Calcium antagonists and islet function. IX. Is extracellular calcium required for insulin release?
The physiological relevance of extracellular Ca2+ and Mg2+ to the process of glucose-induced insulin release is examined in both the isolated perfused rat pancreas and rat isolated pancreatic islets. When no Ca2+ is added to the perfusion or incubation media, both the initial and late phases of insulin release are severely impaired. The effect of Ca2+ deprivation is an immediate and immediately reversible phenomenon, although the length of the period of Ca2+ deprivation prior to stimulation with glucose modulates the severity of the impairment in the secretory response. When Mg2+ is omitted from the media, the response to glucose occurs precociously and at a higher than normal rate, such a facilitation being a rapid phenomenon. When both Ca2+ and Mg2+ are removed from the extracellular milieu, the secretory response may occur in a normal fashion, except for an earlier onset. The rate of secretion is only reduced after or during a prolonged exposure of the endocrine pancreas to media deprived of both Ca2+ and Mg2+. The release of insulin evoked by glucose in the absence of Ca2+ and Mg2+ is enhanced by theophylline, partially inhibited by verapamil, and abolished by the Ca2+-chelator EGTA. It is concluded that Mg2+, in physiological concentration, exerts an inhibitory effect upon the process of glucose-induced insulin release. However, even in the absence of extracellular Mg2+, the secretory process remains apparently dependent on the availability of minute amounts of extracellular Ca2+.
D000818:Animals; D002118:Calcium / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D004347:Drug Interactions; D005110:Extracellular Space / Q000502:physiology*; D005947:Glucose / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000502:physiology*; D008274:Magnesium / Q000494:pharmacology; D051381:Rats; D012636:Secretory Rate / Q000187:drug effects; D013268:Stimulation, Chemical; D013806:Theophylline / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology
D007328:Insulin; D013806:Theophylline; D014700:Verapamil; D008274:Magnesium; D005947:Glucose; D002118:Calcium