EFFECTS OF ALPHA(1)-ADRENERGIC STIMULATION ON L-TYPE CA2+ CURRENT IN RAT VENTRICULAR MYOCYTES

The effect of alpha(1)-adrenergic stimulation on L-type Ca2+ current ( I-Ca,I- L) in adult rat ventricular myocytes was investigated using th ree different methods of current recording. During conventional whole- cell recordings with 5 mM-BAPTA included in the pipette solution, phen ylephrine (20 mu M) did not increase I-Ca,I- L after 10 min of applica tion. With nystatin perforated-patch whole-cell recordings, phenylephr ine potentiated I-Ca,I- L, although there were variations among myocyt es. The most frequent response was a transient suppression of peak I-C a,I- L at similar to 2 min of exposure followed by a sustained increas e of current amplitude evident after 5-10 min exposure. The relative c urrent amplitude 10 min after phenylephrine application was 1.08 +/- 0 .05 compared to control (n = 14 cells, P < 0.05). During cell-attached single channel recordings, phenylephrine (1 mu M) increased the L-typ e Ca2+ channel open probability (NPo) by 2.25 +/- 0.31-fold (n = 21, P < 0.01). It potentiated NPo by increasing the number of openings per sweep and also by promoting longer openings. These effects developed s lowly in similar to 10 min. Phenylephrine had no effect on unitary cur rent amplitude. The potentiation was also elicited by methoxamine (5 m u M) and was blocked by prazosin (1 mu M), indicating that it was medi ated by alpha(1)-adrenergic receptor stimulation. The increase in NPo was suppressed by chelerythrine, a protein kinase C inhibitor. Our res ults demonstrate that I-Ca,I- L can be enhanced by alpha(1)-adrenergic stimulation, and stress the importance of not disturbing the intracel lular environment during studies of the modulation of cardiac I-Ca,I- L by alpha(1)-adrenergic stimulation. (C) 1998 Academic Press.