Thomas A. Stekiel

Region of Epidural Blockade Determines Sympathetic and Mesenteric Capacitance Effects in Rabbits

Background The mechanisms producing hemodynamic changes during epidural anesthesia are incompletely understood. The role of capacitance changes in the splanchnic venous bed can be clarified by comparing blocks of differing segmental distributions. Specifically, we speculated that blocks that include the innervation to the mesenteric circulation alter hemodynamics, sympathetic activity, and venous capacitance to a greater extent than blocks without blockade of sympathetic nerves to this critical vascular bed.

Effects of Epidural and Systemic Lidocaine on Sympathetic Activity and Mesenteric Circulation in Rabbits

BackgroundThe mechanisms producing hemodynamic changes during epidural anesthesia are incompletely understood. This study examines the sympathetic block and splanchnic venodilatation that result from extensive thoracolumbar epidural anesthesia in rabbits using direct measurements of sympathetic efferent nerve activity (SENA) and mesenteric vein diameter (VD). MethodsEpidural catheters were inserted in rabbits anesthetized with α-chloralose, paralyzed with vecuronium, and receiving mechanical ventilation. Arterial pressure was monitored with a femoral cannula, heart rate was determined from the pressure signal, SENA was measured from a postganglionic splanchnic nerve, and VD was measured from segments of ileum externalized in situ. Epidural anesthesia was induced with 0.4 ml/kg lidocaine, using concentrations of either 0.5, 1, or 1.5%. Control animals received intramuscular lidocaine in a dose of either 6 or 15 mg/kg. After recovery from epidural anesthesia, complete sympathetic blockade was induced by systemic administration of the ganglionic blocker hexamethonium (HX). Individual groups included from five to eight animals. ResultsA mild decrease in arterial pressure and SENA followed the larger dose of intramuscular lidocaine, but no changes occurred in VD in the control animals exposed to systemic lidocaine at levels comparable to that in the epidural groups (0.96–3.58 μg/ml). Epidural injectate extended from T2 to L5. All concentrations of epidural lidocaine produced comparable degrees of hypotension (-53.5 to-61.4%), decreased SENA (-82.6 to-95.5%), and increased VD (7.5 to 10.2%). The duration of the changes was greater with more concentrated lidocaine. Hexamethonium produced changes in arterial pressure and VD comparable to those evoked by epidural anesthesia. ConclusionsEpidural anesthesia increases splanchnic venous capacitance by markedly decreasing splanchnic sympathetic nerve activity.

Effects of Volatile Anesthetic Agents on In Situ Vascular Smooth Muscle Transmembrane Potential in Resistance- and Capacitance-regulating Blood Vessels

Introduction This study was designed to compare the inhibitory effect of inhaled volatile anesthetic agents on in situ sympathetic neural versus nonneural regulation of vascular smooth muscle transmembrane potentials as correlates of vascular smooth muscle tone in resistance‐ and capacitance‐regulating blood vessels. Methods Vascular smooth muscle transmembrane potentials were measured in situ with glass microelectrodes in neurally intact, small (200–300 m OD) mesenteric arteries and veins of rats before, during, and after inhaled halothane, isoflurane, or sevoflurane (0.5 or 1.0 minimum alveolar concentration [MAC]). Such transmembrane potentials and their anesthetically induced changes were compared, respectively, with those measured in similar vessel preparations after local sympathetic neural denervation with 6‐hydroxydopamine. Results In neurally intact vessels, transmembrane potentials (in millivolts, mean +/‐ SD) before inhalation of the anesthetic agent were ‐39 +/‐ 2.8 (artery) and ‐43 +/‐ 4.6 (vein). At 1.0 MAC, halothane, isoflurane, and sevoflurane induced respective hyperpolarizations (in millivolts, mean +/‐ SD) of 9 +/‐ 3.1, 6 +/‐ 2.7, and 9 +/‐ 4.0 in arteries and 6 +/‐ 4.4, 2.8 +/‐ 3.0, and 8.7 +/‐ 5.6 in veins. Sympathetic denervation significantly attenuated these hyperpolarizations (except for venous response to isoflurane). At 0.5 MAC, transmembrane potential responses to all three volatile anesthetic agents were small and not consistently significant in either the intact or denervated vessels. Conclusions In resistance‐regulating arteries in situ, inhaled halothane, isoflurane, and sevoflurane (1.0 MAC) attenuate both sympathetic neural and nonneural regulation of vascular smooth muscle transmembrane potentials (and tone). In capacitance‐regulating veins in situ, sevoflurane (1.0 MAC) also attenuates both regulatory mechanisms, whereas halothane and isoflurane primarily attenuate nonneural mechanisms. At 0.5 MAC, none of these agents significantly affected either mode of regulation of vascular smooth muscle transmembrane potentials in arteries or veins.

Potassium channel-mediated hyperpolarization of mesenteric vascular smooth muscle by isoflurane

BACKGROUND A primary source of calcium (Ca2+) necessary for excitation contraction in vascular smooth muscle (VSM) is influx via voltage-dependent Ca2+ channels. Thus, force generation in VSM is coupled closely to resting transmembrane potential, which itself is primarily a function of potassium conductance. Previously, the authors reported that volatile anesthetics hyperpolarize VSM of small mesenteric resistance arteries and capacitance veins. The current study was designed to determine whether isoflurane-mediated hyperpolarization is the result of specific effects on one or more of four types of potassium channels known to exist in VSM. METHODS Transmembrane potentials (Em) were recorded from in situ mesenteric capacitance and resistance vessels in Sprague-Dawley rats weighing 250-300 g. In separate experiments, selective inhibitors of each of four types of potassium channels known to exist in VSM were administered in the superfusate of the vessel preparations to assess their effects on isoflurane-mediated hyperpolarization. RESULTS Resting VSM Em ranged from -38 to -43 mV after local sympathetic denervation. Isoflurane produced a significant hyperpolarization (2.7-4.3 mV), whereas each potassium channel inhibitor significantly depolarized (2.8-8.5 mV) the VSM. Both 100 nM iberiotoxin (inhibitor of high conductance calcium-activated potassium channels) and 1 microM glybenclamide (inhibitor of adenosine triphosphatase-sensitive potassium channels) significantly inhibited VSM hyperpolarization induced by 1 MAC (minimum alveolar concentration) levels of inhaled isoflurane (0.1-0.9 mV Em change, which was not significant). In contrast, isoflurane hyperpolarized the VSM significantly despite the presence of 3 mM 4 aminopyridine (inhibitor of voltage-dependent potassium channels) or 10 microM barium chloride (an inhibitor of inward rectifier potassium channels) (3.7-8.2 mV change in VSM Em). CONCLUSIONS These results suggest that isoflurane-mediated hyperpolarization (and associated relaxation) of VSM can be attributed in part to an enhanced (or maintained) opening of calcium-activated and adenosine triphosphate-sensitive potassium channels but not voltage-dependent or inward rectifier potassium channels.

Mechanisms of isoflurane-mediated hyperpolarization of vascular smooth muscle in chronically hypertensive and normotensive conditions.

Background The purpose of this study was to compare the effects of isoflurane on membrane and intracellular mechanisms that regulate vascular smooth muscle (VSM) transmembrane potential (Em; which is related to VSM tone) in the spontaneously hypertensive rat (SHR) model of essential hypertension and its normotensive Wistar-Kyoto (WKY) control. Methods Vascular smooth muscle Em values were measured in situ in locally denervated, superfused, intact, small (200–300-&mgr;m OD) mesenteric arteries and veins in anesthetized 9– 12-week-old SHR and WKY. Effects of 1.0 minimum alveolar concentration (0.60 mm) superfused isoflurane on VSM Em were measured before and during superfusion with specific inhibitors of VSM calcium-activated (KCa) and adenosine triphosphate–regulated (KATP) potassium channels, and with endogenous mediators of vasodilatation (nitric oxide, cyclic guanosine monophosphate, protein kinase G, cyclic adenosine monophosphate, and protein kinase A). Results Isoflurane significantly hyperpolarized small arteries (5 ± 3.4 mV) and veins (6 ± 4.7 mV) (pooled SHR and WKY, mean ± SD). Inhibition of KCa and KATP channels, cyclic adenosine monophosphate, and protein kinase A, but not nitric oxide, cyclic guanosine monophosphate, and protein kinase G, abolished such hyperpolarization equally in SHR and WKY vessels. Conclusions Isoflurane-induced in situ VSM hyperpolarization in denervated, small mesenteric vessels involves a similar activation of KCa and KATP channels and cyclic adenosine monophosphate, but not nitric oxide or cyclic guanosine monophosphate, second messenger pathways in both SHR and WKY. A greater isoflurane-induced VSM hyperpolarization (observed previously in neurally intact SHR vessels) suggests enhanced inhibition of elevated sympathetic neural input as a major mechanism underlying such hyperpolarization (and coupled relaxation) in this neurogenic model of hypertension.

The mechanisms of propofol-mediated hyperpolarization of in situ rat mesenteric vascular smooth muscle

Previously, we reported that propofol hyperpolarizes vascular smooth muscle (VSM) cells of small arteries and veins. The current study was designed to determine whether propofol-mediated hyperpolarization is the result of specific effects on potassium channels known to exist in VSM and on steps in the intracellular nitric oxide (NO), cyclic guanosine monophosphate (cGMP), and cyclic adenosine monophosphate (cAMP) second messenger pathways. VSM transmembrane potentials (Em) were measured in situ in sympathetically denervated, small mesenteric arteries and veins of Sprague-Dawley rats. Effects of propofol on VSM Em were determined before and during superfusion with specific inhibitors of VSM calcium-activated (KCa), adenosine triphosphate-sensitive (KATP), voltage-dependent (Kv), and inward rectifying (KIR) potassium channels and with endogenous mediators of vasodilation. Propofol significantly hyperpolarized VSM in small mesenteric vessels. This hyperpolarization was abolished on inhibition of KCa and KATP channel activity and on inhibition of NO and cGMP (but not cAMP). Assuming a close inverse correlation between the magnitude of VSM Em and contractile force, these results suggest that propofol induces hyperpolarization and relaxation in denervated, small mesenteric vessels by activation of KCa and KATP channels. Such channel activation may be mediated by activation of NO and cGMP, but not cAMP, second messenger pathways.

The Inhibitory Action of Halothane on Reflex Constriction in Mesenteric Capacitance Veins

Potent inhalational anesthetics depress autonomic reflex responses at multiple sites. Most studies emphasize cardiac chronotropic changes and changes in systemic blood pressure. Recently, active reflex venoconstriction of 500-1,000 microns O.D. mesenteric veins has been demonstrated. In the current study, the effects of halothane on the reflex responses of similar mesenteric veins were measured. Mesenteric vein diameter and intravenous pressure were measured in 500-1,000 microns O.D. veins from the mesentery of segments of terminal ileum externalized in situ from 27 New Zealand white rabbits anesthetized with alpha-chloralose. Mean arterial pressure was measured with femoral arterial cannulation, and heart rate was determined from the arterial pressure signal. In a separate group of six animals, sympathetic efferent nerve activity was measured from a postganglionic splanchnic nerve. Reflex venoconstriction and increases in mean arterial pressure and mesenteric vein pressure in response to bilateral carotid occlusion were attenuated by 0.5% and 1% inhaled halothane but not by superfusate equilibrated with 3% halothane. Decreases in mesenteric vein diameter and increases in mesenteric vein pressure in response to celiac ganglion stimulation were unaffected by both 0.5% inhaled halothane and superfusate equilibrated with 5% halothane. The bilateral carotid occlusion reflex-mediated increase in sympathetic efferent nerve activity was depressed by both 0.5% and 1% inhaled halothane. The effect of inhaled halothane on prestimulation baseline vein diameter was inconsistent. Superfusate equilibrated with 5% but not 3% halothane caused baseline venodilation. These results suggest a mechanism whereby control of venous tone is inhibited by halothane proximal to the postganglionic neuron. This could involve central or ganglionic inhibition.

Mechanism of Mesenteric Venodilatation after Epidural Lidocaine in Rabbits

BackgroundIncreased splanchnic venous capacitance has been observed during extensive thoracolumbar epidural anesthesia in rabbits, but the mechanism is not clear. The present study examines the contributions of intravascular pressure changes, catecholamine levels, neural input, and direct effects of lidocaine to mesenteric venodilatation. MethodsEpidural catheters were inserted in rabbits anesthetized with α-chloralose. Vein diameter was measured by videomicrography from segments of ileum externalized in situ. Plasma epinephrine and norepinephrine levels were measured in animals receiving epidural blockade (0.4 ml/kg lidocaine 1.5%, n = 5) and in control animals given intramuscular lidocaine 15 mg/kg (n = 5). Intraluminal pressure was monitored during the onset of epidural anesthesia (0.4 ml/kg lidocaine 1.0%, n = 9) by a servo-null micropressure technique. The effect of inhibiting norepinephrine release from sympathetic nerves in the mesenteric veins was determined by using topical tetrodotoxin (n = 8) and by assessing the effect of topical lidocaine (10 and 100 μg/ml, n = 5) administered in the solution bathing the mesentery. ResultsEpidural injectate extended from T2 to L5. Plasma epinephrine decreased 68.3 ± 4.4% (mean ± SEM) with epidural anesthesia, and norepinephrine was lower after epidural block than after intramuscular lidocaine (1,868 ± 290 pg/ml vs. 3,049 ± 712 pg/ml). Mesenteric vein pressure decreased 35.3 ± 3.5% and vein diameter increased 10.2 ± 3.3% during epidural blockade. Tetrodotoxin caused mesenteric venodilatation (7.6 ± 2.0%) and prevented venodilatation by subsequent epidural lidocaine. Topical lidocaine 10 μg/kg produced no change in vein diameter, but lidocaine 100 μg/ml increased it 3.5 ± 1.3%. ConclusionsSplanchnic venodilatation during epidural anesthesia is an active process: a decrease in intravenous pressure concurrent with dilatation indicates that vein wall tension diminished. Significant dilatation with tetrodotoxin and lack of dilatation with subsequent epidural block point to a minor role for changes in circulating catecholamines. A direct effect of lidocaine does not contribute to splanchnic venodilatation except when circulating lidocaine concentrations reach very high levels.

Does isoflurane alter mesenteric venous capacitance in the intact rabbit

Volatile anesthetics act at a number of sites to alter cardiovascular function and the response of the cardiovascular system to barostatic reflexes. We examined the effects of isoflurane on reflex regulation of mesenteric venous capacitance vessels. To determine whether isoflurane alters mesenteric venous capacitance, continuous direct observations of mesenteric vein diameter, intravenous pressure, and mesenteric sympathetic efferent nerve activity (SENA) were made in 31 chloralose-anesthetized New Zealand white rabbits. Simultaneous measurements were obtained for aortic pressure and heart rate. The responses to changes in baroreceptor activation by means of either bilateral carotid occlusion (BCO) or aortic nerve stimulation (ANS) were studied in one group of 18 rabbits, while the response to direct electric activation by means of celiac ganglion stimulation (CGS) was studied in another group of 13 rabbits. In both groups, isoflurane vapor was administered at levels of 0.75% or 1.5%, and superfused isoflurane was administered directly to the vessel in doses of either 3% or 5% equilibrated with physiologic salt solution. Anesthetic levels were verified by mass spectrometry for expired gas and by gas chromatography for blood and superfusate levels. Inhaled isoflurane reduced hemodynamic variables and SENA in a dose-dependent fashion, but these same variables were unaffected by superfused isoflurane. One and one-half percent inhaled isoflurane abolished all reflex responses to baroreceptor stimulation in mesenteric capacitance veins and in SENA, but superfused isoflurane produced no corresponding attenuation of reflex responses to baroreceptor stimulation. Neither inhaled nor superfused isoflurane suppressed the reflex venoconstriction in response to CGS. Both inhaled and superfused isoflurane dilated the baseline vein diameter before stimulation. These results indicate that isoflurane does increase the diameter of mesenteric venous capacitance vessels and inhibits reflex-induced constriction of mesenteric veins, whereas mesenteric sympathetic efferent nerve activity decreases and the reflex responses to activation of the carotid sinus and aortic baroreceptors are attenuated by inhaled isoflurane. The mechanism of this action appears to be primarily through the inhibition of central or peripheral sympathetic ganglionic transmission of barostatic control.

The effects of propofol on neural and endothelial control of in situ rat mesenteric vascular smooth muscle transmembrane potentials.

We indirectly assessed the in vivo effect of propofol on sympathetic neural and endothelial control of vascular smooth muscle (VSM) tone in Sprague-Dawley rats by measurement of in situ responses of VSM transmembrane potential (Em) in intact, small mesenteric arteries and veins superfused with physiologic salt solution. Measurements were made before, during, and after propofol infusion (10 and 30 mg · kg−1 · h−1) in sympathetically innervated and locally denervated vessels. Propofol’s effect on Em response to superfusion with acetylcholine (ACh), in physiologic salt solution also containing NG-nitro-l-arginine-methyl-ester and indomethacin, was determined in innervated vessels. At 30 mg · kg−1 · h−1, propofol caused greater arterial VSM hyperpolarization in innervated compared with denervated vessels (4.8 ± 2.0 mV versus 2.8 ± 1.5 mV, respectively). ACh hyperpolarized arterial, but not venous, VSM (e.g., 11.7 ± 2.4 mV at 10−4 M). ACh-induced hyperpolarization was eliminated by 30 mg · kg−1 · h−1 propofol. Assuming a close inverse correlation between magnitude of VSM Em and contractile force, these results suggest that propofol attenuates both sympathetic neural and nonneural regulation of VSM tone. They also suggest that propofol and ACh may act competitively in the second messenger cascade regulating VSM K+ channel activity in mesenteric resistance arteries.