Regulation of cardiac alternans by β-adrenergic signaling pathways

Abstract

In cat atrial myocytes, β-adrenergic receptor (β-AR) stimulation exerts profound effects on excitation-contraction coupling and cellular Ca2+cycling that are mediated by β1- and β2-AR subtypes coupled to G proteins (Gsand Gi). In this study, we determined the effects of β-AR stimulation on pacing-induced Ca2+alternans. Ca2+alternans was recorded from single cat atrial myocytes with the fluorescent Ca2+indicator indo-1. Stable Ca2+alternans occurred at an average pacing frequency of 1.7 Hz at room temperature with a mean alternans ratio of 0.43. Nonselective β-AR stimulation as well as selective stimulation of β1/Gs, β2/Gs+ Gi, and β2/Gscoupled pathways all abolished pacing-induced Ca2+alternans. β1-AR stimulation abolished alternans through stimulation of PKA and Ca2+/calmodulin-dependent protein kinase II, whereas β2-AR stimulation exclusively involved PKA and was mediated via Gs, whereas a known second pathway in cat atrial myocytes acting through Giand nitric oxide production was not involved in alternans regulation. Inhibition of various mitochondrial functions (dissipation of the mitochondrial membrane potential or inhibition of mitochondrial F1/F0-ATP synthase, mitochondrial Ca2+uptake via the mitochondrial Ca2+uniporter, and Ca2+extrusion via mitochondrial Na+/Ca2+exchange) enhanced Ca2+alternans; however, β-AR stimulation still abrogated alternans, provided that sufficient cellular ATP was available. Selective inhibition of mitochondrial or glycolytic ATP production did not prevent β-AR stimulation from abolishing Ca2+alternans. However, when both ATP sources were depleted, β-AR stimulation failed to decrease Ca2+alternans. These results indicate that in atrial myocytes, β-AR stimulation protects against pacing-induced alternans by acting through parallel and complementary signaling pathways.