Functional desensitization to isoproterenol without reducing cAMP production in canine failing cardiocytes

To corroborate alterations in the functional responses to beta -adrenergic receptor (beta -AR) stimulation with changes in beta -AR signaling in faili ng cardiomyocytes, contractile and L-type Ca2+ current responses to isoprot erenol along with stimulated cAMP generation were compared among cardiomyoc ytes isolated from canines with tachycardia-induced heart failure or health y hearts. The magnitude of shortening of failing cardiomyocytes was signifi cantly depressed (by 22 +/- 4.4%) under basal conditions, and the maximal r esponse to isoproterenol was significantly reduced (by 45 +/- 18%). Similar results were obtained when the responses in the rate of contraction and ra te of relaxation to isoproterenol were considered. The L-type Ca2+ current amplitude measured in failing cardiomyocytes under basal conditions was unc hanged, but the responses to isoproterenol were significantly reduced compa red with healthy cells. Isoproterenol-stimulated cAMP generation was simila r in sarcolemmal membranes derived from the homogenates of failing (45 +/- 6.8) and healthy cardiomyocytes (52 +/- 8.5 pmol cAMP. mg protein(-1) . min (-1)). However, stimulated cAMP generation was found to be significantly re duced when the membranes were derived from the homogenates of whole tissue (failing: 67 +/- 8.1 vs. healthy: 140 +/- 27.8 pmol cAMP.mg protein(-1).min (-1)). Total beta -AR density was not reduced in membranes derived from eit her whole tissue or isolated cardiomyocyte homogenates, but the beta (1)/be ta (2) ratio was significantly reduced in the former (faitachycardia-induce d heart failure, reduction in the functional responses of isolated cardiomy ocytes to beta -AR stimulation may be attributed to alterations in the exci tation-contraction machinery rather than to limitation of cAMP generation.