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

Effect of isoflurane on the beta-adrenergic and endothelium-dependent relaxation of pig cerebral microvessels after cardiopulmonary bypass

We examined the direct vasomotor effect of isoflurane as well as its effect on endothelium-dependent and beta-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 degrees C or lowered to 15 degrees 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 approximately 100 microm were dissected and changes in diameter monitored by in vitro videomicroscopy. Following preconstriction with the thromboxane analogue U46619 1 micromol/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 beta-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 micromol/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.

Isoflurane Inhalation After Circulatory Arrest Protects Against Warm Ischemia Reperfusion Injury of the Lungs

BACKGROUND

Non-heart-beating donors are expected to ameliorate shortages of donors for organ transplantation. The issue of preventing warm ischemic injury after circulatory arrest must be investigated. In the current study, we investigated whether isoflurane inhalation during warm ischemia could attenuate ischemia reperfusion injury (IRI) of the lung.

METHODS

An isolated perfused rat lung model was used. The rats were allocated into four groups: the no ischemia group; the ischemia-1 minimum alveolar concentration (MAC) iso group (ventilation with air and 1.38% isoflurane); the Ischemia-3MAC iso group (ventilation with air and 4.2% isoflurane); and the Ischemia-no treatment group (ventilation with only air). Lungs were subjected to 50 min of ischemia at 37 degrees C. Physiological lung functions were measured after reperfusion in experiment one. Mitochondrial control ratio (RCR), cytochrome-c release from mitochondria, and caspase activities just after warm ischemia were measured in experiment two.

RESULTS

Pulmonary functions in the Ischemia-1MAC iso group were significantly greater than those in the Ischemia-no treatment group for experiment one. There were no dose-dependent effects between 1MAC and 3MAC isoflurane. In experiment two, RCR in the Ischemia-1MAC iso group was significantly greater than that in the Ischemia-no treatment group. Cytochrome-c release and caspase-9 activity in the Ischemia-1MAC iso group were significantly decreased compared to those in the Ischemia-no treatment group.

CONCLUSIONS

Isoflurane inhalation attenuates warm IRI with the protection of mitochondria. Our results suggest that isoflurane inhalation after circulatory arrest can be a simple and effective method to protect the lung against warm ischemia.

Nitric oxide in B6 mouse and nitric oxide-sensitive soluble guanylate cyclase in cat modulate acetylcholine release in pontine reticular formation

ACh regulates arousal, and the present study was designed to provide insight into the neurochemical mechanisms modulating ACh release in the pontine reticular formation. Nitric oxide (NO)-releasing beads microinjected into the pontine reticular formation of C57BL/6J (B6) mice significantly (P < 0.0001) increased ACh release. Microdialysis delivery of the NO donor N-ethyl-2-(1-ethyl-2-hydroxy-2-nitrosohydrazino)-ethanamine (NOC-12) to the mouse pontine reticular formation also caused a concentration-dependent increase in ACh release (P < 0.001). These are the first neurochemical data showing that ACh release in the pontine reticular formation of the B6 mouse is modulated by NO. The signal transduction cascade through which NO modulates ACh release in the pontine reticular formation has not previously been characterized. Therefore, an additional series of studies quantified the effects of a soluble guanylate cyclase (sGC) inhibitor, 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), on ACh release in the cat medial pontine reticular formation. During naturally occurring states of sleep and wakefulness, but not anesthesia, ODQ caused a significant (P < 0.001) decrease in ACh release. These results show for the first time that NO modulates ACh in the medial pontine reticular formation of the cat via an NO-sensitive sGC signal transduction cascade. Isoflurane and halothane anesthesia have been shown to decrease ACh release in the medial pontine reticular formation. The finding that ODQ did not alter ACh release during isoflurane or halothane anesthesia demonstrates that these anesthetics disrupt the NO-sensitive sGC-cGMP pathway. Considered together, results from the mouse and cat indicate that NO modulates ACh release in arousal-promoting regions of the pontine reticular formation via an NO-sensitive sGC-cGMP pathway.

Vascular Actions of Calcitonin Gene-Related Peptide and Adrenomedullin

This review summarizes the receptor-mediated vascular activities of calcitonin gene-related peptide (CGRP) and the structurally related peptide adrenomedullin (AM). CGRP is a 37-amino acid neuropeptide, primarily released from sensory nerves, whilst AM is produced by stimulated vascular cells, and amylin is secreted from the pancreas. They share vasodilator activity, albeit to varying extents depending on species and tissue. In particular, CGRP has potent activity in the cerebral circulation, which is possibly relevant to the pathology of migraine, whilst vascular sources of AM contribute to dysfunction in cardiovascular disease. Both peptides exhibit potent activity in microvascular beds. All three peptides can act on a family of CGRP receptors that consist of calcitonin receptor-like receptor (CL) linked to one of three receptor activity-modifying proteins (RAMPs) that are essential for functional activity. The association of CL with RAMP1 produces a CGRP receptor, with RAMP2 an AM receptor and with RAMP3 a CGRP/AM receptor. Evidence for the selective activity of the first nonpeptide CGRP antagonist BIBN4096BS for the CGRP receptor is presented. The cardiovascular activity of these peptides in a range of species and in human clinical conditions is detailed, and potential therapeutic applications based on use of antagonists and gene targeting of agonists are discussed.

Small-dose nitric oxide improves oxygenation during one-lung ventilation: an experimental study

Inhaled nitric oxide (NO) at 20 or 40 ppm does not improve arterial oxygenation during one-lung ventilation (OLV). The authors hypothesized that NO at smaller concentrations might improve oxygenation. Twelve piglets weighing 26 to 32 kg were studied. When PaO(2) had reached a plateau during OLV, NO at doses of 4, 8, 16, and 32 ppm were randomly administered for 30 min. Hemodynamic data were determined by invasive monitoring. Blood gas analysis and, in six animals, ventilation-perfusion analysis by the multiple inert gas elimination technique were used to characterize pulmonary gas exchange. NO at 4, 8, 16, and 32 ppm improved PaO(2) during OLV. NO at 4 ppm had a more intense effect on arterial oxygenation than doses of 8, 16, and 32 ppm (DeltaPaO(2), 42 +/- 35 mm Hg versus 22 +/- 20 mm Hg, 13 +/- 18 mm Hg, and 15 +/- 16 mm Hg; P < 0.05). NO at 4 ppm reduced intrapulmonary shunt flow, whereas a larger concentration exhibited no statistically significant effect. The authors conclude that NO improves arterial oxygenation more effectively at smaller doses than at larger doses. This dose-dependent effect remains to be confirmed in acute hypoxemia during OLV.

IMPLICATIONS

Inhaled nitric oxide at 4 ppm improves arterial oxygenation during one-lung ventilation to a greater extent than larger doses, and this effect is caused by a reduction in intrapulmonary shunt.

ADP-induced pial arteriolar dilation in ovariectomized rats involves gap junctional communication

It was previously shown that, despite the loss of nitric oxide (NO) dependence, ADP-induced pial arteriolar dilation was not attenuated in estrogen-depleted [i.e., ovariectomized (Ovx)] rats. Additional evidence suggested that the NO was replaced by an endothelium-dependent hyperpolarizing factor (EDHF)-like mechanism. To further characterize the nascent EDHF role in Ovx females, the current study was undertaken to test whether, in Ovx rats, ADP-induced pial arteriolar dilation retained its endothelial dependence and whether gap junctions are involved in that response. A closed cranial window and intravital microscopy system was used to monitor pial arteriolar diameter changes in anesthetized rats. The endothelial portion of the ADP-induced dilation was evaluated using light dye endothelial injury (L/D). The study was organized around three experimental approaches. First, the responses of pial arterioles to ADP before and after L/D exposure in intact and Ovx female rats were tested. L/D reduced the ADP response by 50-70% in both groups, thereby indicating that the endothelium dependence of ADP-induced vasodilation is not altered by chronic estrogen depletion. Second, the NO synthase inhibitor N(omega)-nitro-L-arginine (L-NNA) and the prostanoid synthesis inhibitor indomethacin (Indo) were coapplied. In intact females, L-NNA-Indo attenuated the response to ADP by 50%, with no further changes upon the addition of L/D. On the other hand, L-NNA-Indo did not affect ADP reactivity in Ovx rats, but subsequent L/D exposure reduced the ADP response by >50%. The NO-prostanoid-independent, but endothelium-dependent, nature of the response in Ovx females is a hallmark of EDHF participation. Third, gap junctional inhibition strategies were applied. A selective inhibitor of gap junctional function, Gap 27, did not affect ADP reactivity in intact females but reduced the the ADP response by 50% in Ovx females. A similar result was obtained following application of a connexin43 antisense oligonucleotide. These findings suggest that the nascent EDHF dependency of ADP-induced pial arteriolar dilation in Ovx females involves connexin43-related gap junctional communication.

Effect of two anaesthetic regimens on airway nitric oxide production in horses

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 (VNO) 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 microg 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 (VE), pulmonary artery pressure (PPA), fractional inspired oxygen concentration (FIO2), end-tidal carbon dioxide concentration (E'CO2), cardiac output (Q) 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(-1) min(-1); 60 min, 0.7 (0.7) pmol l(-1) kg(-1) min(-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.

Does halothane or isoflurane affect hypoxic and post-hypoxic vascular response in rabbit aorta?

BACKGROUND

Halothane and isoflurane affect differently endothelium-dependent and -independent vasorelaxation at 95% O2. In addition, hypoxic vascular response might involve endothelium-dependent and -independent mechanisms. Therefore, we investigated, in rabbit aortic rings, 1) the influence of halothane and isoflurane on vasodilation at 95% O2 and on hypoxic-induced vasorelaxation at 0% O2 and 2) the influence of halothane and isoflurane on endothelium-dependent and -independent post-hypoxic vascular response.

METHODS

Endothelium-intact and endothelium-denuded rabbit aortic rings were used. Phenylephrine precontracted rings were exposed, at 95% O2, to acetylcholine (ACh, 10(-9) to 10(-4) M) or sodium nitroprusside (SNP, 10(-9) to 10(-4) M) in the presence or absence of anaesthetic at 1 or 2 MAC. Precontracted rings were also exposed to an acute reduction in O2 from 95% to 0% followed by an acute reoxygenation with 95% O2 in the absence or presence of anaesthetic at 1 or 2 MAC.

RESULTS

At 95% O2, halothane decreased endothelium-dependent relaxation to ACh, while endothelium-independent relaxation to SNP was decreased only at 2 MAC. Isoflurane did not modify ACh- or SNP-induced relaxation. At 0% O2, neither halothane nor isoflurane altered the hypoxic vascular relaxation. Post-hypoxic response was not changed either.

CONCLUSION

Our results indicate that halothane and isoflurane do not alter vascular hypoxic response in conductance arteries.

Methylene blue, a soluble guanylyl cyclase inhibitor, reduces the sevoflurane minimum alveolar anesthetic concentration and decreases the brain cyclic guanosine monophosphate content in rats

The nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) signal pathway plays an important role in anesthetic and analgesic effects. We sought to determine the involvement of inhibition of soluble guanylyl cyclase (sGC) in the anesthetic mechanism and site of action of volatile anesthetics. We examined the effect of intracerebroventricular (ICV) administration of methylene blue (MB), a sGC inhibitor, on the minimum alveolar anesthetic concentration (MAC) of sevoflurane and the brain cGMP content in rats in vivo. We also investigated the effect of sevoflurane on NO-stimulated sGC activity in vitro. The rats were divided into three groups. After the ICV administration of MB, sevoflurane MAC and brain cGMP contents were measured in the first group and the second group, respectively. In the third group, brain cGMP contents were determined after sevoflurane anesthesia without the ICV administration of MB to examine the direct effect of sevoflurane on brain cGMP contents. MB significantly decreased sevoflurane MAC and brain cGMP content in a dose-dependent manner. Sevoflurane itself also dose-dependently decreased cGMP contents in brain in vivo and inhibited the NO-stimulated sGC activity in vitro. These results suggest that the inhibition of the NO-cGMP signal pathway at the sGC level could be involved in anesthetic or analgesic effects, and the inhibitory effect of sevoflurane on sGC would be one of the sites of action of this anesthetic.

IMPLICATIONS

Because the nitric oxide-cyclic guanosine monophosphate signal pathway mediates nociception and the site of action of halogenated volatile anesthetics in uncertain, we examined the possible involvement of inhibition of soluble guanylyl cyclase in the anesthetic mechanism. The inhibitory effect of sevoflurane on soluble guanylyl cyclase could be one of sites of this anesthetic.

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.

Suppression of acetylcholine-induced relaxation by local anesthetics and vascular NO-cyclic GMP system

BACKGROUND

Local anesthetics have been demonstrated to attenuate acetylcholine-induced relaxation of vascular smooth muscle, but the mechanism responsible has not been elucidated. The present study was undertaken to ascertain whether this effect of local anesthetics is due to suppression of the vascular nitric oxide (NO)-cyclic GMP (cGMP) system.

METHODS

Isolated rat aortae were cut into helical strips and mounted in bathing solution to measure isometric tension changes. They were precontracted with phenylephrine (0.3 microM) then exposed to cumulative concentrations of relaxants including acetylcholine, sodium nitroprusside (SNP) and papaverine, in the absence or presence of local anesthetics. Aortae for cGMP measurements were cut longitudinally into pairs of strips and bathed in the solution without tension. In the absence or presence of anesthetics, they were stimulated with acetylcholine or SNP, and the cGMP content of each strip was radioimmunoassayed.

RESULTS

Acetylcholine-induced, endothelium-dependent relaxation of phenylephrine-precontracted aortae was attenuated by lidocaine (30-300 microM), tetracaine (10-30 microM), bupivacaine (10-100 microM) and ropivacaine (30-100 microM). SNP-induced relaxation was attenuated by lidocaine (300 microM), tetracaine (30 microM), bupivacaine (10-100 microM) and ropivacaine (30-100 microM). Papaverine-induced relaxation was attenuated by lidocaine (300 microM), bupivacaine (30-100 microM) and ropivacaine (30-100 microM), and augmented by tetracaine (30 microM). Cyclic GMP levels in acetylcholine-stimulated aortae were reduced significantly by lidocaine (300 microM), tetracaine (100 microM) and bupivacaine (300 microM) treatment, but not by ropivacaine (300 microM). SNP-stimulated cGMP levels were reduced by tetracaine (100 microM) but not by any other anesthetics at the concentrations tested.

CONCLUSION

We conclude that lidocaine, tetracaine and bupivacaine suppress acetylcholine-stimulated formation of cGMP. However, the attenuation of acetylcholine-induced relaxation by local anesthetics is not totally ascribable to reduced cGMP levels.

Isoflurane and halothane attenuate endothelium-dependent vasodilation in rat coronary microvessels

Volatile anesthetics attenuate endothelium-dependent vasodilation but the mechanism of attenuation remains controversial. The present study examines the mechanism of isoflurane- and halothane-mediated attenuation of endothelium-dependent vasodilation in Wistar rat coronary microvessels of about 100 microns internal diameter. The vessels were studied in vitro in a pressurized (40 mm Hg), no-flow state using video microscopy. After preconstriction of the vessels with the thromboxane analog U46619 1 microM, concentration response curves to acetylcholine (ACh), the calcium ionophore A23187, sodium nitroprusside (SNP), or the stable cyclic guanosine monophosphate (cGMP) analog 8-bromo-cGMP (Br-cGMP) were obtained in the presence of 0% (control), 1% or 2% isoflurane, or 1% or 2% halothane. Isoflurane 1% and 2% significantly attenuated vasodilation to ACh and A23187. Isoflurane 2%, but not 1%, attenuated vasodilation to SNP. Vasodilation to Br-cGMP was not affected by isoflurane. Halothane attenuated vasodilation to ACh, but had no effect on vasodilation to A23187, SNP, or Br-cGMP. We conclude that isoflurane attenuates endothelium-dependent vasodilation by impairing at least two distinct steps in the nitric oxide (NO)-cGMP pathway, the first being between endothelial increase of calcium and smooth muscle guanylate cyclase and the second being inhibition of soluble guanylate cyclase activity. These two steps appear to have different sensitivities to the effect of isoflurane. Halothane has an effect at the endothelial receptor level, but not any distal steps in the NO-cGMP pathway.

Voltage-clamp studies of gap junctions between uterine muscle cells during term and preterm labor

Gap 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.

Carbon monoxide: an endogenous modulator of the nitric oxide-cyclic GMP signaling system

Carbon monoxide (CO) is an activator of soluble guanylyl cyclase and is implicated as a neuronal messenger. CO production, nitric oxide synthase (NOS) activity, and guanosine 3',5'-monophosphate (cGMP) levels were quantitated in cerebellar granule cell cultures. Metabolic labeling experiments enabled the direct measurement of neuronal CO production in vitro. CO production is significant, and peaked during early stages of culture. NOS activity and cGMP levels synchronously increased as cells matured. Whereas inhibition of NOS depleted cGMP in mature cultures, inhibitors of CO production potentiated the nitric oxide (NO)-mediated cGMP increase. Exogenous CO at similar concentrations to endogenous levels blocked the NO-mediated cGMP increase. These results directly demonstrate that endogenous neuronal CO production is high and indicate that while NO is the major regulator of cGMP in these neurons, CO may modulate the NO-cGMP signaling system.