Vascular effects of halothane and isoflurane: cGMP dependent and independent actions

This study investigated the effects of halothane and isoflurane on cGMP-dependent and independent regulation of vascular contraction of the isolated rat aorta and on NO-stimulated soluble guanylate cyclase (sGC) isolated from the perfused rat liver. For the studies of the aorta, isometric tension of isolated rings, with and without, endothelium was recorded and cGMP content measured. ACh was used to initiate endothelial-dependent relaxation of norepinephrine (NE)-contracted rings while NO was used to directly stimulate isolated aortic ring sGC which catalyzes the isolated aortic ring formation of cGMP. Both halothane and isoflurane interfered with ACh and NO relaxations and with NO-stimulated increases in cGMP. Halothane was more potent, having significant attenuating effects at 0.34 mM (1 MAC) and 0.72 mM (2 MAC) while isoflurane had effects only at 0.53 mM (2 MAC). For the isolated sGC studies, a soluble liver fraction was prepared from perfused rat livers. In the absence of NO stimulation, neither halothane nor isoflurane modified the activity of the sGC. However, during NO-stimulation halothane produced significant, concentration-dependent, inhibition of sGC activity over a wide range of NO concentrations. Isoflurane also inhibited sGC activity, but to a lesser extent than halothane. The mechanism whereby the anesthetics could interfere with sGC from liver and blood vessels is unknown. It could result from anesthetic interaction at hydrophobic sites that may exist in GC. However, the results of both the aorta and liver sGC enzyme studies support the suggestion that these anesthetics can compete with NO for its binding site on the ferrous heme of sGC, with chemical structural differences accounting for the potency variations. Both anesthetics also had cGMP independent effects, causing concentration dependent relaxations of NE-contracted vessels without endothelium. Isoflurane was about 5 times more effective at 1 MAC than halothane. Therefore, the net effects of these anesthetics involve the sum of two opposite effects on tension of vessels with intact endothelium: 1) interference with NO-stimulated cGMP relaxation and 2) direct stimulation of relaxation (not dependent on changes in cGMP).