INCREASED EXPRESSION OF ADENYLYLCYCLASE TYPE-VI PROPORTIONATELY INCREASES BETA-ADRENERGIC RECEPTOR-STIMULATED PRODUCTION OF CAMP IN NEONATAL RAT CARDIAC MYOCYTES

Cellular content of cAMP generated by activation of adenylylcyclase (A C; EC 4.6.1.1) is a key determinant of functional responsiveness in th e heart and other tissues, We have tested two hypotheses regarding the relationship between AC content and beta-adrenergic receptor (beta AR )mediated signal transduction in cardiac myocytes, First, that AC cont ent limits adrenergic signal transduction, and, second, that increased AC, independent of (beta AR) number and G-protein content, yields a p roportional increase in beta AR-mediated transmembrane signaling, We u sed recombinant adenovirus to increase AC isoform VI (AC(VI)) expressi on in neonatal cardiac myocytes, Cells that overexpressed AC(VI) respo nded to agonist stimulation with marked increases in cAMP production i n proportion to protein expressed, In parallel experiments performed o n cells transfected with lacZ (control) or AC(VI), [H-3]forskolin bind ing, used to assess AC protein expression, was amplified 6-fold, while beta AR-stimulated cAMP production from these cells was increased 7-f old. No changes in beta AR number, or in the heterotrimeric GTP-bindin g proteins, Gas or Gait, were observed, Previous studies indicate that increased cardiac expression of beta AR or Gas does not yield proport ional increases in transmembrane adrenergic signaling, In contrast, th e current data demonstrate that increased AC(VI) expression provides a proportional increase in beta-adrenergic signal transduction, Our res ults show that the amount of AC sets a limit on transmembrane beta-adr energic signaling, We speculate that similar functional responses are possible in other cell types in which AC plays an important physiologi cal role.