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

We examined the effect of α₁-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 carboxyseminaphthorhodafluor-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⁺](i) transient amplitudes, myofilament response to Ca²⁺ via cytosolic alkalinization; 3) α₁-AR plus WB-4101 decreased twitch and [Ca²⁺](i) transient amplitudes and also pH(i); 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 pH(i) are due to opposing WB-4101- and CEC-sensitive α₁-AR subtype signaling pathways.