Endothelin-1 inhibits L-type Ca2+ current enhanced by isoprenaline in rat atrial myocytes

Endothelin-1 (ET-1) was shown to exert direct cardiac effects by complex signaling pathways and to interact with neurotransmitter regulation of cardiac activity. The effect of ET-1 was investigated on the beta-adrenergic stimulation of cardiac L-type Ca2+ current (I-CaL) on isolated rat atrial myocytes by using the patch-clamp technique. ET-1 (5x10(-8) M) reversed the increase in I-CaL induced by isoprenaline (10(-6) M) but had no effect on basal I-CaL and on (-) Bay K 8644-increased I-CaL(10(-6) M); so ET-1 might exert an effect only when the Ca2+ channels are phosphorylated. The antiadrenergic action of ET-1, blocked by BQ-123 (10(-6) M) and unaffected by IRL 1038 (3.5x10(-8) M) should be mediated by ET-A receptors. The inhibitory action of ET-1 was still observed when I-CaL was previously increased by forskolin (3x10(-6) M), 8-bromo-cyclic adenosine monophosphate(8-Br-cAMP; 200 mu M), or cAMP (100 mu M) in presence of isobutyl methyl xanthine (IBMX; 10(-6) M), suggesting that the antiadrenergic action of ET-1 on I-CaL was exerted independent of the cAMP-dependent phosphorylation pathway. ET-1 is known to be an activator of phosphoinositide hydrolysis, resulting in an increased production of IP3 and diacylglycerol (DAG). A Ca2+-dependent inhibition of I-CaL. consequently to an elevation of the intracellular Ca2+ pool via IP3 might be excluded in the action of ET-1, because of the presence of EGTA in the intrapipette medium. ET-1 reversed the isoprenaline-induced increase in I-CaL in the presence of protein kinase C inhibitor [PKC(19-31); 100 mu M), making unlikely the involvement of a DAG-dependent activation of PKC. Therefore the antiadrenergic action of ET-1 might also be independent on the phosphoinositide pathway.