Roles of PKA, PI3K, and cPLA2 in the NO-mediated negative inotropic effect of 32-adrenoceptor agonists in guinea pig right papillary muscles

摘要

First published October 17, 2007; doi:10.1152/ajpcell.00231.2007.-Although P2-adrenoceptors represent 15-25% of beta-adrenoceptors in the guinea pig heart, their functionality is controversial. We assessed the inotropic effects of 32-adrenoceptor partial agonists in right papillary muscles. Salbutaraol induced a small but significant concentration-dependent negative inotropic effect (NIE, -5% at 60 nM) followed by a moderate positive inotropic effect (+36% at 6 muM) due to activation of beta,-adrenoceptors. In the presence of 4 muM atenolol, the concentration-dependent NIE (-12% at 6 muM) was biphasic, best described by a double logistic equation with respective EC30 values of 3 and -420 nM, and was insensitive to SR59230A. In muscles from pertussis toxin-treated guinea pigs, the salbutamol-induced positive inotropic effect was sensitive to low concentrations of ICI-118551 in an unusual manner. Experiments in reserpinized animals revealed the importance of the phosphorylation-dephosphorylation processes. PKA inhibition reduced and suppressed the effects obtained at low and high concentrations, respectively, indicating that its activation was a prerequisite to the NIE. The effect occurring at nanomolar concentrations depended upon PKA/phosphatidylinositol 3-kinase/ cytosolic phospholipase A2 (cPLA2) activations leading to nitric oxide (NO) release via the arachidonic acid/cyclooxygenase pathway. NO release via PKA-dependent phosphorylation of the receptor was responsible for the inotropic effect observed at submicromolar concentrations, which is negatively controlled by cPLA2. The possibility that these effects are due to an equilibrium between different affinity states of the receptor (Gs/Gi coupled and G independent with different signaling pathways) that can be displaced by ICI-118551 is discussed. We conclude that beta2-adrenoceptors are functional in guinea pig heart and can modulate the inotropic state.