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RESEARCH PRODUCT

Sulfhydryl G Proteins and Phospholipase A2-Associated G Proteins Are Involved in Adrenergic Signal Transduction in the Rat Pineal Gland

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The rat pineal gland with its circadian noradrenaline-regulated melatonin rhythm is an excellent model for studying adrenergic signal transduction with respect to cAMP and cGMP formation. The stimulatory G(s) proteins play a well-established role in this process. In contrast, the potential roles of the inhibitory G(i) proteins, the functionally unclear other G(o) proteins, and a number of G protein subtypes are not known. The present study examines the effects on beta(1)- and beta(1)-plus-alpha(1)-stimulated cAMP and cGMP formation of a number of G protein modulators in rat pinealocyte suspension cultures. The effects of the nitric oxide donor sodium nitroprusside on cGMP were also examined. The results showed that drugs that activate G proteins of the G(i)/G(o) family, i.e., pertussis toxin, mastoparan, and compound 48/80, had no effect on unstimulated, isoproterenol (beta(1))-stimulated, or combined isoproterenol/phenylephrine (beta(1)-plus()-alpha(1))-stimulated cAMP and cGMP accumulation. However, in this experimental paradigm, the inhibitors of sulfhydryl G proteins (N-ethylmaleimide) and those of phospholipase A2-related G proteins (isotetrandrine) exerted a clear inhibitory effect. Sodium-nitroprusside-stimulated cGMP accumulation was also inhibited. These results confirm a previous report that members of the G(i)/G(o) family, which are present in the rat pineal gland, do not play a major role in adrenergic signal transduction. The new finding that sulfhydryl G proteins and phospholipase A2-associated G proteins exert a clear stimulatory effect on adrenergic signal transduction suggests that they are subtypes of G(s) proteins.