The amount of AC sets a limit on cardiac beta-adrenergic signaling in vivo, and increased AC, independent of betaAR number and G-protein content, provides a means to regulate cardiac responsiveness to betaAR stimulation. Overexpressing an effector (AC) does not alter transmembrane signaling except when receptors are activated, in contrast to receptor/G-protein overexpression, which yields continuous activation and has detrimental consequences. Our findings establish the importance of AC content in modulating beta-adrenergic signaling in the heart, suggesting a new target for safely increasing cardiac responsiveness to betaAR stimulation.

Background-We tested the hypothesis that increased cardiac myocyte adenylyl cyclase (AC) content increases cardiac function and response to catecholamines in cardiomyopathy. Methods and Results-Transgenic mice with cardiac-directed expression of AC type VI (AC VI ) were crossbred with mice with cardiomyopathy induced by cardiac-directed G q expression. G q mice had dilated left ventricles, reduced heart function, decreased cardiac responsiveness to catecholamine stimulation, and impaired ␤-adrenergic receptor (␤AR)-dependent and AC-dependent cAMP production. G q /AC mice showed improved basal cardiac function in vivo (Pϭ0.01) and ex vivo (PϽ0.0005). When stimulated through the ␤AR, cardiac responsiveness was increased (Pϭ0.02), and cardiac myocytes showed increased cAMP production in response to isoproterenol (Pϭ0.03) and forskolin (PϽ0.0001). Key Words: receptors, adrenergic, beta Ⅲ gene therapy A hallmark of dilated cardiomyopathy is decreased generation of cAMP by cardiac myocytes in response to ␤-adrenergic receptor (␤AR) stimulation. However, treatments for clinical heart failure that increase myocardial cAMP content with pharmacological agents that stimulate the ␤AR or decrease the breakdown of cAMP generally have failed, perhaps because of deleterious consequences of unrelenting stimulation of the ␤AR. Indeed, overexpression of cardiac ␤ARs in transgenic mice caused increased basal heart rate, function, and cAMP generation, 1 and mice overexpressing cardiac G s␣ developed cardiomyopathy due to sustained ␤AR stimulation. 2 Cardiac-directed overexpression of ␤ARs failed to improve heart function and increased mortality in murine dilated cardiomyopathy. 3 We recently showed that cardiac myocytes with increased expression of adenylyl cyclase (AC) produce more cAMP when stimulated through the ␤AR or AC. 4 Cardiac-directed expression of AC type VI (AC VI ) results in a phenotypically normal heart with normal basal function and cAMP levels but supranormal responses to catecholamine stimulation. 5 Thus, receptor/G-protein overexpression and standard inotropic therapy yield continuous ␤AR activation and detrimental consequences, whereas overexpression of cardiac AC VI alters transmembrane signaling only when receptors are activated. Conclusions-IncreasingThis could provide increased cAMP generation in heart failure in a manner that circumvents the deleterious consequences of sustained activation.Cardiac-directed expression of G q results in reduced left ventricular (LV) function, decreased cardiac responsiveness to catecholamines, and impaired ␤AR-dependent and ACdependent cAMP production. 6 The exact mechanism for dilation is unknown, but G q is coupled to endothelin, angiotensin II, and ␣ 1 -adrenergic receptors, pathways that influence cardiac myocyte growth and remodeling. This model provides an opportunity to test the hypothesis that cardiacdirected AC expression can increase cAMP generation and restore heart function and response to catecholamines in dilated cardiomyopathy. Methods AnimalsAnimal use followed...

Background-Adenylyl cyclases (ACs) are a family of effector molecules for G-protein-coupled receptors. The 2 ACs most abundantly expressed in cardiac myocytes are types 5 (AC5) and 6 (AC6), which have 65% amino acid homology. It has been speculated that coexpression of 2 AC types in cardiac myocytes represents redundancy, but the specific role of AC6 in cardiac physiology and its differences from AC5 remain to be defined. Methods and Results-We generated transgenic mice with targeted deletion of AC6. Deletion of AC6 was associated with reduced left ventricular contractile function (Pϭ0.026) and relaxation (Pϭ0.041). The absence of AC6 was associated with a 48% decay in ␤-adrenergic receptor-stimulated cAMP production in cardiac myocytes (Pϭ0.003) and reduced protein kinase A activity (Pϭ0.015). In addition, phospholamban phosphorylation was reduced (Pϭ0.015), sarcoplasmic reticulum Ca 2ϩ -ATPase activity was impaired (PϽ0.0001), and cardiac myocytes showed marked abnormalities in calcium transient formation (Pϭ0.001). Conclusions-The

Background-We tested the hypothesis that intracoronary delivery of an adenovirus encoding adenylyl cyclase type VI (Ad.AC VI ) would be associated with increased left ventricular (LV) function in pigs with congestive heart failure. Methods and Results-Pigs (52Ϯ6 kg; nϭ16) underwent placement of pacemakers, LV pressure transducers, and left atrial and aortic catheters. Physiological and echocardiographic studies were obtained from conscious animals 13 days later, and pacing was initiated (220 bpm). Seven days later, measures of LV function were reduced, documenting severe LV dysfunction and dilation. Pigs then received intracoronary Ad.AC VI (1.4ϫ10 12 vp; nϭ7) or saline (PBS; nϭ9) (randomized, blinded), with concomitant infusion of nitroprusside (50 g/min, 6.4 minutes) to increase gene transfer. Pacing was continued for 14 days, and final studies were obtained. The a priori key end point was change in LV dP/dt during isoproterenol infusion (pre-Ad.AC VI value minus value after 21 days of pacing). Pigs receiving Ad.AC VI showed a smaller decrease in both LV ϩdP/dt (Pϭ0.0014) and LV ϪdP/dt (Pϭ0.0008). Serial echocardiography showed that Ad.AC VI treatment was associated with increased LV function and reduced LV dilation and that end-systolic wall stress was reduced. AC-stimulated cAMP production was increased 1.7-fold in LV samples from Ad.AC VI -treated pigs (Pϭ0.006), and B-type natriuretic peptide was reduced (0.035). Gene transfer was confirmed by polymerase chain reaction. Conclusions-AC