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

We examined the effect of alpha(1)-adrenergic receptor !AR) subtypes on contraction, cytosolic Ca2+ concentration ([Ca2+](i)), and cytosolic pH (pH(i)) of rat ventricular myocytes loaded with the Ca2+ indicator indo 1 or the pH indicator carboxy-seminaphthorhodafluor-1. Nonselective alpha(1)-AR stimulation was effected with phenylephrine plus nadolol. alpha(1)-AR subtype stimulation was achieved with alpha(1)-AR and chloroethylclonidine (CEC) or with alpha(1)-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 I) nonselective alpha(1)-AR stimulation increased twitch and [Ca2+](i) transient amplitudes, myofilament response to Ca2+, and pH(i); 2) alpha(1)-AR plus CEC increased twitch and [Ca2+](i) transient amplitudes and also enhanced myofilament response to Ca2+ via cytosolic alkalinization; 3) alpha(1)-AR plus WB-4101 decreased twitch and [Ca2+](i) transient amplitudes and also pH(i); and 4) cytosolic acidification due to alpha(1)-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 alpha(1)-adrenergic stimulation on contraction, [Ca2+](i), and pH(i) are due to opposing WB-4101- and CEC-sensitive alpha(1)-AR subtype signaling pathways.