Opposing effects of α1-adrenergic receptor subtypes on Ca2+ and pH homeostasis in rat cardiac myocytes

Abstract

We examined the effect of α1-adrenergic receptor (AR) subtypes on contraction, cytosolic Ca2+ concentration ([Ca2+]i), and cytosolic pH (pHi) of rat ventricular myocytes loaded with the Ca2+ indicator indo 1 or the pH indicator carboxy-seminaphthorhodafluor-1. Nonselective α1-AR stimulation was effected with phenylephrine plus nadolol. α1-AR subtype stimulation was achieved with α1-AR and chloroethylclonidine (CEC) or with α1-AR and WB-4101. Cells were in bicarbonate buffer with 0.5 mM Ca2+ and were electrically stimulated at 0.5 Hz. Results show that 1) nonselective α1-AR stimulation increased twitch and [Ca2+]itransient amplitudes, myofilament response to Ca2+, and pHi; 2) α1-AR plus CEC increased twitch and [Ca2+]itransient amplitudes and also enhanced myofilament response to Ca2+ via cytosolic alkalinization; 3) α1-AR plus WB-4101 decreased twitch and [Ca2+]itransient amplitudes and also pHi; and 4) cytosolic acidification due to α1-AR plus WB-4101 was abolished by protein kinase C inhibition (staurosporine pretreatment) or downregulation (prolonged exposure to phorbol esters). In summary, the net effects of α1-adrenergic stimulation on contraction, [Ca2+]i, and pHi are due to opposing WB-4101- and CEC-sensitive α1-AR subtype signaling pathways.