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ABSTRACT

The mechanisms involved in inhibition of insulin secretion by somatostatin and noradrenaline were compared in order to establish whether the receptors for these agents are coupled to similar effector systems in the pancreatic B cell. Both agents significantly reduced forskolin-induced adenylate cyclase activity in islet homogenates, although noradrenaline was more effective than somatostatin. The capacity of noradrenaline to inhibit insulin secretion was largely unaffected by agents that increase intracellular cyclic AMP, whereas the effect of somatostatin as an inhibitor was markedly reduced under these conditions. Both noradrenaline and somatostatin inhibited the stimulation of insulin secretion induced by K⁺ depolarization, but different mechanism were involved. Somatostatin significantly inhibited K⁺-stimulated ⁴⁵Ca²⁺ efflux and influx in islets, while noradrenaline exerted only a minor influence on these processes. The data indicate that noradrenaline controls insulin secretion by a mechanism which operates beyond the level of intracellular messenger generation. In contrast, somatostatin exerts at least part of its inhibitory effect on insulin secretion by directly controlling islet cell Ca²⁺ influx in a manner which may be regulated by cyclic AMP.

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 G_i1 and G_i2, reacted with an islet protein having an approximate M _r of 40 000. Antiserum IlC (G_i1 specific) failed to recognize any islet proteins, suggesting that G_i2 is present in much greater amounts than G_i1. Indeed, G_i1 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 G_o-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 G_o are present in rat islets.

Subcellular fractionation indicated that all three G proteins identified in this study (G_i2 and two forms of G_o) are localized to islet membranes. No immunoreactivity could be detected in the cytosolic fraction.

ABSTRACT

Insulin secretion from isolated rat islets of Langerhans in the presence of 4 mm glucose averaged 2·26 ± 0·20 (s.e.m.) ng/islet per 90 min and was significantly (P<0·001; n=30) increased to 3·28 ± 0·21 ng/islet per 90 min by the covalent α-adrenoceptor antagonist benextramine (10 μm). Glucose (20 mm) also increased the secretion rate (to 6·24 ± 6·0 ng/islet per 90 min) but, under these conditions, the response was not further enhanced by benextramine. Clonidine and noradrenaline (1 nm–10 μm) each caused dose-dependent inhibition of glucose-induced insulin secretion which was maximal at 1 μm. Benextramine, when added simultaneously with the agonist, relieved, in a dosedependent manner, the inhibition of secretion induced by either clonidine or noradrenaline with similar sensitivity. Even after a 30-min preincubation with benextramine the antagonist failed to differentiate between noradrenaline, adrenaline and clonidine with respect to inhibition of insulin secretion. In contrast to its effects on adrenergic responses, short-term treatment with benextramine did not significantly affect muscarinic—cholinergic receptor-mediated ⁴⁵Ca²⁺ efflux from rat islets of Langerhans perifused in Ca²⁺-depleted medium. These data suggest that benextramine does not differentiate between clonidine and noradrenaline in rat islets of Langerhans but that it does show preference for α-adrenoceptors in this tissue.

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.