Halothane attenuates nitric oxide relaxation of rat aortas by competition for the nitric oxide receptor site on soluble guanylyl cyclase.

Endothelial cells play an important role in the regulation of vascular activity through the release of endothelium derived relaxing factor (EDRF) now believed to be nitric oxide (NO). NO and the NO donor drug nitroglycerin relax vascular smooth muscle by stimulating soluble guanylyl cyclase leading to elevation of intracellular levels of cyclic guanosine 3',5'-monophosphate (cGMP). Halothane has been shown to inhibit the action of NO on blood vessels. This study was designed to further investigate the mechanisms by which halothane attenuates NO-induced vascular relaxations. This was done by examining the effects of halothane on nitroglycerin and NO-induced relaxations in the presence and absence of the inhibitors of soluble guanylyl cyclase, methylene blue and 6-anilino-5,8-quinolinedione (LY 83583). Thoracic aortas from anesthetized male Sprague-Dawley rats were excised and cut into rings and the endothelium was removed. The aortic rings were suspended in organ baths containing Krebs solution and equilibrated at their optimal passive tension. When a stable plateau of contraction was produced by EC60 concentrations of norepinephrine, increasing concentrations of nitroglycerin or NO were added to the baths to relax the rings. This contraction-relaxation procedure was repeated three or four times. In some baths halothane was administered by a calibrated vaporizer 10 min before beginning the second procedure. Either methylene blue or LY 83583 was added to the baths 20 min before the third procedure. The combination of halothane, methylene blue or LY 83583 was added before the fourth procedure. Halothane, methylene blue or LY 83583 significantly inhibited nitroglycerin-induced relaxation individually. Halothane and LY 83583 also significantly inhibited NO-induced relaxations (5 x 10(-9)-3 x 10(-8) M and 5 x 10(-9)-3 x 10(-5) M, respectively) individually. The combination of halothane and methylene blue or halothane and LY 83583 significantly inhibited nitroglycerin-induced relaxation, also, the combination of halothane and LY 83583 significantly inhibited NO-induced relaxations. Halothane, methylene blue and LY 83583 treatment led to rightward shift in the concentration-effect curves. Halothane, in combination with methylene blue or LY 83583, produced inhibition equivalent to the sum of their individual effects. The present study demonstrates that the halothane, methylene blue and LY 83583 attenuate nitroglycerin and NO-induced relaxations of endothelium-denuded rat aortic rings. This suggests that halothane, methylene blue and LY 83583 may act through competitive antagonism at a common site of action on soluble guanylyl cyclase in the EDRF/NO relaxation pathway.