Both α1A- and α1B-adrenergic receptor subtypes couple to the transient outward current (ITo) in rat ventricular myocytes

1 Regulation of transient outward current (I-To) by alpha(1)-adrenergic (alpha(1)AR) plays a key role in cardiac repolarization. alpha(1)ARs comprise a heterogeneous family; two natively expressed subtypes (alpha(1A) and alpha(1B)) and three cloned subtypes (alpha(1a), alpha(1b) and alpha(1d)) can be distinguished. We have examined the electrophysiological role of each alpha(1)AR subtype in regulating I-To in isolated rat ventricular myocytes, 2 Reverse transcription-PCR study revealed the presence of three subtype mRNAs (alpha(1a), alpha(1b) and alpha(1d)) in rat myocytes. 3 Radioligand binding assay using [I-125]-HEAT showed that the inhibition curves for alpha(1A)AR-selective antagonists (WB4101, 5-methyl-urapidil, (+)-niguldipine and KMD-3213) in rat ventricles best fit a two-site model, with 30% high and 70% low affinity binding sites. The high affinity sites were resistant to 100 mu M chloroethylclonidine (CEC), while the low affinity sites were highly inactivated by CEC. 4 Whole cell voltage clamp study revealed that methoxamine reduced a 4-aminopyridine(4-AP)-sensitive component of I-To in the isolated rat ventricle myocytes. Lower concentrations of KMD-3213 (1 nM) or 5-MU (10 nM) did not affect the methoxamine-induced reduction of I-To. On the other hand, CEC treatment (100 mu M) of isolated myocytes reduced the methoxamine-induced reduction of I-To by 46%, and the remaining response was abolished by lower concentrations of KMD-3213 or 5-MU. 5 The results indicate that rat ventricular myocytes express transcripts of the three alpha(1)AR subtypes (alpha(1a), alpha(1b) and alpha(1d)); however, two pharmacologically distinct alpha(1)AR subtypes (alpha(1A) and alpha(1B)) are predominating in receptor populations, with approximately 30% alpha(1A)AR and 70% alpha(1B)AR. Although both alpha(1A) and alpha(1B)AR subtypes are coupled to the cardiac I-To, alpha(1B)ARs predominantly mediate alpha(1)AR-induced effect.