Isoproterenol stimulates shift of G proteins from plasma membrane to pinocytotic vesicles in rat adipocytes: a possible means of signal dissemination.

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Abstract
Guanine nucleotide-binding regulatory proteins (G proteins) are linked to a large number of surface membrane receptors and appear to regulate a variety of effector systems located both in the plasma membrane and in other parts of the cell. The mechanism of the disseminative actions of G proteins remains obscure. During an investigation of the fate of two types of G proteins, Gs and Gi, in rat adipocytes, we unexpectedly found that isoproterenol, which stimulates cAMP levels and lipolysis in these cells, induces parallel increases in both Gs and Gi in a low-density microsomal fraction rich in endosomes and Golgi bodies. Two plasma membrane constitutive enzymes, adenylyl cyclase and 59-nucleotidase, are also elevated in this fraction. NaF and NaN3, metabolic inhibitors, block the redistribution process. The isoproterenol-stimulated shifts are completely reversible after removal of the hormone, indicating a recycling, endocytic process. The endocytic process seems to be fluid phase endocytosis, or pinocytosis, since isoproterenol stimulates the uptake of both fluorescent-labeled dextran and horseradish peroxidase into the same vesicles containing Gs. However, the vesicles that accumulate in response to isoproterenol seem heterogenous in properties that may reflect the lipolytic process induced by isoproterenol. It is speculated that the "pinosomes" formed in response to lipolytic hormones may continually produce signals within the cellular interior during their processing and cycling. Hence, signal production in response to hormones need not be confined to the cell membrane; circulating pinosomes may be responsible for some of the disseminative effects of hormones.