PubMed1994
To evaluate the effect of chronically elevated adenylyl cyclase, we targeted the expression of a constitutively active mutant alpha-subunit (alpha s+) of Gs to the insulin-producing pancreatic beta-cells of transgenic mice. As assessed by the polymerase chain reaction, expression of alpha s+ mRNA was restricted to the transgenic pancreas. Histological analysis by light microscopy and immunohistochemistry for insulin, glucagon, and somatostatin appeared normal in transgenic islets. Pancreatic insulin content was quantitatively the same for alpha s+ transgenic and control mice. Comparisons of glucose homeostasis, insulin secretion, and islet cAMP revealed the expected differences between alpha s+ transgenic and control mice; in every case, however, responses to glucose alone were normal, and the differences were observed only when measurements were performed in the presence of isobutylmethylxanthine (IBMX), an inhibitor of cAMP phosphodiesterase. 1) In vivo, ip glucose tolerance was normal in alpha s+ transgenics; when ip glucose was preceded by administration of IBMX, the rise in blood glucose was approximately 33% less in the transgenic than in the control mice. 2) Insulin secretion from the perfused pancreas stimulated sequentially with 11 and 22 mM glucose caused characteristic first and second phase insulin release that did not differ between transgenic and control pancreases. IBMX increased biphasic insulin release from all pancreases, but caused a 2-fold greater than normal release from the transgenics. 3) Similarly, batch-incubated alpha s+ and control islets secreted equivalent amounts of insulin in the presence of glucose (22 mM) alone, whereas the combination of glucose plus IBMX was twice as effective on alpha s+ islets. 4) Islet cAMP levels paralleled insulin secretion; in the presence of IBMX, but not glucose alone, cAMP was increased 2-fold more in alpha s+ vs. control islets. We conclude that expression of constitutively active alpha s mutant in pancreatic beta-cells of transgenic mice is functionally effective, causing the physiological phenotype of increased islet cAMP and insulin secretion. However, these changes are uncovered only in the presence of IBMX; without IBMX, glucose homeostasis and islet function appear normal. This normalization, or counterregulation, of cAMP synthesis presumably is accomplished by a compensatory increase in cAMP degradation, possibly via increased activity of cAMP phosphodiesterase.
1-Methyl-3-isobutylxanthine↗