Augmentation of cardiac contractility with no change in L‐type Ca²⁺ current in transgenic mice with a cardiac‐directed expression of the human adenylyl cyclase type 8 (AC8)

Abstract

The beta-adrenergic cascade is severely impaired in heart failure (HF), in part because of a reduction in the activity of the two dominant cardiac adenylyl cyclase (AC) isoforms, AC5 and AC6. Hence, cardiac-directed AC overexpression is a conceivable therapeutic strategy in HF. In this study, we explored the consequences at the cellular and organ level of a cardiac-directed expression of the human AC8 in the transgenic mouse line AC8TG. Unlike AC5 and AC6, which are inhibited by intracellular Ca2+, AC8 is stimulated by Ca2+-calmodulin. Langendorff perfused hearts from AC8TG mice had a twofold higher left ventricular systolic pressure, a 40% faster heart rate, a 37% faster relaxation, and a 30% higher sensitivity to external Ca2+ than nontransgenic control mice (NTG). Cell shortening measured in isolated ventricular myocytes developed 22% faster and relaxed 43% faster in AC8TG than in NTG mice. Likewise, Ca2+ transients measured in fluo-3 AM-loaded myocytes were 30% higher and relaxed 24% faster in AC8TG compared with NTG mice. In spite of the large increase in Ca2+ transients and contraction, expression of AC8 had no effect on the whole-cell L-type Ca2+ current (ICa, L) amplitude. Moreover, ICa, L was unchanged even when AC8 was activated by raising intracellular Ca2+. Thus, cardiac expression of AC8 leads to an increase in cAMP that activates specifically Ca2+ uptake into the sarcoplasmic reticulum but not Ca2+ influx at the sarcolemma, suggesting a strong compartmentation of the cAMP signal.

Disruption of type 5 adenylyl cyclase gene preserves cardiac function against pressure overload

“…Previous studies in Gq͞type 5 or 6 AC double cardiac-specific overexpression mice demonstrated that the addition of AC ameliorated Gq-mediated cardiomyopathy and improved survival in Gq mice (16,18,19). Furthermore, mice with overexpression of type 8 AC also presented no signs of cardiomyopathy, despite a 7-fold increase in basal AC activity and a 4-fold increase in PKA activity in the heart (21,22). These studies, taken together, could be interpreted to indicate that increased AC in the heart is beneficial.…”

“…Although acute sympathetic stimulation and activation of the cAMP-PKA pathway is a major mechanism to improve cardiac function, previous studies using transgenic models demonstrated that chronic activation of these pathways caused by the cardiacspecific overexpression of ␤-AR, Gs␣, and PKA resulted in cardiomyopathy (8)(9)(10)(11)(12)(13)(14)(15). Previous studies in which type 5 or type 6 AC was overexpressed do not support this position (16)(17)(18)(19)(20)(21)(22). Transgenic mice with cardiac-specific overexpression of type 6 AC did not develop cardiac dysfunction and myocardial fibrosis even in 19-mo-old animals, despite a 20-fold increase of type 6 AC protein in the heart (20).…”

“…Most prior experimental studies on this topic have focused on mouse models overexpressing ␤-AR (8-11), Gs␣ (12)(13)(14), or protein kinase A (PKA) (15), and the results from these studies support the use of ␤-AR blockade in heart failure, because these genetically engineered mouse models demonstrated that prolonged ␤-AR stimulation results in cardiomyopathy and that the adverse effects of pressure overload are exacerbated (11). Interestingly, the results from genetically engineered mouse models in which isoforms of adenylyl cyclase (AC) have been overexpressed are at variance with the above results (16)(17)(18)(19)(20)(21)(22). AC plays a pivotal role in ␤-AR signaling, transducing the signal generated from coupling of the ligand, most commonly norepinephrine, with the ␤-AR and Gs␣ to catalyze cAMP, which consequently modulates the L-type Ca 2ϩ channel and augments cardiac contractility.…”

<b>Adipose stem cell sheets improved cardiac function in the rat myocardial infarction, but did not alter cardiac contractile responses to β-adrenergic stim</b><b>ulation </b>

“…Cardiac remodeling and the heart failure lead to β-adrenoceptor downregulation as well as desensitization/uncoupling of β-adrenoceptor (5). This impaired β-adrenergic signaling is reported to be due to either 1) reduced β-receptors in MI hearts, 2) impaired adenylate cyclase activity to reduce cAMP production, or 3) impaired protein kinase A (PKA) downstream effects (1). In the present study, isoproterenol, pimobendan, and dibutyryl c-AMP did not restore cardiac contractile response to β-adrenergic stimulation, indicating that cell sheets could not improve impaired PKA-mediated downstream effects of β-adrenergic stimulation.…”

Cardiac Specific Increase in Aldosterone Production Induces Coronary Dysfunction in Aldosterone Synthase–Transgenic Mice

“…Whole-cell patch-clamp of ventricular myocytes was used to record the L-type Ca 2ϩ current (I Ca,L ) as published previously, 20 and the time-dependent and time-independent components of the potassium transient outward currents, I to1 and I sus , respectively, as published previously. 22 …”