Regulation of cardiac myocyte cyclic AMP production by intracellular α1-adrenergic receptors.

Abstract

Sympathetic regulation of cardiac function involves β1‑adrenergic receptor (β1AR) production of cAMP. However, the effects are typically mediated by the endogenous neurotransmitter norepinephrine (NE), which is also a potent α1-adrenergic receptor (α1AR) agonist. Furthermore, activation of a mitogen-activated protein kinase (MAPK) signaling pathway by nuclear α1ARs is thought to be cardioprotective, although the exact mechanism has yet to be elucidated. Because long-term stimulation of cAMP production by β1ARs contributes to hypertrophy and apoptosis, we tested the hypothesis that the cardioprotective α1AR signaling pathway inhibits β1AR cAMP production. Changes in cAMP activity were measured in adult rat ventricular myocytes (ARVMs) using FRET-based biosensors. We found that the increase in cAMP produced by the βAR agonist isoproterenol could be antagonized by the selective α1AR agonist methoxamine (METH), and the response to METH could be antagonized by the selective α1AR antagonist prazosin (PRAZ). The effects of METH could also be antagonized by blocking the MAPK signaling pathway with non-selective tyrosine kinase inhibitors genistein and lavendustin A as well as the selective MEK inhibitors U0126 and PD98059. Furthermore, NE produced increases in cAMP activity that was enhanced in the presence of PRAZ, demonstrating α1AR inhibition of β1AR cAMP production contributes to the net response. NE stimulation of cAMP production was also significantly enhanced when the organic cation transporter OCT3 was inhibited with cortisol, and addition of PRAZ caused no further increase, suggesting the α1ARs responsible for inhibiting cAMP production are intracellular. Confocal imaging of ARVMs treated with BODIPY-labeled PRAZ confirmed α1ARs are primarily perinuclear in these cells, a result confirmed by the labeling of α1ARs on isolated nuclei. These results suggest the cardioprotective effect of α1AR stimulation may involve nuclear receptor activation of a MAPK signaling mechanism that inhibits β1AR production of cAMP.