Altering the receptor-effector ratio by transgenic overexpression of type V adenylyl cyclase: Enhanced basal catalytic activity and function without increased cardiomyocyte beta-adrenergic signalling

The limiting element in beta-adrenergic receptor (beta AR)-G(s)-adenylyl cy clase (AC) signal transduction in the cardiomyocyte is not known, but it ha s been proposed that the level of adenylyl cyclase expression constrains be ta AR signaling. To alter the above equilibrium, type V AC was overexpresse d in a myocyte-specific manner in the hearts of transgenic mice using the a lpha-myosin heavy chain promoter. Expression of type VAC was similar to 75% over endogenous levels as quantitated by [H-3]forskolin binding. Functiona l activity of the transgene product was evident in cardiac membrane AC stud ies, where basal (45 +/- 11 vs 19 +/- 5 pmol min(-1) mg(-1)) and forskolinMn2+ (695 +/- 104 vs 386 +/- 34 pmol min(-1) mg(-1)) stimulated activities were increased compared to activities in nontransgenic (NTG) littermates. H owever, while isoproterenol stimulated activities were higher (74 +/- 12 vs 46 +/- 9.8 pmol min(-1) mg(-1)), the fold stimulation over basal was not i ncreased in ACV overexpressors compared to NTG (line 14.3 = 2.29 +/- 0.44-f old, line 15.1 = 1.70 +/- 0.1-fold, NTG = 2.62 +/- 0.18-fold). Similarly, i n whole cell patch-clamp studies, beta AR-mediated opening of L-type Ca2+ c hannels was not found to be enhanced in transgenic ACV myocytes (225 +/- 15 vs 216 +/- 10% of basal currents). Basal and isoproterenol stimulated PKA activities were elevated in the ACV mice compared to NTG, but again the ext ent of stimulation over basal was not enhanced. Phosphorylated phospholamba n was similar to 2-fold greater in myocytes from ACV hearts compared to NTG , indicating that distal elements of the contractile cascade are activated by AC overexpression. ACV mice displayed increased heart rates and fraction al shortening as assessed by echocardiography. However, in vivo hemodynamic studies revealed that heart rate and contractility responses to agonist in fusion were not enhanced in ACV mice compared to NTG. We conclude that at n ative stoichiometries, the levels of adenylyl cyclase influence basal activ ities and cardiac function, but do not constrain beta AR signaling in the c ardiomyocyte.