Both alpha(1A)- and alpha(1B)-adrenergic receptor subtypes couple to the transient outward current (I(To)) 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 [(125)I]-HEAT showed that the inhibition curves for alpha(1A)AR-selective antagonists (WB4101, 5-methylurapidil, (+)-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 microM 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 microM) 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.