Effects of Isoflurane Anesthesia on Pulmonary Vascular Response to K (^ATP)⁺Channel Activation and Circulatory Hypotension in Chronically Instrumented Dogs 

All surgical procedures and experimental protocols were approved by our institutional animal care and use committee.

Fifteen microfilaria-free, male mongrel dogs (weight, 26 +/− 1 kg) were premedicated with 10 mg morphine sulfate given intramuscularly and anesthetized with 20 mg/kg intravenous pentobarbital sodium. After insertion of a cuffed endotracheal tube, the lungs were mechanically ventilated and anesthesia was maintained with halothane (1.2% end-tidal concentration). After the dogs were prepared for sterile surgery, a left thoracotomy was performed through the fifth intercostal space, and the heart was exposed via pericardiotomy. Heparin-filled Tygon catheters (1.02 mm inner diameter; Norton, Akron, OH) were inserted into the descending thoracic aorta, left and right atrium, and main pulmonary artery. Hydraulic occluders (18 to 22 mm inner diameter; Jones, Silver Springs, MD) were positioned loosely around the right main pulmonary artery and thoracic inferior vena cava. An electromagnetic flow probe (10 mm inner diameter; Zepeda, Seattle, WA) was placed around the left main pulmonary artery. The pericardial edges were apposed loosely, and the free ends of the catheters, occluders, and flow probe were threaded through the chest wall and tunneled subcutaneously to a final position between the scapulae. A chest tube placed in the left thorax before closure was removed on the first postoperative day. Postoperative analgesia was achieved with 10 mg morphine sulfate given intramuscularly as required. Antibiotics were administered during operation and for 10 days afterward. The dogs were allowed to recover for at least 2 weeks before experimentation, during which time they became familiar with the laboratory environment.

Vascular pressures were measured by attaching the fluid-filled catheters to strain-gauge manometers (P23 inner diameter; Gould Electronics, Eastlake, OH). The transducers were positioned at the midchest at the level of the spine, and all pressures were referenced to atmospheric pressure. Heart rate was calculated from the phasic systemic arterial pressure recording. Left pulmonary blood flow (LQ) was measured by connecting the flow probe to an electromagnetic flowmeter (SWF-4rd; Zepeda). The flow probe was calibrated in vivo on a weekly basis using the thermal dilution technique. Calibration was achieved by acutely inserting a 7-French gauge balloon-tipped thermal dilution catheter into the pulmonary artery through a percutaneous jugular puncture after topical anesthesia (2% lidocaine spray). The tip of the catheter was positioned 2 or 3 cm beyond the pulmonic valve. The implanted perivascular hydraulic occluder was inflated to completely occlude the right main pulmonary artery, which directed total pulmonary blood flow through the left pulmonary artery (and flow probe). The LQ was measured by thermal dilution (9520A; American Edwards, Irvine, CA) with multiple 5-ml sterile iced injectates of 5% dextrose in water. Values for LQ were referenced to body weight (ml [middle dot] min^-1[middle dot] kg^-1). The aortic and pulmonary arterial catheters were used to obtain blood samples to measure systemic arterial and mixed venous blood gases, respectively. Systemic arterial and mixed venous pH, carbon dioxide tension (P^CO(2)), and oxygen tension (P^O(2)) were measured according to standard methods (ABL-3, Radiometer, Copenhagen, Denmark). Oxyhemoglobin saturation (S^O(2)) was measured using a Hemoximeter (OSM-3; Radiometer).

All experiments were performed with each healthy dog fitted with instruments for long-term monitoring and lying on its right side in a quite laboratory environment. Conscious dogs were not sedated. Continuous left pulmonary vascular pressure-flow (LPQ) plots were used to assess the effects of the various experimental interventions on the pulmonary circulation. The LPQ plots were generated by continuously measuring the pulmonary vascular pressure gradient (pulmonary arterial pressure [PAP]- left atrial pressure [LAP]) and LQ during gradual (approximately 1 min) inflation of the hydraulic occluder implanted around the right main pulmonary artery. This technique to measure the LPQ relation is highly reproducible and has little or no effect on systemic hemodynamics, blood gases, or the zonal condition of the lung. [15] 

Protocol 1: Effect of Isoflurane Anesthesia on the Pulmonary Vascular Response to Lemakalim. We evaluated the effect of isoflurane anesthesia on the pulmonary vascular response to cumulative doses of the K (^ATP)⁺channel agonist, lemakalim, after preconstriction with the thromboxane analog, U46619 (9, 11-dideoxy-11 [small alpha, Greek], 9 [small alpha, Greek]-epoxymethano-prostaglandin F²([small alpha, Greek]), a gift of Cayman Chemical, Ann Arbor, MI). Because the pulmonary circulation is dilated maximally under baseline conditions, pulmonary vasomotor tone must be increased (i.e., preconstricted) to assess the response to a putative pulmonary vasodilator (i.e., lemakalim). The dose of U46619 was titrated to double, approximately, the pulmonary vascular pressure gradient (PAP-LAP) at any given level of LQ. We tested the hypothesis that the pulmonary vasodilator response to lemakalim would be attenuated during isoflurane anesthesia. A baseline LPQ plot was first obtained in each conscious dog (n = 7). U46619 was then administered (0.08 +/− 0.01 [micro sign]g [middle dot] kg^-1[middle dot] min^-1given intravenously) to preconstrict the pulmonary circulation before the administration of lemakalim. The LPQ plots were obtained during U46619 preconstriction alone and then again with each dose of lemakalim (0.1, 1, and 5 [micro sign]g [middle dot] kg^-1[middle dot] min^-1given intravenously) during its cumulative administration (approximately 15 min at each dose) while the infusion of U46619 was continued. We verified that pulmonary vasoconstriction induced by U46619 is stable during the course of this protocol. [16]On a separate day, this protocol was repeated in the same seven dogs during isoflurane anesthesia. Anesthesia with isoflurane was induced by mask and supplemented with a subanesthetic dose of thiopental sodium (3 mg/kg given intravenously) to minimize excitatory behavior. The trachea was intubated (9 mm inner diameter), and ventilation was controlled with a respirator (Harvard Apparatus, Natick, MA) with zero end-expiratory pressure. Muscle relaxants were not used in these studies. Immediately after intubation, 2% isoflurane (Anaquest, Madison, WI) was delivered by a vaporizer (Isotec 3, Ohmeda, Madison, WI). Fresh gas flow was set at 100 ml [middle dot] min^-1[middle dot] kg^-1. Tidal volume was fixed at 15 ml/kg. Systemic arterial blood gases were matched to values measured in the conscious state by adjusting the respiratory rate to between 10 to 13 breaths/min and by administering supplemental oxygen (fractional inspiratory oxygen approximately 0.26). The end-tidal carbon dioxide level measured at the adapter end of the endotracheal tube was monitored continuously during the experiment (model 78356A; Hewlett-Packard, Andover, MA). After induction, isoflurane was allowed to equilibrate for at least 1 h to achieve steady state conditions. This method of isoflurane anesthesia in dogs results in end-tidal isoflurane concentrations (Nellcor, Hayward, CA) of approximately 1.65% and 1.75% after 1 and 2 h, respectively, which represents approximately 1.2 minimum alveolar concentration in dogs. [17]The plasma thiopental sodium concentration is negligible 1 h after administration. [18]The LPQ plots were obtained during isoflurane anesthesia, after preconstriction with U46619, and during the cumulative administration of lemakalim. The dose of U46619 (0.07 +/− 0.01 [micro sign]g [middle dot] kg^-1[middle dot] min (^-1) given intravenously) was titrated to achieve the same degree of preconstriction induced in the conscious state. This technique allowed us to assess the pulmonary vasodilator response to lemakalim at the same level of vasomotor tone in the conscious and isoflurane-anesthetized states.

Protocol 2: Pulmonary Vascular Response to Circulatory Hypotension in Intact Conscious Dogs. To confirm our previous results, [14]we tested the hypothesis that circulatory hypotension would result in pulmonary vasoconstriction in intact conscious dogs. For each intact conscious dog (n = 8), the LPQ plots were generated on the same day at baseline (during normotension) and during circulatory hypotension, which was achieved by gradual (approximately 15 min) inflation of the hydraulic occluder implanted around the thoracic inferior vena cava until the mean systemic arterial pressure (SAP) was reduced to approximately 50 mmHg. The SAP remained stable at this new steady state value while the LPQ plot was generated. Aside from modest hyperpnea, the dogs remained quiet during hypotension.

Protocol 3: Effect of K^ATP+Channel Inhibition on the Pulmonary Vascular Response to Circulatory Hypotension in Conscious Dogs. We evaluated the effect of the K^ATP+antagonist, glibenclamide, on the magnitude of the pulmonary vasoconstrictor response to circulatory hypotension in conscious dogs. We tested the hypothesis that K^ATP+channel inhibition would potentiate the pulmonary vasoconstrictor response to circulatory hypotension. These experiments used the same eight dogs studied in protocol 2. The LPQ plots were generated on the same day at baseline during normotension without previous drug administration, 15 min after the intravenous administration of glibenclamide (3 mg/kg), and during hypotension in the presence of the K^ATP+channel inhibitor. This dose of glibenclamide attenuates the pulmonary vasodilator response to lemakalim in conscious dogs. [7] 

Protocol 4: Effect of Isoflurane Anesthesia on the Pulmonary Vascular Response to Circulatory Hypotension. We investigated the effect of isoflurane anesthesia on the magnitude of the pulmonary vasoconstrictor response to circulatory hypotension. We tested the hypothesis that isoflurane anesthesia would attenuate the magnitude of the pulmonary vasoconstrictor response to circulatory hypotension. These experiments used the same eight dogs studied in protocols 2 and 3. Isoflurane anesthesia was induced as described in protocol 1. The LPQ plots were generated on the same day in isoflurane-anesthetized dogs at baseline and during circulatory hypotension, as described in protocol 2.

Protocol 5: Effect of K^ATP+Channel Inhibition on the Pulmonary Vascular Response to Circulatory Hypotension during Isoflurane Anesthesia. We evaluated the effect of isoflurane anesthesia on the glibenclamide-induced potentiation of the pulmonary vasoconstrictor response to circulatory hypotension. We tested the hypothesis that glibenclamide would not potentiate the pulmonary vasoconstrictor response to circulatory hypotension during isoflurane anesthesia. These experiments used the same eight dogs studied in protocols 2 to 4. Isoflurane anesthesia was induced as described in Protocol 1. The LPQ plots were generated on the same day in isoflurane-anesthetized dogs at baseline, 15 min after the intravenous administration of glibenclamide, and during circulatory hypotension.

Phasic and mean vascular pressures and LQ were displayed continuously on an eight-channel strip-chart recorder (2800; Gould, Eastlake, OH). Mean pressures and LQ, measured at end-expiration, were obtained using passive electronic filters with a 2-s time constant. All vascular pressures were referenced to atmospheric pressure before and after each LPQ plot. The analog pressure and LQ signals also were digitally converted and multiplexed (PCM-8; Medical Systems, Greenvale, NY) and stored on videotape (videocassette recorder AG-1260; Panasonic, Secaucus, NJ) for later playback and analysis. The LPQ relation was measured continuously over the empirically measured range of LQ in each experiment. In all protocols, the LPQ relation was linear by inspection over the empirically measured range of LQ. Therefore, linear regression analysis was used to calculate the slope and intercept for PAP-LAP (or PAP-0 if LAP <or= to 0 mmHg) as a function of LQ in each experiment. The correlation coefficient for the LPQ relation in each protocol averaged 0.97 or greater. The composite LPQ plots summarized in the figures were generated using the regression parameters from each continuously measured LPQ plot to calculate PAP-LAP at 10-ml [middle dot] min^-1[middle dot] kg^-1intervals of LQ over the empirically measured range of LQ. The minimum and maximum values of LQ in each composite LPQ plot represent the average minimum and maximum values of LQ for the dogs studied in that protocol. Multivariate analysis of variance in the form of Hotelling's T²was used to assess the effects of the various experimental interventions (e.g., U46619, lemakalim, isoflurane, hypotension, glibenclamide, and so forth) on the regression parameters obtained in each experiment within each specific protocol. [19]Two-way analysis of variance was used to assess the effects of isoflurane anesthesia on the pulmonary vascular responses to lemakalim and circulatory hypotension and steady state hemodynamics and blood gases. The pulmonary vasodilator response to lemakalim at LQ = 75 ml [middle dot] min^-1[middle dot] kg^-1was expressed as the percentage decrease in U46619 preconstriction, [2,3,7,11,15]which was calculated using the following formula:Equation 1Thus, a lemakalim-induced decrease in PAP-LAP of 100% represents a complete reversal of U46619 preconstriction and a full return to the baseline LPQ relation. All values are presented as mean +/− SEM.

All solutions were prepared on the day of the experiment. U46619 was suspended in 95% ethanol and stored as a stock solution at -20 [degree sign]C. On the day of the experiment, 360 [micro sign]g was dissolved in 60 ml 0.9% saline. Lemakalim (BRL 38227, a gift from SmithKline Beecham, Herts, UK) was dissolved in 95% ethanol and diluted in sterile water. Glibenclamide (Sigma Chemical Co., St. Louis, MO) was dissolved in 0.1N NaOH and diluted in 5% dextrose.

Isoflurane had no net effect on the baseline LPQ relation compared with the conscious state (Figure 1). The dose of U46619 was titrated to achieve the same degree of preconstriction in the conscious and isoflurane-anesthetized states (Figure 1). In the presence of U46619 preconstriction lemakalim (1 [micro sign]g [middle dot] kg^-1[middle dot] min^-1given intravenously) induced pulmonary vasodilation (P < 0.05) in the conscious state and during isoflurane anesthesia (Figure 2). Figure 3summarizes the pulmonary vascular dose-response relations for lemakalim in the conscious and isoflurane-anesthetized states. Lemakalim caused dose-dependent pulmonary vasodilation (P < 0.05) in both conditions. However, the magnitude of the pulmonary vasodilator response to lemakalim was attenuated (P < 0.05) during isoflurane anesthesia compared with the conscious state.

(Table 1and Table 2) summarize the steady state hemodynamics and blood gases, respectively. Baseline SAP was decreased during isoflurane compared with the conscious state. U46619 increased SAP and PAP in both conditions. Lemakalim decreased SAP and increased heart rate in both conditions. Lemakalim increased LQ in the conscious state, whereas LQ was unchanged during isoflurane anesthesia. Baseline blood gases were similar in conscious and isoflurane-anesthetized dogs (Table 2). Lemakalim increased mixed venous oxygen tension and S^O(2) in the conscious state but not during isoflurane anesthesia.

Circulatory hypotension was achieved by acute (approximately 15 min) inflation of the hydraulic occluder implanted around the thoracic inferior vena cava. In intact conscious dogs, the SAP was decreased from 112 +/− 4 mmHg to 49 +/− 2 mmHg during hypotension (Table 3). As summarized in Figure 4, hypotension resulted in increases (P < 0.05) in the pulmonary vascular pressure gradient (PAP-LAP) at values of LQ = 40, 50, and 60 ml [middle dot] min^-1[middle dot] kg^-1compared with values measured during normotension. Thus, in intact conscious dogs, hypotension resulted in pulmonary vasoconstriction. Hypotension-induced pulmonary vasoconstriction was observed in all eight intact conscious dogs.

(Table 3and Table 4) summarize steady state hemodynamics and blood gases, respectively. The PAP, LAP, and LQ were decreased during hypotension. Hypotension was associated with an increase in systemic arterial pH and a decrease in carbon dioxide tension, whereas mixed venous oxygen tension and S^O(2) were decreased.

Compared with intact conscious dogs, glibenclamide had no effect on the baseline LPQ relation (Figure 5A). After pre-treatment with glibenclamide, the SAP was decreased from 119 +/− 3 mmHg to 52 +/− 2 mmHg during hypotension (Table 3). As summarized in Figure 6, glibenclamide increased (P < 0.05) the magnitude of the hypotension-induced increases (P < 0.05) in PAP-LAP at LQ = 40, 50, and 60 ml [middle dot] min^-1[middle dot] kg^-1compared with responses measured in intact dogs. Thus, K^ATP+channel inhibition potentiated the pulmonary vasoconstrictor response to circulatory hypotension.

During normotension, glibenclamide increased LAP and decreased heart rate and LQ compared with those in intact dogs (Table 3). Glibenclamide had no effect on systemic arterial blood gases, increased mixed venous carbon dioxide tension, and decreased mixed venous oxygen tension and S^O(2) compared with those in intact dogs (Table 4). During hypotension, LQ and mixed venous pH and S^O(2) decreased to lesser values in glibenclamide-pretreated dogs compared with those in intact dogs (Table 3and Table 4).

In isoflurane-anesthetized dogs, SAP was decreased from 69 +/− 3 mmHg to 51 +/− 1 mmHg during hypotension (Table 3). As summarized in Figure 7, isoflurane decreased (P < 0.05) the magnitude of the hypotension-induced increases (P < 0.05) in PAP-LAP at LQ = 40, 50, and 60 ml [middle dot] min^-1[middle dot] kg^-1compared with responses measured while dogs were intact and conscious. Thus, isoflurane attenuated the pulmonary vasoconstrictor response to circulatory hypotension.

Steady state PAP, LAP, and LQ were decreased during hypotension in isoflurane-anesthetized dogs (Table 3). Systemic arterial blood gases were unchanged during hypotension, whereas mixed venous oxygen tension and S^O(2) were decreased (Table 4).

Glibenclamide had no effect on the baseline LPQ relation in isoflurane-anesthetized dogs (Figure 5B). After pretreatment with glibenclamide, SAP was decreased from 94 +/− 3 mmHg to 51 +/− 1 mmHg during hypotension. As summarized in Figure 8, glibenclamide increased (P < 0.05) the magnitude of the hypotension-induced increases (P < 0.05) in PAP-LAP at LQ = 40, 50, and 60 ml [middle dot] min^-1[middle dot] kg^-1compared with responses measured in otherwise intact isoflurane-anesthetized dogs. Thus, K^ATP+channel inhibition potentiated the pulmonary vasoconstrictor response to circulatory hypotension in isoflurane-anesthetized dogs.

After glibenclamide, PAP, LAP, LQ, and mixed venous pH and S^O(2) decreased to lower values during hypotension compared with isoflurane anesthesia alone (Table 3and Table 4).

The overall goal of this study was to evaluate the effects of isoflurane anesthesia on exogenous and endogenous regulation of the pulmonary circulation by K^ATP+channel activation. Dogs fitted with instruments for long-term monitoring were used so the pulmonary vascular response to the various experimental interventions could be assessed on separate days in the same animal in the conscious and isoflurane-anesthetized states. This experimental model obviates the effects of acute surgical trauma and the use of background anesthetics, which can modify neural, [2,20]humoral, [21,22]and local [3,23]mechanisms of pulmonary vascular regulation. The dose of U46619 was titrated to achieve the same degree of preconstriction in the conscious and isoflurane-anesthetized states, which allowed us to compare the magnitude of the pulmonary vasodilator response to lemakalim at the same level of pulmonary vasomotor tone. Responses to the pharmacologic and physiologic interventions were assessed by measuring their effects on the LPQ relation. This technique avoids the difficulties inherent in the interpretation of single-point calculations of pulmonary vascular resistance, which can reflect either active (vasoactive) or passive (flow dependent) changes in the pulmonary circulation in response to an experimental intervention. [24]Our results indicate that compared with the conscious state, the pulmonary vasodilator response to exogenous K^ATP+channel activation and the pulmonary vasoconstrictor response to circulatory hypotension were attenuated during isoflurane anesthesia. In addition, inhibition of endogenous K^ATP+channels with glibenclamide resulted in potentiation of the pulmonary vasoconstrictor response to circulatory hypotension in both conscious and isoflurane-anesthetized dogs.

The mechanism responsible for the attenuated response to lemakalim during isoflurane anesthesia could involve activation of reflex vasoconstrictor mechanisms. Lemakalim resulted in systemic hypotension in the conscious state, and this effect was even more pronounced during isoflurane anesthesia. In conscious dogs, systematic hypotension results in reflex pulmonary vasoconstriction that is mediated primarily by sympathetic [small alpha, Greek]^{1-adrenoreceptor}activation. [14]We recently found that the inhibitory effect of desflurane anesthesia on lemakalim-induced pulmonary vasodilation can be reversed in part by prazosin, a sympathetic [small alpha, Greek]^{1-adrenoreceptor}antagonist. [11]Furthermore, we have shown that desflurane potentiates the pulmonary vasoconstrictor response to sympathetic [small alpha, Greek]^{1-adrenoreceptor}activation. [25]However, in contrast to desflurane, isoflurane does not potentiate the pulmonary vasoconstrictor response to sympathetic [small alpha, Greek]^{1-adrenoreceptor}activation. [2]In addition, combined neurohumoral block has no effect on the pulmonary vasodilator response to lemakalim in the conscious state, nor does it reverse the attenuated pulmonary vasodilator response to lemakalim in halothane-anesthetized dogs. [7]Finally, in the current study, we observed that the pulmonary vasoconstrictor response to circulatory hypotension was attenuated in isoflurane-anesthetized dogs. Thus, it appears unlikely that the attenuating effect of isoflurane on lemakalim-induced pulmonary vasodilation involves activation of reflex vasoconstrictor mechanisms.

K^ATP+channels are expressed in pulmonary vascular smooth muscle [5,6]and endothelial cells. [26]Recently, we showed in isolated canine pulmonary arterial rings that the pulmonary vasorelaxant response to lemakalim involves an endothelium-dependent and vascular smooth muscle component. [10]The endothelium-dependent component of the response to lemakalim does not involve nitric oxide but is mediated by cyclooxygenase metabolites. [10]Halothane attenuated lemakalim-induced pulmonary vasorelaxation by inhibiting the endothelium-dependent, cyclooxygenase-mediated component of the response. [10]Isoflurane anesthesia could exert similar inhibitory actions, although it has been shown actually to enhance the vasodilator influence of cyclooxygenase metabolites during alveolar hypoxia. [1]Isoflurane selectively attenuates endothelium-dependent relaxation in canine pulmonary arteries by interfering with a glibenclamide-sensitive, synergistic interaction between nitric oxide and prostacyclin. [27] 

Isoflurane could exert a direct inhibitory effect on K^ATP+channels. Isoflurane suppresses the Ca^2+-dependentand -independent K (⁺) channel current in canine cerebral arteries. [28,29]Isoflurane could alter K^ATP+channel gating during agonist-induced activation. In contrast to inhibition, several recent studies have shown that isoflurane-induced coronary vasodilation is mediated by K^ATP+channel activation [12,13]However, isoflurane had no net effect on the baseline LPQ relation, which suggests that isoflurane did not activate K^ATP+channels in the pulmonary circulation. Furthermore, K^ATP+channel inhibition with glibenclamide had no effect on the baseline LPQ relation in either conscious or isoflurane-anesthetized dogs, which suggests that K^ATP+channels are not tonically active in the pulmonary circulation under these conditions.

As we reported before, [14]circulatory hypotension resulted in pulmonary vasoconstriction in intact conscious dogs. We can only speculate about the homeostatic role of hypotension-induced pulmonary vasoconstriction. This response could result in the redistribution of blood volume from the pulmonary to systemic circulation, which would provide a “transfusion” of blood to the systemic circulation during hypotension. Glibenclamide markedly potentiated the pulmonary vasoconstrictor response to hypotension, which suggests that endogenous K^{ATP+-mediated}vasodilation modulated the pulmonary vasoconstrictor response to this stimulus. It is unlikely that this effect was mediated directly by a decrease in intracellular ATP, with subsequent opening of K^ATP+channels. Rather, it is more likely that hypotension stimulated the release of endogenous mediators, which activated K^ATP+channels as part of their effect. For example, K (^ATP)⁺channels contribute to sympathetic [small beta, Greek]-adrenoreceptor-mediated and adenosine-mediated pulmonary vasodilation. [30]Circulatory hypotension clearly results in sympathetic nervous system activation and would also likely cause the release of adenosine from underperfused tissues (e.g., cardiac and skeletal muscle). Both of these effects could result in K^ATP+channel activation, which would blunt the pulmonary vasoconstrictor response to circulatory hypotension in the intact animal.

The magnitude of the pulmonary vasoconstrictor response to hypotension was reduced during isoflurane anesthesia. Although anesthesia reduces the systemic vascular response to hypotension, [31,32]the effects of anesthesia on the pulmonary vascular response to hypotension had not been investigated. The technique that we used to induce hypotension resulted in unloading of arterial baroreceptors and cardiopulmonary baroreceptors. In addition, vena caval occlusion activates hepatic baroreceptors and increases renal and cardiopulmonary sympathetic efferent nerve activity. [33]Thus, the pulmonary vasoconstrictor response to hypotension represents the integrative response to activation of these reflex pathways. Isoflurane could attenuate the pulmonary vasoconstrictor response to hypotension by altering the afferent or efferent limbs of these reflex pathways, or by modifying central nervous system signal processing. Alternatively, the hypotensive stimulus may have been smaller during isoflurane, because the baseline SAP was lower during isoflurane compared with the conscious state (69 vs. 112 mmHg, respectively). We previously found that isoflurane potentiates the pulmonary vasodilator response to sympathetic [small beta, Greek]-adrenoreceptor activation, whereas it has no effect on the pulmonary vasoconstrictor response to sympathetic [small alpha, Greek]^{1-adrenoreceptor}activation. [2]These differential effects of isoflurane on sympathetic nervous system regulation of the pulmonary circulation could be responsible for the attenuated pulmonary vasoconstrictor response to hypotension during isoflurane anesthesia. However, isoflurane did not modify the influence of endogenous K^ATP+channel activation on the pulmonary vascular response to hypotension, because glibenclamide potentiated the hypotension-induced vasoconstrictor response to the same extent in both conscious and isoflurane-anesthetized dogs. This result is consistent with the possibility that in the intact condition sympathetic [small beta, Greek]-adrenoreceptor activation may stimulate K^ATP+channel-mediated pulmonary vasodilation during hypotension.

The 3 mg/kg dose of glibenclamide was chosen because it causes a rightward shift in the dose-response curve for lemakalim-induced pulmonary vasodilation. [7]This dose of glibenclamide had no effect on the baseline LPQ relation in either the conscious or isoflurane-anesthetized states. Furthermore, this dose of glibenclamide has no effect on the pulmonary vasodilator response to SIN-1, a nitric oxide donor. [3]As expected, glibenclamide decreased blood glucose levels from 92 +/− 2 mg/dl to 72 +/− 3 mg/dl 30 to 45 min after administration. This hypoglycemic effect was treated with intravenous glucose at the end of the experiment. The dogs recovered promptly from the hypotension experiments and showed no adverse effects from acute hypotension or K^ATP+channel inhibition.

One possible limitation of the hypotension experiments is that it was necessary to extrapolate values obtained from the regression parameters to compare pressure gradients at the same level of flow (40 to 60 ml [middle dot] min^-1[middle dot] kg^-1) during normotension and hypotension. This assumes that the LPQ relation is linear under the conditions of these experiments. In the current study, we have direct, empirically measured evidence that the LPQ relation is linear over a flow range from 10 to 170 ml [middle dot] min^-1[middle dot] kg^-1.

In conclusion, our results indicate that K^{ATP+-mediated}pulmonary vasodilation and the pulmonary vasoconstrictor response to circulatory hypotension are attenuated during isoflurane anesthesia. Endogenous K^ATP+channel activation modulates the pulmonary vasoconstrictor response to hypotension in the conscious and isoflurane-anesthetized states.