ENDOTHELIN-1 INHIBITS THE SLOW COMPONENT OF CARDIAC DELAYED RECTIFIERK-TOXIN-SENSITIVE MECHANISM( CURRENTS VIA A PERTUSSIS)

Endothelin-1 (ET-1) is a 21-amino acid peptide hormone released from m yocardial and endothelial cells, whose receptors (both ETA and ETB) ar e expressed in the myocardium. We report here that ET-1 inhibits the c ardiac delayed rectifier K+ current (I-K) via a pertussis toxin (PTX)- sensitive mechanism. Ventricular myocytes enzymatically isolated from guinea pig hearts were voltage-clamped by the conventional whole-cell and nystatin-perforated patch technique (intrapipette and extrapipette K+ concentrations, 150 and 5.4 mmol/L, respectively) in the presence of nifedipine (2 mu mol/L). Amplitudes of tail and steady state (2-sec ond pulse) currents were measured as I-K. ET-1 suppressed the basal I- K by 20.9 +/- 2.3% in a concentration-dependent manner, with an IC50 o f 1.1 +/- 0.3 nmol/L (n = 19), although it did not suppress the basal I-K using the nystatin method. E-4031 (5 mu mol/L), a blocker of the r apid component of I-K (I-Kr), did not prevent the inhibitory action of ET-1. ET-1 reduced by 63.4 +/- 6.5% the slow component of I-K (I-Ks) that had been enhanced to approximate to 2-fold by isoproterenol (ISO, 20 nmol/L). The action was concentration dependent, with an IC50 of 0 .7 +/- 0.4 nmol/L, (n=22), and was also observed using the nystatin me thod. The effect of ET-1 appeared to be mediated by an ETA receptor, b ecause it was prevented by FR139317, an ETA-selective antagonist (1 mu mol/L, n=4), and sarafotoxin S6c, an ETB-selective agonist (100 nmol/ L, n=4), could not inhibit the ISO-enhanced I-K. ET-1 antagonized I-Ks enhanced by histamine (250 nmol/L, n=7) and forskolin (500 nmol/L, n= 7) but did not inhibit I-Ks enhanced by the internal application of cA MP (100 mu mol/L, n=6). Preincubation of myocytes with PTX (5 mu g/mL for >60 minutes at 36 degrees C) completely abolished the inhibitory a ction of ET-1 on the ISO-enhanced I-Ks (n = 4). Thus, nanomolar ET-1 i nhibits I-Ks via the ETA receptor/PTX-sensitive G protein/PKA pathway.