beta(2)-adrenergic cAMP signaling is uncoupled from phosphorylation of cytoplasmic proteins in canine heart

Background-Recent studies of beta-adrenergic receptor (beta-AR) subtype sig naling in in vitro preparations have raised doubts as to whether the cAMP/p rotein kinase A (PKA) signaling is activated in the same manner in response to beta(2)-AR versus beta(1)-AR stimulation. Methods and Results-The present study compared, in the intact dog, the magn itude and characteristics of chronotropic, inotropic, and lusitropic effect s of cAMP accumulation, PKA activation, and PKA-dependent phosphorylation o f key effector proteins in response to beta-AR subtype stimulation. In addi tion, many of these parameters and L-type Ca2+ current (I-Ca) were also mea sured in single canine ventricular myocytes. The results indicate that alth ough the cAMP/PKA-dependent phosphorylation cascade activated by beta(1)-AR stimulation could explain the resultant modulation of cardiac function, su bstantial beta(1)-AR-mediated chronotropic, inotropic, and lusitropic respo nses occurred in the absence of PKA activation and phosphorylation of nonsa rcolemmal proteins, including phospholamban, troponin I-Ca C protein, and g lycogen phosphorylase kinase. However, in single canine myocytes, we found that beta(2)-AR-stimulated increases in both I-Ca and contraction were abol ished by PKA inhibition. Thus, the beta(2)-AR-directed cAMP/PKA signaling m odulates sarcolemmal L-type Ca2+ channels but does not regulate PKA-depende nt phosphorylation of cytoplasmic proteins. Conclusions-These results indicate that the dissociation of beta(2)-AR sign aling from cAMP regulatory systems is only apparent and that beta(2)-AR-sti mulated cAMP/PKA signaling is uncoupled from phosphorylation of nonsarcolem mal regulatory proteins involved in excitation-contraction coupling.