stimulation of the rat pancreatic acini induced by secretin, forskolin, and dibutyryl cyclic AMP (dbcAMP) was studied using W-7, a calmodulin antagonist, and a low Ca²⁺ medium. The time course of amylase secretion was studied in a perifusion system using dispersed rat pancreatic acini. The amylase release patterns of each secretagogue were as follows: a biphasic amylase release pattern under the stimulation of secretin, a one peak pattern during the stimulation of forskolin and a rapid response after cessation of the stimulation, and a gradual increased pattern during the stimulation of dbcAMP followed by a rapid response. The amylase release under the stimulation by forskolin and dbcAMP was slightly weaker as compared with that of secretin stimulation. The amylase secretion stimulated by secretin (5×10^-7M), forskolin (50μM), and dbcAMP (2mM) was inhibited by W-7 (50μM). In a low Ca²⁺ medium (4.7-5.1×10^-6M), the secretory rate did not increase during the stimulation by secretin, forskolin, and dbcAMP, and a rapid amylase response remained after cessation of the stimulation of forskolin and dbcAMP. The pretreatment with EDTA (1mM) suppressed both the gradual amylase release and the rapid response induced by dbcAMP in a low Ca²⁺ medium. These results suggested that each secretagogue, via cyclic AMP (cAMP), induced a different amylase secretory pattern dependent on an intracellular Ca²⁺ content, and was mediated by the Ca²⁺-calmodulin complex.