Modulation of islet G-proteins, alpha-glucosidehydrolase inhibition and insulin release stimulated by various secretagogues.
Guanine nucleotide-binding proteins (G-proteins) are known to act as important modulators of insulin release from the islets of Langerhans. We have recently found that the deoxynojirimycin-derivative emiglitate, a recognized inhibitor of intestinal alpha-glucosidehydrolase activity, is a powerful inhibitor of glucose-induced insulin release. With the use of isolated mouse islets the present investigation was performed in a primary attempt to elucidate whether this inhibitory mechanism in some way was linked to the beta-cell G-protein system. Treatment of freshly isolated islets with pertussis toxin (PTX), which is known to inactivate the G (i)-proteins, abolished the inhibitory effect of the alpha(2)-adrenoceptor agonist clonidine on insulin release stimulated by the phosphodiesterase inhibitor IBMX in the presence of the protein kinase C activator TPA and even changed it into an increase. Emiglitate did not display any inhibitory action on insulin release induced by these secretagogues. Similarly, clonidine-induced inhibition of glucose stimulated insulin release was reversed by PTX. However, PTX did not influence the suppressive action of emiglitate on glucose-induced insulin secretion. In contrast, the adenylate cyclase activator forskolin totally abolished the inhibitory effect of emiglitate, but not that of the glucose analogue mannoheptulose, on glucose-induced insulin release. Moreover, the stimulatory effect of forskolin and cholera toxin (CTX) (activator of G (s)-proteins) on the secretion of insulin was markedly enhanced in the presence of emiglitate. In conclusion, our results suggest that the inhibitory effect of emiglitate on glucose-induced insulin release is not directly related to the G(s)-proteins, but most likely exerted solely through the selective suppression of lysosomal aglucosidehydrolase activity, a step in between the proximal and the distal G(i)-proteins, in glucose induced stimulus-secretion mechanisms. Our data also suggests that the inhibitory action of emiglitate on glucose stimulated insulin release can be compensated for by an increased sensitivity of the cyclic AMP-protein kinase A pathway. Hence, emiglitate might indirectly elicit an increased activity of the G(s)-proteins to facilitate the secretory process.
Salehi
A
A
Department of Pharmacology, University of Lund, Lund, Sweden.
Lundquist
I
I
D017485:1-Deoxynojirimycin / Q000031:analogs & derivatives; D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D037361:Adenylate Cyclase Toxin; D000262:Adenylyl Cyclases / Q000378:metabolism; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D002772:Cholera Toxin / Q000494:pharmacology; D003000:Clonidine / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D019204:GTP-Binding Proteins / Q000378:metabolism*; D005944:Glucosamine / Q000031:analogs & derivatives / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology; D006026:Glycoside Hydrolases / Q000037:antagonists & inhibitors*; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D008356:Mannoheptulose / Q000494:pharmacology; D051379:Mice; D037342:Pertussis Toxin; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D011493:Protein Kinase C / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D037361:Adenylate Cyclase Toxin; D000316:Adrenergic alpha-Agonists; D004791:Enzyme Inhibitors; D007328:Insulin; D010726:Phosphodiesterase Inhibitors; D010566:Virulence Factors, Bordetella; D017485:1-Deoxynojirimycin; D005576:Colforsin; D008356:Mannoheptulose; D002772:Cholera Toxin; D037342:Pertussis Toxin; D011493:Protein Kinase C; D006026:Glycoside Hydrolases; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases; D005947:Glucose; D003000:Clonidine; D005944:Glucosamine; D013755:Tetradecanoylphorbol Acetate; C045622:emiglitate; D015056:1-Methyl-3-isobutylxanthine