Regulation of phosphorylation of Gi2α protein controls the secretory response to isoproterenol in rat parotid tissues

Abstract

Treatment of rat parotid tissues with 1 microM isoproterenol (IPR) for 10 min caused a 60% decrease in pertussis toxin (IAP)-catalyzed ADP-ribosylation of Gi alpha and resulted in supersensitivity of amylase secretion from the tissues. However, conversely, IPR treatment for 30 min caused a 40% increase in IAP-catalyzed ADP-ribosylation of Gi alpha, coupled with desensitization of amylase secretion. No changes in Gs function were observed in IPR-induced phenomena. Pretreatment with okadaic acid induced enhancement of the supersensitivity of amylase secretion and disappearance of the desensitization. These phenomena were accompanied with decreases in IAP-catalyzed ADP-ribosylation of Gi alpha. IPR treatment for 30 min caused a 50% decrease in phosphorylation of Gi2 alpha immunoprecipitated with anti-G protein antiserum (AS/7) from [32P]Pi-labeled cells, but such treatment for 10 min caused a 40% increase in phosphorylation in the cells pretreated with okadaic acid. Phosphorylation and dephosphorylation of immunoprecipitates with AS/7 by protein kinase A (PKA) and alkaline phosphatase caused decreases and increases in IAP-catalyzed ADP-ribosylation, respectively, indicating the presence of PKA-mediated phosphorylation sites on Gi2 alpha. Thus, the control of the phosphorylation of Gi2 alpha is of importance and relevance in the regulation of biological processes and cellular responses.

Water Channels and Zymogen Granules in Salivary Glands

“…PKA type I is the membrane-bound form that is activated during β-ADR stimulation and is accompanied by an increase in specific membrane phosphorylation during amylase secretion induced by β-ADR agonists (64,141). These findings suggest that Gi2α protein is selectively phosphorylated by PKA type I, resulting in decreased function and supersensitivity of amylase secretion (6,64,65). 2.…”

“…The phosphorylation causes some conformational changes in Gi protein trimers and decreases in both the IAP-catalyzed ADPribosylation of Gi protein and their dissociation into α-and βγ-subunits (157,158). In parotid tissues supersensitized with IPR, there is a 60% decrease in IAPcatalyzed ADP-ribosylation of Giα (64). Conversely, however, a 40% increase in IAP-catalyzed ADPribosylation of Giα is coupled with the desensitization of amylase and mucin secretion from rat parotid and submandibular tissues induced by IPR and histamine, respectively (6,64,65).…”

“…In parotid tissues supersensitized with IPR, there is a 60% decrease in IAPcatalyzed ADP-ribosylation of Giα (64). Conversely, however, a 40% increase in IAP-catalyzed ADPribosylation of Giα is coupled with the desensitization of amylase and mucin secretion from rat parotid and submandibular tissues induced by IPR and histamine, respectively (6,64,65). In agonist-induced desensitization of AC stimulation in MDCK cells (159) and rat heart muscle cells (160), there are also increases in the level and function of Gi proteins, as measured by IAPcatalyzed ADP-ribosylation and GTP-binding capacities of these proteins.…”

“…Each regulatory subunit comprises R Iα, R Iβ, and R IIα and R IIβ, respectively (139,140). In parotid glands, there are two PKA isozymes, type I and II (64). PKA type I is the membrane-bound form that is activated during β-ADR stimulation and is accompanied by an increase in specific membrane phosphorylation during amylase secretion induced by β-ADR agonists (64,141).…”

“…IAP-sensitive G proteins, termed Gi1, Gi2, and Gi3, comprise a family of Gi proteins and Gi2 inhibits adenylate cyclase activity (156). Giα protein is phosphorylated by PKA (64,157,158). The phosphorylation causes some conformational changes in Gi protein trimers and decreases in both the IAP-catalyzed ADPribosylation of Gi protein and their dissociation into α-and βγ-subunits (157,158).…”