Vascular effects of halothane and isoflurane: cGMP dependent and independent actions
Abstract
This study investigated the effects of halothane and isoflurane on eGMP-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).
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Effect of two anaesthetic regimens on airway nitric oxide production in horses
2001, British Journal of AnaesthesiaCitation Excerpt :Therefore, the present study supports the hypothesis that halothane causes in vivo suppression of NOS. However, failure of exogenous inhaled NO to reduce pulmonary artery pressure during halothane anaesthesia implies either that NO is not important for the regulation of pulmonary vascular tone under these conditions or that there is inhibition of the action of NO, possibly through interference with cGMP-mediated relaxation.10 In conclusion, it is possible that the increased mean pulmonary artery pressure in these horses during halothane anaesthesia may be linked to the observed differences in NO production.
There is evidence that halothane inhibits nitric oxide synthase in vitro, but the effect of intravenous anaesthetic agents is less clear. This study was undertaken to compare the rate of exhaled nitric oxide production (V²NO) in spontaneously breathing horses anaesthetized with halothane or an intravenous regimen. Seven adult horses were studied twice in random order. After premedication with romifidine 100 μg kg−1, anaesthesia was induced with ketamine 2.2 mg kg−1 and maintained with halothane in oxygen (HA) or by an intravenous infusion of ketamine, guaiphenesin and romifidine (IV). Inhaled and exhaled nitric oxide (NO) concentrations, respiratory minute ventilation (V²E), pulmonary artery pressure (PPA), fractional inspired oxygen concentration (FiO2), end-tidal carbon dioxide concentration (E′CO2), cardiac output (Q²) and partial pressures of oxygen and carbon dioxide in arterial blood (PaO2, PaCO2) were measured. Exhaled nitric oxide production rate was significantly lower (40 min, P<0.01; 60 min, P<0.02) during HA [40 min, 1.4 (sd 1.4) pmol l−1 kg−1min−1; 60 min, 0.7 (0.7) pmol l−1 kg−1min−1] than during IV [40 min, 9.3 (9.9) pmol l−1 kg−1 min−1; 60 min, 12.5 (13.3) pmol l−1 kg−1 min−1). Mean pulmonary artery pressure was significantly higher (40 min, P<0.01; 60 min, P<0.001) during HA [40 min, 5.9 (1.1) kPa; 60 min, 5.9 (0.9) kPa] compared with IV (40 min, 4.4 (0.4) kPa; 60 min, 4.4 (0.5) kPa]. NO is reduced in the exhalate of horses anaesthetized with halothane compared with an intravenous regimen. It is suggested that increased mean pulmonary artery pressure during halothane anaesthesia may be linked to the differences in NO production.
The nitric oxide-cyclic 3',5'-guanosine monophosphate signal transduction pathway in the mechanism of action of general anesthetics
1998, Toxicology Letters(1) The nitric oxide–cyclic 3′,5′-guanosine monophosphate (NO–cGMP) system is a major signaling transduction pathway implicated in a wide range of physiologic and pathophysiologic functions of the cardiovascular, respiratory, gastrointestinal, nervous or immune systems. (2) Evidence is provided that, at anesthetic concentrations, volatile and intravenous anesthetics interact with the NO–cGMP system. They have been shown to produce a decrease in cGMP in neuronal and vascular tissue. (3) Inhibition of NO synthesis produces a dose-dependent reversible decrease in the minimum anesthetic requirement and in the ED50 for the loss of righting reflex induced by general anesthetics. Volatile anesthetics also inhibit the NO-mediated relaxation in many vascular beds. (4) The selective alpha-2 adrenergic agonist, dexmedetomidine, which has potent sedative/hypnotic, anesthetic sparing and analgesic properties, produces a dose-dependent, reversible decrease in cGMP in mouse cerebellum at concentrations that decrease the anesthetic requirement of volatile anesthetics or induce a loss of righting reflex, an effect eliminated when NO synthase is inhibited. The site and mechanism by which the anesthetics interact with the NO–cGMP system is not yet clear and may vary with the anesthetic.
Effects of halothane and urethane-chloralose anaesthesia on the pressor and cerebrovascular responses to 7-nitroindazole, an inhibitor of nitric oxide synthase
1998, Pharmacological ResearchWe examined the effect of 7–nitroindazole (7NI), a reportedly relatively specific inhibitor of the neuronal isoform of nitric oxide synthase (nNOS), on mean arterial blood pressure and on cerebral blood flow in rats under three different types of anaesthesia: urethane–chloralose, halothane, or urethane preceded by induction of anaesthesia with halothane. In rats under urethane–chloralose anaesthesia, 7NI induced an increase in mean systemic arterial blood pressure. In contrast, halothane used for induction and maintenance of anaesthesia eliminated the 7NI–induced systemic pressor effect, while halothane used only for induction of anaesthesia greatly attenuated the 7NI–induced systemic pressor effect. Cerebral blood flow, as measured by Laser Doppler flowmetry, decreased significantly to 85–72% of baseline within 5–10 min after i.p. 7NI injection regardless of the type of anaesthesia. Blockade of the systemic pressor effect of 7NI by halothane but not of the reduction in cerebral blood flow produced by 7NI is consistent with prior evidence that: (1) the cerebral vasculature and the peripheral vasculature differ in the isoforms of NOS involved in maintaining vascular tone, with nNOS more important in the former and endothelial NOS (eNOS) in the latter; and (2) halothane interferes with eNOS–mediated vascular tone but not nNOS–mediated control of cerebral blood flow. The fact that 7NI yields a pressor effect that can be attenuated by halothane, as also true for isoform–non–selective NOS inhibitors, raises the possibility that 7NI may to some extent inhibit endothelial NO formation.
Effect of isoflurane on the β-adrenergic and endothelium-dependent relaxation of pig cerebral microvessels after cardiopulmonary bypass
1998, Journal of Stroke and Cerebrovascular DiseasesWe examined the direct vasomotor effect of isoflurane as well as its effect on endothelium-dependent and β-adrenergic vasodilation of cerebral microcirculation following either normothermic cardiopulmonary bypass (CPB) or profoundly hypothermic CPB with circulatory arrest. Pigs were placed on CPB; the systemic temperature was either maintained at 37°C or lowered to 15°C with 60 minutes of circulatory arrest. After 2 hours of CPB, the animals were separated from CPB; 15 minutes later the brain was quickly harvested in cold Krebs solution. Control animals were not instrumented and their brains were similarly harvested. Arteries of ≈100 μm were dissected and changes in diameter monitored by in vitro videomicroscopy. Following preconstriction with the thromboxane analogue U46619 1 μmol/L, percent relaxation to the endothelium-dependent dilator adenosine diphosphate (ADP) 10−9 to 10−4 mol/L, the endothelium-independent dilator sodium nitroprusside (SNP) 10−9 to 10−4 mol/L, or the β-adrenergic agonist isoproterenol 10−12 to 10−4 mol/L was measured either in the presence or absence of isoflurane 2%. Additionally, with or without preconstriction with U46619 1 μmol/L, vessel diameter changes were monitored with increasing concentrations of isoflurane 0–3%. Dose-response curves were compared by two-way analysis of variance. Vasodilation to ADP or isoproterenol, but not SNP, was attenuated after normothermic CPB (N-CPB) or profoundly hypothermic CPB (PH-CPB). Although isoflurane attenuated vasodilation of control vessels to ADP or isoproterenol, isoflurane did not further attenuate vasodilation to ADP or isoproterenol after N-CPB or PH-CPB. The direct vasomotor effect of isoflurane depended on the preexisting tone of the vessels, constricting vessels without preconstriction and dilating them after preconstriction. These findings may have implications on the incidence of neuropsychological dysfunction after CPB and use of isoflurane.
Halothane attenuates nitric oxide relaxation of rat aortas by competition for the nitric oxide receptor site on soluble guanylyl cyclase
1998, European Journal of PharmacologyEndothelial 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 (5×10−9–3×10−8 M and 5×10−9–3×10−5 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.
Voltage-clamp studies of gap junctions between uterine muscle cells during term and preterm labor
1996, Biophysical JournalGap junctions between myometrial cells increase dramatically during the final stages of pregnancy. To study the functional consequences, we have applied the double-whole-cell voltage-clamp technique to freshly isolated pairs of cells from rat circular and longitudinal myometrium. Junctional conductance was greater between circular muscle-cell pairs from rats delivering either at term (32 +/- 16 nS, mean +/- SD, n = 128) or preterm (26 +/- 17 nS, n = 33) compared with normal preterm (4.7 +/- 7.6 nS, n = 114) and postpartum (6.5 +/- 10 nS, n = 16); cell pairs from the longitudinal layer showed similar differences. The macroscopic gap junction currents decayed slowly from an instantaneous, constant-conductance level to a steady-state level described by quasisymmetrical Boltzmann functions of transjunctional voltage. In half of circular-layer cell pairs, the voltage dependence of myometrial gap junction conductance is more apparent at smaller transjunctional voltages (< 30 mV) than for other tissues expressing mainly connexin-43. This unusual degree of voltage dependence, although slow, operates over time intervals that are physiologically relevant for uterine muscle. Using weakly coupled pairs, we observed two unitary conductance states: 85 pS (85–90% of events) and 25 pS. These measurements of junctional conductance support the hypothesis that heightened electrical coupling between the smooth muscle cells of the uterine wall emerges late in pregnancy, in preparation for the massive, coordinate contractions of labor.