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Am J Physiol Lung Cell Mol Physiol 291: L636-L643, 2006. First published May 25, 2006; doi:10.1152/ajplung.00063.2006
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Propofol and thiopental attenuate adenosine triphosphate-sensitive potassium channel relaxation in pulmonary veins

Woon-Seok Roh, Xueqin Ding, and Paul A. Murray

Center for Anesthesiology Research, The Cleveland Clinic Foundation, Cleveland, Ohio

Submitted 20 February 2006 ; accepted in final form 19 May 2006


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Pulmonary veins (PV) make a significant contribution to total pulmonary vascular resistance. We investigated the cellular mechanisms by which the intravenous anesthetics propofol and thiopental alter adenosine triphosphate-sensitive potassium (KATP+) channel relaxation in canine PV. The effects of KATP+ channel inhibition (glybenclamide), cyclooxygenase inhibition (indomethacin), nitric oxide synthase inhibition (L-NAME), and L-type voltage-gated Ca2+ channel inhibition (nifedipine) on vasorelaxation responses to levcromakalim (KATP+ channel activator) alone and in combination with the anesthetics were assessed. The maximal relaxation response to levcromakalim was attenuated by removing the endothelium and by L-NAME, but not by indomethacin. Propofol (10–5, 3 x 10–5, and 10–4 M) and thiopental (10–4 and 3 x 10–4 M) each attenuated levcromakalim relaxation in endothelium-intact (E+) rings, whereas propofol (3 x 10–5 and 10–4 M) and thiopental (3 x 10–4 M) attenuated levcromakalim relaxation in endothelium-denuded (E–) rings. In E+ rings, the anesthesia-induced attenuation of levcromakalim relaxation was decreased after pretreatment with L-NAME but not with indomethacin. In E-strips, propofol (10–4 M) and thiopental (3 x 10–4 M) inhibited decreases in tension and intracellular Ca2+ concentration ([Ca2+]i) in response to levcromakalim, and these changes were abolished by nifedipine. These findings indicate that propofol and thiopental attenuate the endothelium-dependent component of KATP+ channel-induced PV vasorelaxation via an inhibitory effect on the nitric oxide pathway. Both anesthetics also attenuate the PV smooth muscle component of KATP+ channel-induced relaxation by reducing the levcromakalim-induced decrease in [Ca2+]i via an inhibitory effect on L-type voltage-gated Ca2+ channels.

Ca2+ influx; anesthetics

ADENOSINE TRIPHOSPHATE-SENSITIVE potassium (KATP+) channels play an important role in the regulation of vascular smooth muscle tone (27). Activation of KATP+ channels causes an increase in K+ efflux, membrane hyperpolarization, inhibition of Ca2+ influx through L-type voltage-gated Ca2+ channels (VGCCs), and subsequent vascular smooth muscle relaxation. Our laboratory and others (2, 6, 8, 22, 28, 29, 34, 36) have demonstrated that KATP+ channel agonists can cause marked pulmonary vasodilation, which is reversed by glybenclamide, a specific KATP+ channel inhibitor. Moreover, endogenous KATP+ channel-induced vasorelaxation is functionally significant, because it has been shown to modulate the pulmonary vasoconstrictor responses to hypoxia (23) and systemic hypotension (8).

KATP+ channels are present not only in vascular smooth muscle cells but also have been demonstrated in vascular endothelial cells (13, 14). In endothelial cells, KATP+ channel activation results in hyperpolarization, an increase in Ca2+ influx (18, 19), and the production of endothelium-derived relaxing factors (17, 20) such as nitric oxide and prostacyclin. We have recently demonstrated that KATP+ channel-induced pulmonary arterial vasorelaxation involves both endothelium-dependent and vascular smooth muscle components (28, 34). However, there is very little information about KATP+ channel-induced vasorelaxation in pulmonary veins (PV). Pulmonary venous resistance is an important component of total pulmonary vascular resistance (1). Recent evidence indicates that there are marked regional differences in reactivity to vasodilators in arterial and venous segments of isolated pulmonary vessels (9, 15, 31).

Propofol and thiopental are widely used intravenous anesthetics for induction and maintenance of cardiac and noncardiac anesthesia. Both anesthetics have some benefits for brain protection (5, 21). The effects of these anesthetics on KATP+ channel-induced vasorelaxation have not been investigated in PV. Moreover, underlying mechanisms responsible for the anesthesia-induced attenuation of the smooth muscle component of KATP+ channel-induced relaxation have not been elucidated. Because propofol and thiopental have been shown to inhibit L-type VGCCs in vascular smooth muscle (12, 37), tracheal smooth muscle cells (38), and myocardial cells (3), it is reasonable to hypothesize that these anesthetics could attenuate the vascular smooth muscle component of KATP+ channel-mediated relaxation via an inhibitory effect on L-type VGCCs. Because PV constriction can increase pulmonary capillary pressure and transvascular fluid flux to cause pulmonary edema, an anesthesia-induced attenuation of a PV vasodilator mechanism could result in an increase in pulmonary capillary pressure, pulmonary edema formation, and congestive heart failure. There has been a suggestion that propofol and thiopental may be associated with pulmonary edema (26, 35).

The overall goal of this in vitro study was to investigate the effects of propofol and thiopental on the PV vasorelaxant response to the KATP+ channel agonist levcromakalim. On the basis of our previous results in pulmonary arteries (28, 34), we tested the hypothesis that these anesthetics would attenuate the endothelium-dependent component of vasorelaxation in response to levcromakalim. We also tested the hypothesis that propofol and thiopental would attenuate the vascular smooth muscle component of levcromakalim-induced vasorelaxation by reducing the agonist-induced decrease in intracellular Ca2+ concentration ([Ca2+]i) via an inhibitory effect on L-type VGCCs.


   MATERIALS AND METHODS
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 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
All experimental procedures and protocols were approved by the Institutional Animal Care and Use Committee of The Cleveland Clinic Foundation (Cleveland, OH).

Preparation of PV rings and strips. Healthy male mongrel dogs (24–30 kg) were anesthetized with pentobarbital sodium (30 mg/kg, intravenously) and fentanyl citrate (15 µg/kg, intravenously). After tracheal intubation, the dogs were placed on positive-pressure ventilation. After the administration of heparin (6,000 units), a catheter was inserted into the right femoral artery for exsanguination by controlled hemorrhage. A left lateral thoracotomy was performed through the fifth intercostal space, and the heart was arrested with electrically induced ventricular fibrillation. The heart and lungs were removed en bloc. The right and left lower intralobar PV (3rd and 4th generation: 2- to 4-mm inner diameter) were carefully dissected free and immersed in cold modified Krebs-Ringer bicarbonate (KRB) solution of the following composition (in mM): 118.3 NaCl, 4.7 KCl, 1.2 KH2PO4, 2.5 CaCl2, 25 NaHCO3, 0.016 Ca-EDTA, and 11.1 glucose. After removal of connective tissue, the veins were cut into ring segments 4–5 mm in length, with care taken not to damage the endothelium. For protocols using pulmonary venous strips, first and second generation veins were cut into strips (4 x 8 mm). Larger veins were used for the strip studies compared with the ring studies to accommodate the equipment used for fluorescence measurements (described below). In some rings and all strips, the endothelium was intentionally denuded by gently rubbing the inner surface with a cotton swab. Denudation was verified by >90% attenuation in the relaxation response to bradykinin (10–8 M).

Isometric tension experiments. PV rings were vertically suspended between two stainless steel hooks in organ chambers filled with 25 ml of modified KRB solution (37°C) gassed with 95% O2 and 5% CO2. One of hooks was anchored, and the other was connected to a strain gauge (Grass Model FT03, Quincy, MA) to measure isometric tension. Rings were stretched at 10-min intervals in increments of 0.5 g to reach optimal resting tension. Optimal resting tension was defined as the minimum level of stretch required to achieve the largest contractile response to 60 mM KCl and was determined in preliminary experiments to be 1.5 g for the size of the veins used in these experiments. After the rings had been stretched to their optimal resting tension, the contractile response to 60 mM KCl was measured. After a washing out of KCl from the organ bath and the return of isometric tension to prestimulation values, each ring was precontracted with the thromboxane analog U46619 [GenBank] .

In preliminary studies, we observed that U46619 [GenBank] -induced increases in pulmonary venous tension were not sustained, so cumulative concentration-response studies for the KATP+ channel agonist levcromakalim (10–7-10–5 M) could not be performed. Instead, a concentration-response curve to U46619 [GenBank] (10–11-10–6 M) was performed. Then we used the same concentration of U46619 [GenBank] [~75% maximal effective concentration (EC75)] sequentially. After washout, each ring was again pretreated with U46619 [GenBank] , and the relaxation response to the next highest concentration of levcromakalim was assessed. Five repetitive treatments with U46619 [GenBank] resulted in highly reproducible preconstriction. The effects of propofol (10–5, 3 x 10–5, and 10–4 M) and thiopental (10–4 and 3 x 10–4 M) on the concentration-response relationship for levcromakalim were assessed by comparing vasorelaxant responses in the presence and absence of the anesthetics. The anesthetics were added directly to the organ bath 15 min before U46619 [GenBank] precontraction.

To investigate the roles of the nitric oxide synthase or cyclooxygenase pathways on levcromakalim-induced vasorelaxation, the response to each concentration of levcromakalim in endothelial-intact or -denuded rings was assessed 15 min after either the nitric oxide synthase inhibitor L-NAME (10–4 M) or the cyclooxygenase inhibitor indomethacin (10–5 M) was added to the bath, either alone or after combined pretreatment with the anesthetics. To investigate the role of L-type VGCCs on levcromakalim-induced vasorelaxation, levcromakalim-induced relaxation in endothelium-denuded rings was assessed 15 min after the L-type VGCC blocker nifedipine (10–5 M) was added to the bath, either alone or after combined pretreatment with the anesthetics. Finally, PV rings with or without endothelium were pretreated with the selective KATP+ channel antagonist glybenclamide (10–5 M), and the levcromakalim-induced relaxation was assessed 15 min after glybenclamide pretreatment.

Simultaneous measurement of tension and [Ca2+]i. Pulmonary venous strips without endothelium were loaded with 5 x 10–6 M fura-2 AM solution for 4 h in a temperature-regulated (37°C) chamber. A nontoxic detergent, 0.05% cremorphor EL, was added to solubilize the fura-2 AM in the solution. After fura-2 loading, the strips were washed with KRB buffer and mounted between two stainless steel hooks in a temperature-controlled cuvette (volume = 3 ml) that was continuously perfused (12 ml/min) with KRB solution bubbled with 95% O2 and 5% CO2 (pH 7.4). One hook was anchored, and the other was connected to a strain gauge transducer to measure isometric tension. The resting tension was adjusted to 1.5 g, which was determined in preliminary studies to be optimal for achieving a maximum contractile response to 40 mM KCl. We used a lower concentration of KCl in the strip studies compared with the ring studies, because the higher concentration was associated with a prolonged washout period before tension and [Ca2+]i returned to baseline values. Fluorescence measurements were performed using a dual-wavelength spectrofluorometer (Deltascan RFK6002; Photon Technology International, South Brunswick, NJ) at excitation wavelengths of 340 and 380 nm and an emission wavelength of 510 nm. The 340-to-380 fluorescence ratio was used as an indicator of [Ca2+]i. The temperature of all solutions was maintained at 37°C with the use of a water bath. Just before data acquisition, background fluorescence was measured and subtracted automatically from the subsequent experimental measurement. Fura-2 fluorescence signals (340, 380, and 340-to-380 ratio) were continuously monitored at a sampling frequency of 2 Hz and collected with the use of a software package from Photon Technology International. After measurement of changes in tension and [Ca2+]i in response to 40 mM KCl, the strips were washed with fresh KRB. The strips were then pretreated for 30 min with propofol (10–5 and 10–4 M) or thiopental (3 x 10–5 and 3 x 10–4 M) or without anesthetic and then precontracted with U46619. [GenBank] After tension and [Ca2+]i increased to new steady-state values, levcromakalim (10–5 M) was added to the perfusate. Changes in tension and [Ca2+]i in response to levcromakalim are expressed as a percentage of peak increases in tension and intracellular Ca2+ in response to U46619 [GenBank] .

Solution and chemicals. All drugs were of the highest purity commercially available: U46619 [GenBank] (Cayman Chemical, Ann Arbor, MI); levcromakalim (Tocris Cookson, Ellisville, MO); L-NAME, indomethacin, propofol, thiopental, nifedipine, and glybenclamide (Sigma Chemical, St. Louis, MO); and fura-2/AM (Texas Fluorescence Labs, Austin, TX). All concentrations are expressed as the final molar concentration in the study chamber. U46619 [GenBank] , propofol, glybenclamide, and fura-2/AM were dissolved in dimethyl sulfoxide and diluted with distilled water. The final concentration of dimethyl sulfoxide in the study chamber was <0.1% (vol/vol). Thiopental and indomethacin were dissolved in NaHCO3 and diluted in distilled water (final study chamber NaHCO3 concentration, 2 x 10–4 M). None of the agents or solutions caused significant shifts in isometric tension or the 340-to-380 ratio at the concentrations used in these studies.

Data analysis. All data are expressed as means ± SD. Vasorelaxant responses to levcromakalim are expressed as the percent relaxation of precontraction induced by U46619. [GenBank] The maximal relaxation response (Rmax) to levcromakalim was measured, with Rmax = 100% indicating complete reversal of U46619 [GenBank] precontraction. Statistical analysis was performed using Student's t-test for paired comparisons or one-way analysis of variance followed by Bonferroni correction. Differences were considered statistically significant at P < 0.05; n refers to the number of dogs from which PV rings or strips were studied in each protocol.


   RESULTS
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
Effects of sequential application of U46619. U46619 (10–8 M) was approximately the log EC75 for both endothelium-intact and -denuded rings. Figure 1A illustrates changes in tension in endothelium-intact PV rings in response to U46619. [GenBank] The U46619 [GenBank] -induced increase in tension was not sustained, so cumulative concentration-response studies for levcromakalim could not be performed. Each ring was precontracted with U46619 [GenBank] , and the relaxation response to each concentration of levcromakalim was assessed in a sequential fashion. Changes in tension in response to sequential treatment of U46619 [GenBank] were highly reproducible (Fig. 1B).


Figure 1
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  		Fig. 1. A: in canine pulmonary venous rings, the increase in tension in response to U46619 (10–8 M) was not sustained, so cumulative concentration-response studies for levcromakalim could not be performed (n = 6). B: increases in tension in response to sequential treatment of U46619 (10–8 M) were highly reproducible in endothelium-intact and endothelium-denuded pulmonary venous rings. U46619-induced increases in tension are expressed as %contractile response to 60 mM KCl. Values are presented as means ± SD in all figures.

 
Effects of endothelial denudation on levcromakalim-induced vasorelaxation. As summarized in Fig. 2, levcromakalim vasorelaxation in PV was attenuated (P < 0.05) by removing the endothelium. Levcromakalim vasorelaxation was essentially abolished by the KATP+ channel antagonist glybenclamide in both endothelium-intact and -denuded PV (Fig. 3).


Figure 2
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  		Fig. 2. Levcromakalim dose-response relationship was attenuated (*P < 0.05) by removing the endothelium in pulmonary venous rings. Relaxation response to levcromakalim is expressed as a percentage of U46619 precontraction.

 

Figure 3
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  		Fig. 3. The adenosine triphosphate-sensitive potassium (KATP+) channel antagonist glybenclamide essentially abolished (*P < 0.05) levcromakalim relaxation in endothelium-intact (A) and endothelium-denuded (B) pulmonary venous rings. Glybenclamide was dissolved in dimethylsulfoxide (DMSO).

 
Effects of propofol and thiopental on levcromakalim vasorelaxation. To achieve the same degree of precontraction observed in control rings, a higher concentration (~10–7 M) of U46619 [GenBank] was required in rings treated with high-dose anesthetic. As summarized in Figs. 4 and 5, propofol (10–5, 3 x 10–5, and 10–4 M) and thiopental (10–4 and 3 x 10–4 M) attenuated levcromakalim relaxation in endothelium-intact PV in a dose-dependent manner. Only higher concentrations of propofol (3 x 10–5 and 10–4 M) and thiopental (3 x 10–4 M) attenuated levcromakalim relaxation in endothelium-denuded PV.


Figure 4
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  		Fig. 4. Propofol attenuated (*P < 0.05) levcromakalim relaxation in a dose-dependent fashion in endothelium-intact (A) and endothelium-denuded (B) pulmonary venous rings.

 

Figure 5
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  		Fig. 5. Thiopental attenuated (*P < 0.05) levcromakalim relaxation in a dose-dependent fashion in endothelium-intact (A) and endothelium-denuded (B) pulmonary venous rings.

 
Effects of cyclooxygenase inhibition or nitric oxide synthase inhibition on propofol- and thiopental-induced changes in levcromakalim vasorelaxation. We tested the hypothesis that propofol and thiopental attenuated levcromakalim relaxation by inhibiting vasodilator metabolites of the cyclooxygenase pathway or the nitric oxide synthase pathway. Indomethacin (10–5 M) had no effect on levcromakalim relaxation compared with the no-drug condition (Fig. 6A). Moreover, propofol (3 x 10–5 M)- or thiopental (10–4 M)-induced attenuation of levcromakalim pulmonary relaxation was still observed after indomethacin pretreatment. In contrast, L-NAME (10–4 M) attenuated levcromakalim relaxation (Fig. 6B). Moreover, the anesthesia-induced attenuation of levcromakalim relaxation was either reduced (propofol) or abolished (thiopental) after L-NAME pretreatment (Fig. 7). These results indicate that propofol and thiopental attenuated the endothelium-dependent component of KATP+ channel-mediated pulmonary venous relaxation via an inhibitory effect on the nitric oxide synthase pathway but not the cyclooxygenase pathway.


Figure 6
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  		Fig. 6. Cyclooxygenase inhibition with indomethacin had no effect on levcromakalim relaxation (A), whereas nitric oxide synthase inhibition with L-NAME attenuated (*P < 0.05) levcromakalim relaxation (B) in endothelium-intact pulmonary venous rings.

 

Figure 7
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  		Fig. 7. Attenuated relaxation response to levcromakalim induced by propofol (A) and thiopental (B) was not observed in endothelium-intact pulmonary venous rings pretreated with L-NAME.

 
Effects of propofol and thiopental on levcromakalim-induced changes in tension and [Ca2+]i. As summarized in Fig. 8, levcromakalim decreased tension and [Ca2+]i in U46619 [GenBank] -precontracted strips. Propofol (10–4 M) and thiopental (3 x 10–4 M) attenuated these levcromakalim-induced decreases in tension and [Ca2+]i (Fig. 8). Lower concentrations of propofol (10–5 M), but not thiopental (3 x 10–5 M), also attenuated the levcromakalim-induced decreases in tension and [Ca2+]i.


Figure 8
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  		Fig. 8. In the absence of anesthetic, levcromakalim (10–5 M) decreased tension and intracellular Ca2+ concentration ([Ca2+]i) in endothelium-denuded U46619-precontracted pulmonary venous strips. Propofol and thiopental attenuated (*P < 0.05) the levcromakalim-induced decreases in tension and [Ca2+]i.

 
Effects of L-type VGCC inhibition on propofol- and thiopental-induced changes in levcromakalim vasorelaxation. We tested the hypothesis that propofol and thiopental could attenuate the vascular smooth muscle component of levcromakalim relaxation via an inhibitory effect on L-type VGCCs. As summarized in Fig. 9, nifedipine (10–5 M) attenuated levcromakalim relaxation in endothelium-denuded PV rings. Neither propofol (10–4 M) nor thiopental (3 x 10–4 M) had any further inhibitory effect on levcromakalim relaxation after nifedipine pretreatment. These findings indicate that propofol and thiopental attenuate the vascular smooth muscle component of levcromakalim relaxation by an inhibitory effect on L-type VGCCs.


Figure 9
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  		Fig. 9. The L-type voltage-gated calcium channel blocker nifedipine attenuated (*P < 0.05) levcromakalim relaxation in endothelium-denuded pulmonary venous rings. Attenuated relaxation response to levcromakalim induced by propofol (A) and thiopental (B) was not observed in endothelium-denuded pulmonary venous rings pretreated with nifedipine.

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
This is the first study to assess the effects of propofol and thiopental on KATP+ channel-induced relaxation in PV. Our results demonstrate that levcromakalim causes endothelium-dependent, glybenclamide-sensitive, and L-type VGCC-dependent PV vasorelaxation. Propofol and thiopental attenuate the endothelium-dependent component of KATP+ channel-induced relaxation via an inhibitory effect on the nitric oxide pathway. Moreover, both anesthetics attenuate the PV vascular smooth muscle component of KATP+ channel-induced relaxation by reducing the decrease in [Ca2+]i via an inhibitory effect on L-type VGCCs.

As in previous in vivo (8, 22, 29) and in vitro studies (28, 34), we used levcromakalim to activate KATP+ channels. In the current study, levcromakalim-induced relaxation was inhibited by glybenclamide, a highly selective KATP+ channel inhibitor, which indicates that levcromakalim causes KATP+ channel-dependent vasorelaxation in PV. Moreover, this relaxation by levcromakalim was attenuated by endothelial denudation, suggesting that KATP+ channel-induced relaxation involves both endothelium-dependent and vascular smooth muscle components.

Recent reports indicate that KATP+ channel agonists such as levcromakalim and pinacidil relax vascular smooth muscle by opening KATP+ channels on endothelial cells (13, 14), suggesting that KATP+ channel agonists could modulate the release of endothelium-derived relaxing factors (17–20) such as nitric oxide or prostacyclin. We previously demonstrated in canine pulmonary artery that the endothelium-dependent component of levcromakalim-induced vasorelaxation requires the activity of the cyclooxygenase pathway but is independent of nitric oxide synthase activity (28, 34). In contrast to pulmonary arteries, cyclooxygenase inhibition had no effect on levcromakalim-induced relaxation in PV, whereas nitric oxide synthase inhibition attenuated levcromakalim-induced relaxation. Although the cellular mechanism responsible for these differences has not been identified, nitric oxide has been reported to play a larger role in PV relaxation compared with pulmonary artery (9).

Both propofol and thiopental attenuated the PV vasorelaxant response to levcromakalim. At lower concentrations, propofol (10–5 M) and thiopental (10–4 M) only inhibited the vasorelaxation response to levcromakalim in endothelium-intact rings, whereas higher concentrations of propofol (3 x 10–5 and 10–4 M) and thiopental (3 x 10–4 M) inhibited relaxation in both endothelium-intact and -denuded rings. Moreover, the endothelium-dependent inhibitory effects of lower concentrations of propofol and thiopental were abolished by L-NAME. It would appear that low concentrations of propofol and thiopental selectively inhibit levcromakalim-induced vasorelaxation mediated by nitric oxide. We recently reported that etomidate and ketamine attenuated vasorelaxant responses to acetylcholine and bradykinin by inhibiting both nitric oxide- and endothelium-derived hyperpolarizing factor-mediated components of the response (25). In the same study (25), these anesthetics attenuated increases in endothelial Ca2+ concentration in response to bradykinin. It is possible that propofol and thiopental inhibited a levcromakalim-induced increase in endothelial Ca2+ concentration, which in turn would decrease the production of nitric oxide and the endothelium-dependent component of the response.

A previous study in endothelium-denuded rat aorta (16) and the results of the current study indicate that propofol and thiopental also attenuated the smooth muscle component of KATP+ channel-induced relaxation. However, the underlying mechanism for this effect has not been previously investigated. Because propofol and thiopental have been shown to inhibit L-type VGCCs in vascular smooth muscle (12, 37), tracheal smooth muscle cells (38), and myocardial cells (3), we hypothesized that propofol and thiopental could attenuate levcromakalim-induced PV relaxation via an inhibitory effect on L-type VGCCs. To test this hypothesis, we assessed the effects of the anesthetics on changes in [Ca2+]i induced by levcromakalim. The maintenance of vasomotor tone depends on steady-state Ca2+ entry through L-type VGCCs. Membrane hyperpolarization by levcromakalim results in the closing of L-type VGCC and causes decreases in [Ca2+]i and tension (24). Propofol (10–4 M) and thiopental (3 x 10–4 M) reduced the levcromakalim-induced decreases in PV [Ca2+]i and tension. Moreover, anesthetic-pretreated rings or strips required higher concentrations of U46619 [GenBank] to achieve similar precontraction values compared with no anesthetic. Taken together, these findings suggest that propofol and thiopental may have inhibitory effects on L-type VGCCs in PV. To confirm this possibility, we assessed the effects of the L-type VGCC blocker nifedipine on levcromakalim relaxation with and without anesthetic pretreatment. We observed that nifedipine attenuated levcromakalim-induced relaxation, and propofol (10–4 M) and thiopental (3 x 10–4 M) had no further attenuating effect on levcromakalim relaxation after nifedipine pretreatment. It should be noted that nifedipine did not completely prevent the levcromakalim-induced relaxation, indicating that levcromakalim causes L-type VGCC-dependent and -independent vasorelaxation. Finally, it should be noted that we have recently reported that propofol and thiopental attenuate capacitative Ca2+ entry (CCE) in pulmonary venous smooth muscle cells (32). However, CCE is generally considered not to be voltage dependent, so it is unlikely that this mechanism is involved in the attenuating effect of these anesthetics on the vasorelaxant response to levcromakalim observed in the present study.

The plasma concentration of propofol in patients during maintenance of general anesthesia has been reported to be in the range of 10–5 to 10–4 M (33). Peak serum concentrations of thiopental immediately after induction reach 4.5 x 10–4 M (4). Because 97–98% propofol (30) and 75% thiopental (10) are bound to plasma proteins, the free concentrations of propofol and thiopental are estimated to be 10–6-10–5 and 5 x 10–6-5 x 10–5 M, respectively. However, it was recently reported that 28% propofol is taken up by the lung during a single passage through the lung and released back into the circulation by back diffusion (11). This results in a higher concentration of propofol in the pulmonary circulation than in the systemic circulation (7). In this study, propofol (10–5 M) and thiopental (10–4 M) significantly attenuated levcromakalim-induced relaxation, indicating that propofol can attenuate KATP+ channel-mediated relaxation at a clinically relevant concentration, whereas thiopental only attenuates KATP+ channel-mediated relaxation at a supraclinical concentration.

In summary, propofol and thiopental attenuated KATP+ channel-induced PV vasorelaxation. At lower concentrations, propofol and thiopental attenuated the endothelium-dependent component of vasorelaxation in response to KATP+ channel activation, and this effect was dependent on the nitric oxide pathway. On the other hand, both anesthetics attenuated the smooth muscle component of KATP+ channel-induced relaxation via reducing the KATP+ channel-mediated decrease in PV [Ca2+]i by an inhibitory effect on L-type VGCCs. These results demonstrate that experimental findings of vascular regulation in pulmonary artery cannot necessarily be extrapolated to PV.


   GRANTS
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
This work was funded by National Heart, Lung, and Blood Institute Grant HL-38291.


   FOOTNOTES
 

Address for reprint requests and other correspondence: P. A. Murray, Center for Anesthesiology Research, NE63, The Cleveland Clinic Foundation, 9500 Euclid Ave., Cleveland, OH 44195 (e-mail: murrayp{at}ccf.org)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

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X. Ding and P. A. Murray
The Differential Effects of Intravenous Anesthetics on Myofilament Ca2+ Sensitivity in Pulmonary Venous Smooth Muscle
Anesth. Analg., November 1, 2007; 105(5): 1278 - 1286.
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