CaMKII-mediated increased lusitropic responses to beta-adrenoreceptor stimulation in ANP-receptor deficient mice.

OBJECTIVE: Mice with genetic disruption of the guanylyl cyclase-A (GC-A) receptor for atrial natriuretic peptide (ANP), have chronic arterial hypertension and marked cardiac hypertrophy. Intriguingly, despite pronounced remodeling, cardiac contractile functions and cardiomyocyte Ca(2+)-handling are preserved and even enhanced. The present study aimed to characterize the specific molecular mechanisms preventing cardiac failure.

METHODS AND RESULTS: Contractile function and expression as well as phosphorylation of regulatory proteins were evaluated in isolated perfused working hearts from wild-type and GC-A KO mice under baseline conditions and during beta(1)-adrenergic stimulation. Ca(i)(2+)-transients were monitored in Indo-1 loaded isolated adult cardiomyocytes. Cardiac contractile, especially lusitropic responsiveness to beta-adrenergic stimulation was significantly increased in GC-A KO mice. This was concomitant to enhanced expression and activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), increased dual-site phosphorylation of phospholamban (PLB) at Ser(16) and Thr(17), enhanced amplitude of Ca(i)(2+) transients, and accelerated Ca(i)(2+) decay. In contrast, the expression of cardiac ryanodine receptors and phosphorylation at Ser(2809) and Ser(2815) was not altered. Pharmacological inhibition of CaMKII-but not of protein kinase A-mediated PLB phosphorylation totally abolished the increased effects of beta-adrenergic stimulation on cardiac contractility and Ca(i)(2+)-handling. Thus, acceleration of sarcoplasmic reticulum Ca(2+)-uptake and increased availability of Ca(2+) for contraction, both secondary to increased CaMKII-mediated PLB phosphorylation, seem to mediate the augmented responsiveness of GC-A KO hearts to catecholamines.

CONCLUSION: Our observations show that increased CaMKII activity enhances the contractile relaxation response of hypertrophic GC-A KO hearts to beta-adrenergic stimulation and emphasize the critical role of CaMKII-dependent pathways in beta(1)-adrenoreceptor modulation of myocardial Ca(2+)-homeostasis and contractility.