The effect of nitric oxide synthase blockade on responses to morphine in rat aortic rings

Summary 1 It has been suggested that opioids may play an indirect role in the regulation of the peripheral circulation through the control of nitric oxide (NO) release in vascular tissue. The current study was undertaken to investigate the effect of nitric oxide synthase (NOS) blockade on responses to morphine in phenylephrine (PE)‐ or KCl‐precontracted rat aortic rings. 2 Morphine (3 × 10−8−3 × 10−5 m) administration did not cause any significant effect on basal tonus of endothelium‐intact or endothelium‐denuded preparations. Morphine produced concentration‐dependent relaxation responses in endothelium‐intact as well as in endothelium‐denuded rat aortic rings precontracted by PE or KCl. Removal of endothelium did not significantly alter the relaxation responses to morphine. 3 The relaxant responses to morphine were significantly and partially inhibited by pretreatment of tissues with naloxone (NAL, 3 × 10−5 m) for 5 min. The inhibitory effect of NAL on relaxant responses to morphine in PE‐ or KCl‐precontracted rings did not differ significantly between endothelium‐intact and endothelium‐denuded preparations. 4 Incubation of endothelium‐intact or endothelium‐denuded rat aortic rings with NOS inhibitor, Nω‐nitro‐l‐arginine methyl ester (l‐NAME, 10−4 m) for 20 min did not cause a significant inhibition on relaxation responses to morphine. 5 These findings confirmed the presence of opiate receptors in rat thoracic aorta, but suggested that mechanisms other than NO release play a role in the relaxant effect of morphine on rat aortic rings.