Loss of contractile activity of endothelin‐1 induced by electrical field stimulation‐generated free radicals

Electrical field stimulation (EFS; 10 V, 10 Hz, 2 ms) of porcine coronary artery strips precontracted with 10 nm endothelin‐1 (ET‐1) for 5 min caused a biphasic response, consisting of a slight contraction during EFS and a marked and irreversible relaxation just after EFS. This irreversible relaxation after EFS has never been investigated. In the present study, we have investigated the mechanism of the relaxation after EFS. The EFS‐induced response was not affected by the presence or absence of endothelium and was insensitive to 10 μm tetrodotoxin (TTX). In the presence of free radical scavengers (40 u ml⁻¹ superoxide dismutase (SOD), 1200 u ml⁻¹ catalase or 80 mm d‐mannitol), the relaxation after EFS was significantly inhibited. Moreover, relaxation after EFS was not observed in porcine coronary artery strips precontracted with 20 mm KCl. In a cascade experiment, EFS of Krebs‐Ringer solution containing 10 nm ET‐1 induced marked suppression of the contractile activity of ET‐1 in porcine coronary artery strips, which was in accord with the observed decrease in release of immunoreactive ET‐1 (ir‐ET‐1). This effect of EFS was significantly inhibited by each of the free radical scavengers, 3 mm vitamin C, 40 u ml⁻¹ SOD, 1200 u ml⁻¹ catalase and 80 mm d‐mannitol. The exchange of 95% O₂/5% CO₂ gas for 95% N₂/5% CO₂ gas significantly inhibited the EFS‐induced decrease in release of ir‐ET‐1. Neither superoxide anions generated by xanthine (10 μm) plus xanthine oxidase (0.1 u ml⁻¹) nor hydrogen peroxide (10 μm) exogenously added to Krebs‐Ringer solution containing 10 nm ET‐1 affected the level of ir‐ET‐1. Generation of hydroxyl radicals was detected in the EFS‐applied Krebs‐Ringer solution. The EFS‐induced generation of hydroxyl radicals was dependent on the period of stimulation and O₂‐bubbling, and significant generation of hydroxyl radicals was detectable with stimulation of over 5 min. Moreover, hydroxyl radicals generated in 50 mm NaCl solution containing 10 nm ET‐1 by H₂O₂ plus Fe²⁺, i.e. the Fenton reaction, significantly decreased the level of ir‐ET‐1. These findings suggest that oxygen‐derived hydroxyl radicals generated by EFS of porcine coronary artery strips inactivate ET‐1, probably by structural modification. Thus, porcine coronary artery strips precontracted with ET‐1 are potently relaxed by EFS. 1994 British Pharmacological Society