Functional Evidence for a Cyclic-AMP Related Mechanism of Action of the β2-adrenoceptor in Human Ventricular Myocytes

The human ventricle contains both β1- andβ1 -adrenoceptors (AR) and both have been shown to be present on a single myocyte. In animal ventricular myocardium there is evidence that β1ARs 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 β2AR responses in human ventricular myocytes isolated from cardiac biopsies or explants. Concentration-response curves to isoproterenol (Iso) were constructed in the absence and presence of aβ1 AR antagonist, CGP 207 12A (300 nmol/l) to determine the contribution of theβ2 AR to contraction. The cells were rechallenged with sub-maximal dose of Iso underβ2 AR-specific conditions and Rp -cAMPS (100–200 μ mol/l) or carbachol (1–3 μ 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=7). Rechallenge with Iso alone reversed the effects of Rp -cAMPS, and subsequent addition of the β1AR antagonist ICI 118,551 reduced the response to baseline (1.6±0.3,n =4) confirming β2AR involvement. Similarly, carbachol decreased Iso-stimulated contraction from 7.5±1.2% 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 β1ARs in human ventricular myocardium, although a small contribution from a non-cAMP dependent pathway may occur.