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ABSTRACT
Inhibition of insulin secretion from rat islets of Langerhans is known to involve at least one pertussis toxin-sensitive guanine-nucleotide binding (G) protein. We have used antisera raised against unique antigenic determinants of different members of the family of pertussis toxin-sensitive G proteins to identify these proteins in rat islets. Antiserum SG1, which recognizes both Gi1 and Gi2, reacted with an islet protein having an approximate M r of 40 000. Antiserum IlC (Gi1 specific) failed to recognize any islet proteins, suggesting that Gi2 is present in much greater amounts than Gi1. Indeed, Gi1 levels were below the detection limit of a sensitive immunogold/silver-staining method, indicating that it may be absent from the cells of rat islets.
Two different antisera were used to identify Go-like G proteins in rat islet homogenates. Both antisera reacted with a protein band which, under appropriate conditions, could be resolved to reveal two separate proteins of M r 39–40 000. Thus, at least two molecular forms of Go are present in rat islets.
Subcellular fractionation indicated that all three G proteins identified in this study (Gi2 and two forms of Go) are localized to islet membranes. No immunoreactivity could be detected in the cytosolic fraction.
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ABSTRACT
The selective β2-adrenergic agonist clenbuterol was ineffective as a stimulus for insulin secretion when isolated rat pancreatic islets were incubated with glucose at concentrations between 4 and 20 mM. Inclusion of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine led to potentiation of glucose-induced insulin secretion, but did not facilitate stimulation by clenbuterol. Furthermore, maintenance of isolated rat islets for up to 3 days in tissue culture also failed to result in the appearance of a secretory response to β-agonists. By contrast, clenbuterol induced a dose-dependent increase in insulin release from isolated human islets incubated with 20 mm glucose. Clenbuterol did not increase the basal rate of insulin secretion (4 mm glucose) in human islets. Under perifusion conditions, the secretory response of human islets to clenbuterol was rapid, of similar magnitude to that seen under static incubation conditions and could be sustained for at least 30 min. The increase in insulin secretion induced by clenbuterol was inhibited by propranolol, indicating that the response was mediated by activation of β-receptors. In support of this, a similar enhancement of glucose-induced insulin secretion was elicited by a different β2-agonist, salbutamol, in human islets. The results indicate that the B cells of isolated rat islets are unresponsive to β-agonists, whereas those of human islets are equipped with functional β-receptors which can directly influence the rate of insulin secretion.
