Increased expression of adenylylcyclase type VI proportionately increases β-adrenergic receptor-stimulated production of cAMP in neonatal rat cardiac myocytes

Cellular content of cAMP generated by activation of adenylylcyclase (AC; EC 4.6.1.1) is a key determinant of functional responsiveness in the heart and other tissues. We have tested two hypotheses regarding the relationship between AC content and β-adrenergic receptor (βAR)-mediated signal transduction in cardiac myocytes. First, that AC content limits adrenergic signal transduction, and, second, that increased AC, independent of (βAR) number and G-protein content, yields a proportional increase in βAR-mediated transmembrane signaling. We used recombinant adenovirus to increase AC isoform VI (AC(VI)) expression in neonatal cardiac myocytes. Cells that overexpressed AC(VI) responded to agonist stimulation with marked increases in cAMP production in proportion to protein expressed. In parallel experiments performed on cells transfected with lacZ (control) or AC(VI), [(3)H]forskolin binding, used to assess AC protein expression, was amplified 6-fold, while βAR-stimulated cAMP production from these cells was increased 7-fold. No changes in βAR number, or in the heterotrimeric GTP-binding proteins, Gαs or Gαi(2), were observed. Previous studies indicate that increased cardiac expression of βAR or Gαs does not yield proportional increases in transmembrane adrenergic signaling. In contrast, the current data demonstrate that increased AC(VI) expression provides a proportional increase in β-adrenergic signal transduction. Our results show that the amount of AC sets a limit on transmembrane β-adrenergic signaling. We speculate that similar functional responses are possible in other cell types in which AC plays an important physiological role.