Effects of Androctonus crassicauda scorpion venom on endothelium-dependent and -independent vascular responses of rabbit aorta.

The effects of Androctonus crassicauda scorpion venom on acetycholine (ACh)-induced relaxations and contractions of rabbit thoracic aorta were studied. Endothelium-dependent relaxations induced by ACh in phenylephrine-precontracted arteries were enhanced by the scorpion venom. ACh-induced contractions in endothelium-intact open aortic rings were less than those obtained in denuded preparations (n = 6, P < 0.05, ANOVA). Venom (5, 10 and 30 micrograms/ml) potentiated ACh-induced contractions in intact and denuded segments. In the denuded segments, this potentiation was inhibited by indomethacin (10 microM). Thromboxane synthase inhibitor, BW 149H (100 microM) and thromboxane A2 (TXA2) receptor antagonist, R 68070 (10 microM) partly inhibited venom-induced potentiation. NG-nitro-L-arginine (100 microM) increased venom-potentiated ACh responses in intact arterial segments. Venom increased the basal tone by 25-35% at 30 micrograms/ml. These results suggest that A. crassicauda venom may release a relaxing factor from endothelium and contracting factor from the smooth muscle of rabbit isolated thoracic aorta. The contracting factor may be a cyclooxygenase-like product, most likely TXA2. The increase in basal tone by 30 micrograms/ml venom was inhibited by phentolamine (10 microM) and guanethidine (10 microM), indicating a venom-induced release of a neurotransmitter from adrenergic nerve endings.