Role of nitric oxide and potassium channels in the cholinergic relaxation of rabbit ear and femoral arteries: effects of cooling

Physiological features of cells and microvasculature under the local hypothermia influence

Hypothermia or cold therapy is the local or systemic application of cold for therapeutic purposes. Local application of cold is used to control inflammation: pain and swelling, hematoma and trismus reduction. Despite the frequent use of cooling in prosthodontic rehabilitation and in physical therapy, as evidenced by many reports in the literature, there is scientific documentation that suggests disadvantages of using this treatment in maxillofacial surgery and oral surgery. Also the clinical studies that have been carried out in maxillofacial surgery and oral surgery have been conducted in an empirical manner, which casts doubt on the results. In view of this, it is relevant to study the mechanisms of microcirculatory preconditioning and hypothermia. This physiological process is so interesting for the development of medical devices of controlled hardware hypothermia to prevent inflammatory symptoms at the stage of rehabilitation by targeting the vascular and cellular component of the inflammatory process in different areas of the human body. To date, the use of local hardware controlled hypothermia in various pathological conditions in humans is a topical trend in medicine. Microcirculatory bloodstream is directly related to temperature factors. Although there are concepts of vascular spasm or dilatation in the microcirculatory bloodstream during systemic hypothermia, there are no reliable data on the cellular and vascular reactions during local hypothermia. In this paper, a search for fundamental and current scientific work on the topic of cellular and vascular changes under the influence of hypothermia was conducted. The search for data revealed that the mechanisms of intracellular hypothermia are of particular interest for the development of therapeutic treatments after surgical interventions in areas with extensive blood supply. With this in mind, it is relevant to investigate several areas: the role of endothelium, glycocalyx and blood cells in microcirculatory-mediated preconditioning and intracellular hypothermia, and in the molecular mechanism that regulates these processes, whether they occur in the same way in all tissues.

Physiological features of cells and microvasculature under the local hypothermia influence

Hypothermia or cold therapy is the local or systemic application of cold for therapeutic purposes. Local application of cold is used to control inflammation: pain and swelling, hematoma and trismus reduction. Despite the frequent use of cooling in prosthodontic rehabilitation and in physical therapy, as evidenced by many reports in the literature, there is scientific documentation that suggests disadvantages of using this treatment in maxillofacial surgery and oral surgery. Also the clinical studies that have been carried out in maxillofacial surgery and oral surgery have been conducted in an empirical manner, which casts doubt on the results. In view of this, it is relevant to study the mechanisms of microcirculatory preconditioning and hypothermia. This physiological process is so interesting for the development of medical devices of controlled hardware hypothermia to prevent inflammatory symptoms at the stage of rehabilitation by targeting the vascular and cellular component of the inflammatory process in different areas of the human body. To date, the use of local hardware controlled hypothermia in various pathological conditions in humans is a topical trend in medicine. Microcirculatory bloodstream is directly related to temperature factors. Although there are concepts of vascular spasm or dilatation in the microcirculatory bloodstream during systemic hypothermia, there are no reliable data on the cellular and vascular reactions during local hypothermia. In this paper, a search for fundamental and current scientific work on the topic of cellular and vascular changes under the influence of hypothermia was conducted. The search for data revealed that the mechanisms of intracellular hypothermia are of particular interest for the development of therapeutic treatments after surgical interventions in areas with extensive blood supply. With this in mind, it is relevant to investigate several areas: the role of endothelium, glycocalyx and blood cells in microcirculatory-mediated preconditioning and intracellular hypothermia, and in the molecular mechanism that regulates these processes, whether they occur in the same way in all tissues.

Moderate hypothermia and responses to calcium channel blockers - Role of the nitric oxide

Moderate hypothermia (25-31 °C) may have a significant influence on vascular tone. At present, very little is known about the role of endothelial nitric oxide on the hypothermia-induced responses. In this study, we investigated the effect of hypothermia (to 28 °C) on the vasodilatation induced by verapamil, a phenylalkylamine calcium channel blocker (10^-9-3 × 10^-4 M) and dihydropyridines, amlodipine (10^-9-3 × 10^-4 M), and benidipine (10^-9-10^-3 M) on 5-hydroxytryptamine (5-HT or serotonin) precontracted calf cardiac veins. Furthermore, the role of nitric oxide in the hypothermia-induced responses was analyzed. Ring preparations of veins obtained from calf hearts were suspended in organ baths containing 15 ml of Krebs-Henseleit solution, maintained at 37 °C, and continuously gassed with 95% O₂-5% CO₂. After a resting period, verapamil, amlodipine, and benidipine were applied cumulatively on serotonin (10^-6 M) precontracted calf cardiac vein rings and induced concentration-dependent relaxations. In another part of the study, the medium temperature was decreased to 28 °C after the preparations were contracted with 5-HT, then cumulative concentrations of verapamil, amlodipine, or benidipine were added. During hypothermia, the pIC₅₀ value, but not the maximal response, to all blockers were significantly higher than at 37 °C. Hypothermia in the presence of N^G-nitro-l-arginine methyl ester (L-NAME, 10^-4 M) decreased the pIC₅₀ and E_max values to verapamil, amlodipine, and benidipine. Only one blocker was tested in each preparation. These results suggest that nitric oxide may play a role in the hypothermia-induced changes in vasodilation caused by verapamil, amlodipine, and benidipine in calf cardiac vein, but further research is needed to explain the complete mechanism.

Enhancing vascular relaxing effects of nitric oxide-donor ruthenium complexes

Ruthenium-derived complexes have emerged as new nitric oxide (NO) donors that may help circumvent the NO deficiency that impairs vasodilation. NO in vessels can be produced by the endothelial cells and/or released by NO donors. NO interacts with soluble guanylyl-cyclase to produce cGMP to activate the kinase-G pathway. As a result, conductance arteries, veins and resistance arteries dilate, whereas the cytosolic Ca2+ levels in the smooth muscle cells decrease. NO also reacts with oxygen or the superoxide anion, to generate reactive oxygen species that modulate NO-induced vasodilation. In this article, we focus on NO production by NO synthase and discuss the vascular changes taking place during hypertension originating from endothelial dysfunction. We will describe how the NO released from ruthenium-derived complexes enhances the vascular effects arising from failed NO generation or lack of NO bioavailability. In addition, how ruthenium-derived NO donors induce the hypotensive effect by vasodilation is also discussed.

Role of the nitric oxide on diazoxide-induced relaxation of the calf cardiac vein and coronary artery during cooling

The effects of cooling (to 28 degrees C) on the vasodilation induced by diazoxide (10(-9)-3 x 10(-4) M) on carbachol-pre-contracted calf cardiac vein and coronary artery and the role of nitric oxide in these effects were analyzed. Diazoxide produced concentration-dependent relaxation of calf cardiac vein and coronary artery rings pre-contracted with carbachol (10(-6) M). During cooling, the pIC(50) values, but not the maximal responses, to diazoxide were significantly lower than at 37 degrees C in both preparations. Cooling to 28 degrees C in the presence of N(G)-nitro-L-arginine methyl ester (10(-4) M) did not modify the effect of temperature both in cardiac vein and coronary artery. These results suggest that cooling-induced changes of diazoxide in calf cardiac vein and coronary artery are independent of nitric oxide.

The effect of glibenclamide on cutaneous laser-Doppler flux

The K(ATP) channels play a crucial role in regulation of vascular tone in conditions of hypoxia. Whether they contribute to peripheral blood flow regulation in human cutaneous microcirculation during a non-hypoxic state is the matter of conflicting in vivo studies that have used plethysmographic method. Our aim was therefore to elucidate the role of K(ATP) channels in human skin microcirculation in three different conditions that evoke different interplays of vascular mechanisms; during resting conditions, during the postocclusive vasodilatation and in the vasoconstriction response to local cold exposure. The laser-Doppler (LD) skin response was monitored in 12 healthy volunteers on the skin of the fingertips of both hands at rest, after the release of an 8-min digital arteries occlusion, and during local cooling of one hand at 15 degrees C. We compared the direct (at the measuring site) and the indirect (at the contralateral non-cooled hand) LD flux response after intradermal microinjection of saline solution (1 mul) and after a microinjection of the K(ATP) channel blocker glibenclamide (8 muM saturated solution) at the measuring site after obtaining the dose-dependent effect of glibenclamide. The effect of the saline solution was used as a reference value. There was a statistically significant lower resting LD flux after the microinjection of glibenclamide 273.6+/-36 PU when compared to the values obtained after the application of the saline solution 375.8+/-31 PU (paired t-test, p=0.016). Glibenclamide also significantly reduced the relative area under the LD flux curve during the PRH response 14551+/-2508 PU*s vs. 6402+/-1476 PU*s (paired t-test, p=0.01) and increased the principal frequency of postocclusive PRH oscillations 0.0931+/-0.01 Hz vs. 0.1309+/-0.02 Hz (p=0.01). In addition, glibenclamide significantly decreased the LD flux during both the direct and indirect response to local cold exposure when compared to the application of saline solution (paired t-test, p<0.01). Our results support the conjecture that ATP sensitive K(+) channels are importantly involved in blood flow regulation of human skin microcirculation in PRH response, in resting conditions as well as in microvascular local cold response.

Reversible impairment of endothelium-dependent relaxation in golden hamster carotid arteries during hibernation

The effects of hibernation on endothelium-dependent vasodilatation were investigated in the golden hamster carotid artery, paying special attention to hibernating body temperature (10 degrees C). To record mechanical and electrical membrane responses, we applied pharmacological (organ bath) and electrophysiological (microelectrode) techniques, using acetylcholine (ACh; 0.001-100 microM) and ATP (0.01-1000 microM) for endothelium-dependent vasodilatation and sodium nitroprusside (SNP; 0.05-10 microM) for endothelium-independent vasodilatation. At 34 degrees C, ACh, ATP and SNP each induced a relaxation or a hyperpolarization, and these responses were similar in all the preparations from control and hibernated animals. At 10 degrees C, on the other hand, ACh-induced relaxations and hyperpolarizations were reduced to approximately 35 % and 50 % of the euthermic level in controls and 1 % and 4 % of the euthermic level in hibernated animals, respectively. In contrast, at 10 degrees C, ATP induced only a contraction or depolarization in all preparations with no significant difference between control and hibernated animals. SNP-induced relaxations and hyperpolarizations obtained at 34 degrees C were not attenuated by cooling to 10 degrees C. In the presence of a P2X receptor blocker, pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 5 microM), at 34 degrees C ATP-induced relaxations and hyperpolarizations were significantly enhanced whereas no responses were induced by ATP at 10 degrees C. After endothelium removal, on the other hand, ATP induced only a contraction or depolarization at both 34 degrees C and 10 degrees C. These results suggest that depression of endothelium-dependent vasodilator responses to ACh and ATP may occur in the hibernating golden hamster carotid artery.

Role of elastic fibers in cooling-induced relaxation

The objective of this work was to confirm the main role of elastic fibers in differing responses of certain vessels during cooling from 37 to 8 degrees C. Previous results have shown that the nature of the vessel (conduit vessel vs muscular vessel) determines the different behavior (dilatation vs contraction) of isolated vessel segments when temperature decreases from 37 to 8 degrees C. In this work, it has been demonstrated that vessels with a great amount of elastic fibers show a dilatation when cooling. On the other hand, muscular vessels with fewer elastic fibers, such as the renal artery, undergo a contraction. The output of calcium from intracellular stores causes contraction of the renal artery during cooling. In this vessel, vasodilatation occurs only when mechanisms of smooth muscle contraction are inactive, as is the case with vessels that have undergone a cold storage period of 48 h. The results presented in this work confirm that there are two main effects, which directly depend on the vessel origin. In conduit arteries, the decrease of temperature induces a vascular relaxation, dependent on the elastic component of the vessel wall. In muscular vessels, the predominant effect is cooling-induced contraction due to an increase of intracellular calcium. This cooling-induced contraction needs the vessel to be in optimal conditions with an active metabolism of the muscular cells. These results are a crucial issue in the sense of explaining several biomedical mechanisms where hypothermia is implicated. The type of vessel implicated in procedures, such as isolated organ perfusion, extracorporeal circulation, and bypass surgery, must be taken into account.

Guanylate cyclase and not ATP-dependent K(+) channels seems temperature-dependent in smooth muscle relaxation of human umbilical arteries

The effects of K(+) channel opener, nicorandil [N-(2-hydroxyethyl)-nicotinamide nitrate], on isolated human umbilical arteries were investigated at two temperatures: 37 degrees C and 25 degrees C. The purpose of this investigation was: (1) to confirm the relaxant effect of nicorandil, (2) to elucidate the influence of endothelium and temperature on nicorandil-induced relaxation, (3) to determine which of guanylate cyclase or ATP-sensitive K(+) channels was implicated in temperature-induced relaxation of smooth muscles. Rings, 3-mm-wide, were suspended in organ chambers for isometric force measurement. All solutions were aerated with 95% O(2)-5% CO(2) and maintained at 37 degrees C or 25 degrees C (cooling), pH 7.4. The presence of an intact endothelium was confirmed by immunohistochemistry. During the first set of experiments after contraction with 5-hydroxytryptamine (5-HT 10(-5) M), nicorandil (10(-9)-10(-4) M) was added to the organ chambers with controls and in with rings incubated with L-arginine, N-nitro-L-arginine (L-NNA) an inhibitor of nitric oxide (NO) synthase, [1-H-(1,2,4) oxadiazole (4,3-a) quinoxalin-1-one] (ODQ), a specific inhibitor of guanylate cyclase, or glibenclamide, an antagonist of nicorandil, all at 10(-5) M. In another set of experiments, rings were contracted with 5-HT (10(-5) M) and relaxed with 3-morpholinosydnonimine [SIN-1 (10(-9)-3x10(-5) M) or cromakalim (10(-9)-3x10(-5) M)]. Our results showed that nicorandil induced concentration-dependent relaxation. At 37 degrees C, in the control, the maximum relaxation was 90+/-5%, and 60+/-8% at 25 degrees C (P<0.01). However, the relaxation at 37 degrees C or 25 degrees C remained unchanged after pretreatment with L-arginine, L-NNA, this suggests that the same concentration of drugs used in this type of vessel does not appear to modulate the relaxant effect of nicorandil. On the other hand, we observed that the relaxant effect of SIN-1 was 72+/-5% at 37 degrees C and only 28+/-7% at 25 degrees C (P<0.01). However, relaxations with cromakalim were partly influenced by cooling. In the presence of ODQ, the nicorandil-induced relaxation observed at 37 degrees C or 25 degrees C was less than that in the control and in the rings incubated with glibenclamide. These results for human umbilical arteries indicate that cooling decreases the relaxation response of smooth muscles and that this is partly due to a decreased response to guanylate cyclase.

Paradoxical effects of temperature on vascular tone

Temperature may have significant influence on vascular tone in such cases as organ preservation, coronary bypass surgery, and extracorporeal circulation. The aim of this research was to study the direct effect of temperature variation on vascular tone in an attempt to elucidate the mechanisms involved. In a first series of experiments, the isometric tension of two different vessels (rat thoracic aorta and pig renal branch artery) was studied at different temperatures. To study the role of calcium in this response, a second series of experiments was performed. In this the vessels were incubated with the intracellular chelator BAPTA/AM. Further experiments were performed to test the effect of cold storage. Our results show that changes in temperature lead to different results in pig renal artery and rat aorta. A decrease in temperature induced a highly reproducible relaxation in rat aorta, whereas pig renal artery presented cooling-induced contraction. Moreover, whereas calcium depletion failed to inhibit cooling-induced relaxation in rat aorta, it did not provoke cooling-induced contraction in pig renal artery. Similar responses were obtained with cold storage and calcium depletion. We intend to demonstrate that, just as the effect of temperature variation on pig renal artery is due to a metabolic mechanism, its effect on rat aorta may be due to structural factors. This hypothesis is supported by the result of histological studies which demonstrate a higher proportion of elastin fibres in rat aorta than in pig renal artery.

Relaxant effects of sodium nitroprusside and NONOates in goat middle cerebral artery: delayed impairment by global ischemia-reperfusion

Global cerebral ischemia and subsequent reperfusion induce early impairment of the vasodilator responses to hypercapnia and vasoactive substances. Nitric oxide (NO) is involved in the regulation of cerebral blood flow (CBF) in both health and disease. The present study was designed to assess possible changes in the cerebrovascular reactivity to NO donors induced by cerebral ischemia-reperfusion in goats. Female goats (n = 9) were subjected to 20 min global cerebral ischemia under halothane/N2O anesthesia. Sixteen additional goats were sham-operated as a control group. One week later the effects of ischemia-reperfusion on relaxations to NO donors sodium nitroprusside (SNP), diethylamine/NO (DEA/NO), diethylenetriamine/NO (DETA/NO), and spermine/NO (SPER/NO) were studied in rings of middle cerebral artery (MCA) isolated in an organ bath for isometric tension recording. SNP, DEA/NO, DETA/NO, and SPER/NO induced concentration-dependent relaxations of MCA precontracted with KCl (DEA/NO > SPER/NO > SNP > DETA/NO) or with endothelin-1 (DEA/NO > SNP > SPER/NO > DETA/NO). Relaxations were always higher in endothelin-1-precontracted arteries. One week after cerebral ischemia concentration-response curves to SNP and DEA/NO were displaced to the right, indicating a reduction in relaxant potency of NO donors. The classical nitrovasodilator SNP and NONOates induce relaxation of isolated goat MCA which is partially inhibited by arterial depolarization. Global cerebral ischemia followed by reperfusion induces delayed impairment of the relaxant effects of NO on cerebrovascular smooth muscle, which results in reduced vasodilatory potency of NO donors in large cerebral arteries.

Comparative relaxant effects of the NO donors sodium nitroprusside, DEA/NO and SPER/NO in rabbit carotid arteries

1. Sodium nitroprusside (SNP, 10(-9)-3x10(-4) M), diethylamine/NO complex (DEA/NO, 10(-9)-10(-4) M) and spermine/NO complex (SPER/NO, 10(-8)-3x10(-4) M) induced concentration-dependent relaxation of isolated rabbit carotid arteries precontracted with KCl (50 mM) or with histamine (3x10(-6) M). 2. In KCl-precontracted arteries the order of potency was SNP=DEA/NO>SPER/NO, and in histamine-precontracted arteries the order of potency was SNP>DEA/NO>SPER/NO. Relaxations to the three NO donors were significantly higher in histamine-precontracted arteries than in KCl-precontracted arteries. 3. The guanylyl cyclase inhibitor methylene blue (10(-5) M) significantly inhibited relaxations to the three NO donors in histamine-precontracted arteries and, to a lesser extent, in KCl-precontracted arteries. 4. In conclusion, the NO donors SNP, DEA/NO and SPER/NO induce quantitatively different relaxation of rabbit carotid artery. Both, lower relaxant effects in depolarized arteries and inhibition of relaxation by methylene blue indicate the mediation of cGMP formation in the relaxant effects of the three NO donors.

Effect of heating on the hemodynamic responses to vasoactive agents

During hyperthermia, vasoconstrictor tone in the viscera is lost despite high levels of sympathetic neural outflow and plasma catecholamines, suggesting that vascular responsiveness to adrenergic receptor stimulation is reduced. The purpose of this study was to determine whether adrenoceptor-mediated control of vascular resistance is altered at high body core temperatures. The hemodynamic responses to adrenoceptor agonists were examined in chloralose-anesthetized rats heated to colonic temperatures (Tco) of 37, 39, and 41.5 degrees C. Elevating Tco to 39 degrees C did not alter the hemodynamic responses to any of these agents. Further heating to 41.5 degrees C markedly attenuated the hemodynamic responses to alpha- and beta-adrenoceptor agonists. Similarly, the regional and systemic hemodynamic responses to ANG II and endothelin were also reduced at 41.5 degrees C. In contrast, the hemodynamic responses to endothelium-dependent and -independent vasodilator agents were unchanged or slightly reduced at 41.5 degrees C. The blunted hemodynamic responses observed at 41.5 degrees C indicate that vascular reactivity to vasoconstrictor agents is reduced with hyperthermia and suggest that this nonspecific change in vascular responsiveness may contribute the circulatory collapse associated with high body temperatures.

Effect of heating on vascular reactivity in rat mesenteric arteries

Vasoconstriction in the viscera is one of the primary cardiovascular adjustments to heating. Local temperature can influence vascular responsiveness to catecholamines and sympathetic nerve activity. Therefore, we hypothesized that heating would alter vascular reactivity in rat mesenteric arteries. Concentration-response curves to norepinephrine, phenylephrine, potassium chloride (KCl), calcium, acetylcholine, and sodium nitroprusside were obtained in vascular ring segments from rat mesenteric arteries at 37 and 41 degrees C. In some rings, basal tension increased slightly during heating. Heating to 41 degrees C did not alter the contractile responses to norepinephrine in endothelium-intact or -denuded rings but augmented the responses to KCl and calcium in endothelium-intact rings. The potentiating effect of heating on the responses to KCl and calcium was eliminated after endothelium removal. In contrast, the relaxant responses to acetylcholine and sodium nitroprusside were significantly attenuated at 41 degrees C. Collectively, these results demonstrate that heating alters vascular reactivity in rat mesenteric arteries. Furthermore, these data imply that heating reduces the ability of vascular smooth muscle to relax, possibly due to a decrease in sensitivity to nitric oxide.

Role of endothelin receptors, calcium and nitric oxide in the potentiation by endothelin-1 of the sympathetic contraction of rabbit ear artery during cooling

1. To examine further the potentiation by endothelin-1 on the vascular response to sympathetic stimulation, we studied the isometric response of isolated segments, 2 mm long, from the rabbit central ear artery to electrical field stimulation (1-8 Hz), under different conditions, at 37 degrees C and during cooling (30 degrees C). 2. Electrical stimulation produced frequency-dependent contraction, which was reduced (about 63% for 8 Hz) during cooling. At 30 degrees C, but not at 37 degrees C, endothelin-1 (1, 3 and 10 nM) potentiated the contraction to electrical stimulation in a dose-dependent way (from 43 +/- 7% to 190 +/- 25% for 8 Hz). 3. This potentiation by endothelin-1 was reduced by the antagonist for endothelin ETA receptors BQ-123 (10 microM) but not by the antagonist for endothelin ETB receptors BQ-788 (10 microM). The agonist for endothelin ETB receptors IRL-1620 (0.1 microM) did not modify the contraction to electrical stimulation. 4. The blocker of L-type Ca2+ channels verapamil (10 microM l-1) reduced (about 72% for 8 Hz) and the unspecific blocker of Ca(2+)-channels NiCl2 (1 mM) practically abolished (about 98%), the potentiating effects of endothelin-1 found at 30 degrees C. 5. Inhibition of nitric oxide synthesis with NG-nitro-L-arginine (L-NOARG, 0.1 mM) increased the contraction to electrical stimulation at 30 degrees C more than at 37 degrees C (for 8 Hz, this increment was 297 +/- 118% at 30 degrees C, and 66 +/- 15% at 37 degrees C). Endothelium removal increased the contraction to electrical stimulation at 30 degrees C (about 91% for 8 Hz) but not at 37 degrees C. Both L-NOARG and endothelium removal abolished the potentiating effects of endothelin-1 on the response to electrical stimulation found at 30 degrees C. 6. These results in the rabbit ear artery suggest that during cooling, endothelin-1 potentiates the contraction to sympathetic stimulation, which could be mediated at least in part by increasing Ca2+ entry after activation of endothelin ETA receptors. This potentiating effect of endothelin-1 may require the presence of an inhibitory tone due to endothelial nitric oxide.

Role of Na+/K+ ATPase on the relaxation of rabbit ear and femoral arteries

The role of Na+/K+ ATPase in vascular relaxation has been studied by determining its inhibitory effects on 2-mm segments from rabbit central ear and femoral arteries, mounted for isometric tension recording. Acetylcholine (10(-8)-10(-4) M), the nitric oxide donor sodium nitroprusside (10(-8)-3 x 10(-4) M), the potassium channel agonist cromakalim (10(-8) x 10(-5) M), histamine (10(-8)-10(-4) M) in the presence of the H1 antagonist chlorpheniramine (10(-5) M), and papaverine (10(-6)-3 x 10(-4) M) all produced arterial relaxation in ear and femoral arteries precontracted with endothelin 1. Addition of potassium (6 x 10(-3)-1.2 x 10(-2) M) caused relaxation of the same arteries preincubated in potassium-free medium. Ouabain (10(-5) M) an inhibitor of Na+/K+ ATPase, reduced the relaxation of ear arteries, but not of femoral arteries, in response to acetylcholine; it also reduced the response to sodium nitroprusside, cromakalim or histamine, and abolished the relaxation to potassium, but did not modify the response to papaverine, in both types of artery. These results suggest that Na+/K+ ATPase might play a role in the relaxation of ear and femoral arteries to nitrovasodilators, to potassium channel openers and to activation of histamine receptors, and that Na+/K+ ATPase might play a role in the cholinergic relaxation of ear, but not femoral arteries, suggesting that the mechanism of cholinergic relaxation might differ in each type of artery.