Stimulation of cyclic AMP-dependent protein kinase by catecholamines and its relationship to alpha-amylase release in rat parotid gland.

Abstract

The activation of cyclic AMP-dependent protein kinase by catecholamines was measured to study the involvement of this enzyme in a-amylase release in the rat parotid gland. Norepinephrine (NE) rapidly increased the activity of protein kinase, and the stimulated activity of protein kinase as well as cyclic AMP accumulation rapidly returned to a basal level after addition of alprenolol. Isoproterenol was about 10 times more potent than NE in both activating protein kinase and stimulating α-amylase release. Isobutyl-methylxanthine, which slightly increased the protein kinase activity, markedly increased the effect of NE. The maximum level of protein kinase activity obtained by the combined stimulation by NE plus isobutyl-methylxanthine was significantly higher than that by NE alone.β-Adrenergic antagonists inhibited more strongly the NE-induced activation of protein kinase than NE-induced α-amylase release. Inhibition of α-amylase release byβ-adrenergic antagonists was almost completely overcome by in creasing the concentration of NE, whereas that of protein kinase activity was not. Ten mm tolbutamide inhibited the effect of NE on α-amylase release without significantly changing the cyclic AMP accumulation and the activation of protein kinase by NE, although 20 mm tolbutamide was inhibitory to the effect of NE on all these parameters. Ten mm tolbutamide inhibited a-amylase release caused by carbamylcholine and methacholine. These results thus revealed discrepancies between the stimulation of α-amylase release and the activation of protein kinase by NE in some experimental conditions. It is suggested that, even though a cyclic AMP-protein kinase system is responsible for the regulation of α-amylase release by catecholamines, the process of α-amylase release may also be modulated by cyclic AMP-independent mechanisms.