Greater inotropic and cyclic AMP responses evoked by noradrenaline through Arg389 β1-adrenoceptors versus Gly389 β1-adrenoceptors in isolated human atrial myocardium

1 We studied the biochemical and contractile responses of isolated human myocardial tissue expressing native receptor variants of the 389G>R beta(1)-adrenoceptor polymorphism. 2 Right atrial appendage was obtained from homozygous RR patients (n=37) and homozygous GG patients (n = 17) undergoing elective cardiac surgery. The positive inotropic effect of noradrenaline in these tissues, mediated through beta(1)-adrenoceptors, was studied using electrically stimulated (1 Hz) atrial strips, as well as the effects of noradrenaline on cyclic AMP levels and cyclic AMP-dependent protein kinase. 3 Tissue from RR homozygotes (n = 14) showed significantly increased inotropic potency to noradrenaline (-log EC50, m=6.92+/-0.12) compared to GG homozygotes (n=8, -log EC50, m=6.36+/-0.11, P<0.005). This difference was not dependent on tissue basal force. 4 Tissue cyclic AMP levels (pmol mg(-1)) were also greater in RR homozygotes (basal 34.8+/-3.7 n = 12, 300 nM noradrenaline 41.4+/-7.6 n = 9, 30 muM noradrenaline 45.2 +/- 3.2 n = 22, 0.2 mm isoprenaline 48.3+/-4.2 n 16) compared to GG homozygotes (basal 30.7+/-4.4 n=5, 300nM noradrenaline 32.6+/-6.92 n=5, 30 muM noradrenaline 38.1+/-3.1 n = 8, 0.2 mM isoprenaline 42.6+/-5.2 n=6, P = 0.007). There were no differences between the variants in terms of cyclic AMP-dependent protein kinase activity. 5 These data provide the first evidence that enhanced G-protein coupling of the R389 beta(1)-adrenoceptor variant reported in rodent fibroblast expression systems is also present in native human receptors. The functional consequence of this is to significantly alter the inotropic potency of beta(1)-adrenoceptor activation depending on its genotype at the 389 position.