Characterization of the endothelin-1-induced regulation of L-type Ca2+ current in rabbit ventricular myocytes

The effects of endothelin-1 (ET-1) on the L-type Ca2+ current (I(Ca)) and the interaction of ET-1 with β-adrenoceptor stimulation were investigated in rabbit ventricular myocytes by the whole-cell patch-clamp technique. ET-1 (10-8 M) had a biphasic effect on I(Ca) (direct effect), causing a transient decrease that was followed by a long-lasting increase which is much smaller than the increase induced by isoprenaline (ISO). The effect of ET-1 on I(Ca) was abolished by a selective ET(A) receptor antagonist, FR139317 (10-6 M). The increase in I(Ca) induced by ET-1 (10- 8 M) was enhanced by a selective ET(B) receptor antagonist, BQ-788 (10-6 M), as the transient decrease but not the increase in I(Ca) induced by ET-1 (10-8 M) was suppressed by BQ-788. In the presence of ISO (10-6 M), ET-1 elicited a more pronounced inhibitory effect: at 10-9-10-7 M ET-1 inhibited the ISO-induced increase in I(Ca) in a concentration-dependent manner (anti-adrenergic effect). The maximum inhibition induced by ET-1 at 10-7 M was approximately 80% of the ISO-induced response, and the IC50 value for anti-adrenergic effect of ET-1 was 4.2x10-9 M. The anti- adrenergic effect of ET-1 (10-8 M) was antagonized by the ET(A) antagonist FR139317 (10-9 10-6 M) in a concentration-dependent manner and was partially inhibited by the ET(B) antagonist BQ-788 (10-6 M). The anti- adrenergic effect of ET-1 was markedly attenuated by pretreatment of ventricular myocytes with pertussis toxin. The increases in I(Ca) induced by forskolin (10-6 M), 3-isobutyl-1-methylxanthine (10-4 M), and 8-bromo- cyclic AMP (3x10-4 M) were also suppressed by ET-1 (10-8 M). In summary, ET-1 has a differential effect on I(Ca) in the absence and in the presence of ISO: ET-1 has a feeble biphasic action on the baseline I(Ca) and, in addition, it elicits a pronounced anti-adrenergic effect on the ISO-induced increase in I(Ca). Pertussis toxin-sensitive G protein is responsible for the anti-adrenergic effect of ET-1 on I(Ca), but the anti-adrenergic effect of ET-1 may involve also the regulation at the level of signaling process beyond the cyclic AMP generation. Antiadrenergic effect of ET-1 on I(Ca) is mainly due to activation of ETA receptors but ET(B) receptors are also involved partially in the anti-adrenergic effect of ET-1 on I(Ca) in rabbit ventricular myocytes.