CONSTITUTIVELY ACTIVE STIMULATORY G-PROTEIN ALPHA(S) IN BETA-CELLS OFTRANSGENIC MICE CAUSES COUNTERREGULATION OF THE INCREASED ADENOSINE-3',5'-MONOPHOSPHATE AND INSULIN-SECRETION

To evaluate the effect of chronically elevated adenylyl cyclase, we ta rgeted the expression of a constitutively active mutant alpha-subunit (alpha(s)(+)) of G(s) to the insulin-producing pancreatic beta-cells o f transgenic mice. As assessed by the polymerase chain reaction, expre ssion of alpha(s)(+) mRNA was restricted to the transgenic pancreas. H istological analysis by light microscopy and immunohistochemistry for insulin, glucagon, and somatostatin appeared normal in transgenic isle ts. 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 be tween alpha(s)(+) transgenic and control mice; in every case, however, responses to glucose alone were normal, and the differences were obse rved only when measurements were performed in the presence of isobutyl methylxanthine (IBMX), an inhibitor of cAMP phosphodiesterase. 1) In v ivo, ip glucose tolerance was normal in alpha(s)(+) transgenics; when ip glucose was preceded by administration of IBMX, the rise in blood g lucose was -33% less in the transgenic than in the control mice. 2) In sulin secretion from the perfused pancreas stimulated sequentially wit h 11 and 22 mM glucose caused characteristic first and second phase in sulin release that did not differ between transgenic and control pancr eases. IBMX increased biphasic insulin release from all pancreases, bu t caused a 2-fold greater than normal release from the transgenics. 3) Similarly, batch-incubated alpha(s)(+) and control islets secreted eq uivalent 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 express ion of constitutively active alpha(s) mutant in pancreatic 8-cells of transgenic mice is functionally effective, causing the physiological p henotype of increased islet cAMP and insulin secretion. However, these changes are uncovered only in the presence of IBMX; without IBMX, glu cose 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 a ctivity of cAMP phosphodiesterase.