b2-Adrenergic cAMP Signaling Is Uncoupled From Phosphorylation of Cytoplasmic Proteins in Canine Heart

Background—Recent studies of b-adrenergic receptor (b-AR) subtype signaling in in vitro preparations have raised doubts as to whether the cAMP/protein kinase A (PKA) signaling is activated in the same manner in response to b2-AR versus b1-AR stimulation. Methods and Results—The present study compared, in the intact dog, the magnitude and characteristics of chronotropic, inotropic, and lusitropic effects of cAMP accumulation, PKA activation, and PKA-dependent phosphorylation of key effector proteins in response to b-AR subtype stimulation. In addition, many of these parameters and L-type Ca current (ICa) were also measured in single canine ventricular myocytes. The results indicate that although the cAMP/PKA-dependent phosphorylation cascade activated by b1-AR stimulation could explain the resultant modulation of cardiac function, substantial b2-AR–mediated chronotropic, inotropic, and lusitropic responses occurred in the absence of PKA activation and phosphorylation of nonsarcolemmal proteins, including phospholamban, troponin I, C protein, and glycogen phosphorylase kinase. However, in single canine myocytes, we found that b2-AR–stimulated increases in both ICa and contraction were abolished by PKA inhibition. Thus, the b2-AR–directed cAMP/PKA signaling modulates sarcolemmal L-type Ca channels but does not regulate PKA-dependent phosphorylation of cytoplasmic proteins. Conclusions—These results indicate that the dissociation of b2-AR signaling from cAMP regulatory systems is only apparent and that b2-AR–stimulated cAMP/PKA signaling is uncoupled from phosphorylation of nonsarcolemmal regulatory proteins involved in excitation-contraction coupling. (Circulation. 1999;99:2458-2465.)