Cardiac Alpha1a-adrenoceptor Stimulation Inhibits L-type Ca2+ Current In The Presence Of Beta-adrenoceptor Stimulation Through Tyrosine Kinase

Abstract

Introduction: We previously showed that cardiac α1-adrenoceptor (AR) stimulation alone potentiates L-type Ca2+ current (ICa) through α1A-AR-PLC-PKC pathway (O-Uchi J et al. PNAS., 2005 and Circ Res., 2008). However, the interaction of α1- and β-AR signalings for ICa regulation was not fully clarified. In the present study, we examined the effect of α1-AR stimulation on ICa when β-AR is stimulated. Methods: Perforated patch-clamp was used for recording ICa from isolated adult rat ventricular myocytes. Cells were at first treated with β-AR agonist (100 nM isoproterenol) for 5 min and then α1-AR agonist (100 μM phenylephrine) was applied in the continuous presence of isoproterenol. Holding potential was set at -40 mV and depolarization pulse to 0 mV was applied every 10 sec. Results: Phenylephrine significantly inhibited ICa in the presence of isoproterenol by 19.6±7.6%. The α1A-AR selective antagonist (WB4101) blocked this inhibitory effect by phenylephrine, but α1B-AR selective antagonist (L-765,314) did not, confirming that only α1A-AR is involved in this inhibitory effect. Phenylephrine had no effect on ICa activated by forskolin. In addition, inhibition of Gq signaling by PLC inhibitor (U73122) or inhibition of Gi/o signaling by pertussis toxin did not blocked the phenylephrine-induced inhibition of ICa. The tyrosine kinase inhibitor (lavendustin A) attenuated the response of phenylephrine during β-AR stimulation. Conclusion: α1A-AR stimulation inhibits ICa in the presence of β-AR stimulation, which is opposite to the effect observed in the absence of β-AR stimulation. This effect is not mediated through Gq and Gi/o but through tyrosine kinase activity, which inhibits the upstream of β-AR signaling (at the level of β-AR or Gs). The inhibitory effect of α1A-AR stimulation could serve as one of the regulatory feedback mechanisms when catecholamine level increases under pathophysiological conditions.