In an attempt to evaluate the role of selective α-adrenergic receptors on Langerhans' islet cells, insulin and glucagon secretion from rat isolated islets were determined with epinephrine, phenylephrine (α
1-stimulator), clonidine (α
2-stimulator), prazosin (α
1-blocker) and yohimbine (a
2-blocker). A new hypoglycemic drug, which was recently reported to act as a α
2 blocker, was also studied in, this connectin.
As already known, epinephrine (5μM) and clonidine (2μM) significantly inhibited insulin secretion and augmented glucagon secretion in the presence of low concentrations of glucoce (5-10 mM). I these conditions, when 100μM of prazosin were added, insulin secretion recovered well, but glucagon secretion was not affected. And when 100μM of yohimbine or DG 5128 were added, insulin or glucagon secretion was significantly increased or decreased, respectively, proving the blocking effect against epinephrine or clonidine. The dose-response curve of insulin and glucagon secreton induced by glucose (0, 5, 10, 15 and 20 mM) were shifted to the left in the presence of DG 5128 (100 and 200μM), changing the maximal response, namely, increaing insulin secretion in high glucose and decreasing glucagon secretion in low glucose.
Thus, glucagon as well as insulin secretion could be regulated mainly through α
2 adrenergic receptors in a tonic fashion. In addition, DG 5128, a new hypoglycemic drug, seems to act as a α
2 blocker in its effect on the glucose metabolism.
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