Functional evidence for a cyclic-AMP related mechanism of action of the β2-adrenoceptor in human ventricular myocytes

The human ventricle contains both beta(1)- and beta(1)-adrenoceptors (AR) and both have been shown to he present on a single myocyte, In animal ventricular myocardium there is evidence that beta(1)ARs increase cardiac contraction by non-cAMP-dependent mechanisms. We have used the anti-adrenergic effects of carbachol and the cAMP antagonist Rp-cAMPS to investigate the functional contribution of cAMP to beta(2)AR responses in human ventricular myocytes isolated from cardiac biopsies or explants. Concentration-response curves to isoproterenol (Iso) wore constructed in the absence and presence of a beta(1)AR antagonist, CGP 207 12A (300 nmol/l) to determine the contribution of the beta(2)AR to contraction. The cells were rechallenged with submaximal dose of Iso under beta(2)AR-specific conditions and Rp-cAMPS (100-200 mu mol/l) or carbachol (1-3 mu M/l) added, Rp-cAMPS significantly decreased contraction amplitude (% shortening; Iso 7.1 +/- 0.7, Iso + Rp-cAMPS 3.5 +/- 0.5, n = 7, P<0.001) though not completely to the baseline (2.2 +/- 0.6, n = T). Rechallenge with Iso alone reversed the effects of Rp-cAMPS, and subsequent addition of the beta(1)AR antagonist ICI 118,551 reduced the response to baseline (1.6 +/- 0.3, n = 4) confirming beta(2)AR involvement. Similarly, carbachol decreased Iso-stimulated contraction from 7.5 +/- 1.1% to 3.2 +/- 0.9% (P<0.05 n = 4), but not completely to basal levels (1.6 +/- 0.3%). These results provide functional evidence for a predominantly cAMP-mediated mechanism of contractile stimulation by beta(1)AKs in human Ventricular myocardium, although a small contribution from a non-cAMP dependent pathway may occur, (C) 2000 Academic Press.