beta-Adrenergic regulation of protein phosphorylation and its relationship to exocrine secretion in dispersed rat parotid gland acinar cells.

The possible role of cyclic AMP-mediated phosphorylation events in the regulation of exocrine secretion after beta-adrenergic stimulation was examined in vitro in dispersed acinar cell aggregates from rat parotid gland. l-Isoproterenol, a beta-adrenergic agonist, stimulated endogenous activity of cyclic AMP-dependent protein kinase, alterations in the 32P content of 3 parotid phosphoproteins (increased 32P in 2, Mr = 27,000 and 14,000; decreased 32P in the remaining, Mr = 13,600), and amylase secretion in a dose-dependent manner. All responses were half-maximal within a range of l-isoproterenol concentrations of approximately 4 X 10(-8) to 5 X 10(-7) M. Examination of the time course of these 3 processes revealed that by 30 s after addition of l-isoproterenol, significant elevations in cyclic AMP-dependent protein kinase activity and alterations in the 32P content of the 3 parotid proteins had occurred, whereas secretion of amylase from cells was first detected 1-2 1/2 min after hormonal stimulation. Dibutyryl cyclic AMP (2 mM) elicited the same changes in parotid protein 32P content as l-isoproterenol. Our results support the concept of a role for cyclic AMP-regulated protein phosphorylation in the sequence of cellular events leading to exocrine protein secretion from the rat parotid gland following beta-adrenergic stimulation.