Electrophysiologic mechanism underlying action potential prolongation by sevoflurane in rat ventricular myocytes

BACKGROUND: Despite prolongation of the QTc interval in humans during sevoflurane anesthesia, little is known about the mechanisms that underlie these actions. In rat ventricular myocytes, the effect of sevoflurane on action potential duration and underlying electrophysiologic mechanisms were investigated. METHODS: The action potential was measured by using a current clamp technique. The transient outward K current was recorded during depolarizing steps from -80 mV, followed by brief depolarization to -40 mV and then depolarization up to +60 mV. The voltage dependence of steady state inactivation was determined by using a standard double-pulse protocol. The sustained outward current was obtained by addition of 5 mm 4-aminopyridine. The inward rectifier K current was recorded from a holding potential of -40 mV before their membrane potential was changed from -130 to 0 mV. Sevoflurane actions on L-type Ca current were also obtained. RESULTS: Sevoflurane prolonged action potential duration, whereas the amplitude and resting membrane potential remained unchanged. The peak transient outward K current at +60 mV was reduced by 18 +/- 2% (P < 0.05) and 24 +/- 2% (P < 0.05) by 0.35 and 0.7 mm sevoflurane, respectively. Sevoflurane had no effect on the sustained outward current. Whereas 0.7 mm sevoflurane did not shift the steady state inactivation curve, it accelerated the current inactivation (P < 0.05). The inward rectifier K current at -130 mV was little altered by 0.7 mm sevoflurane. L-type Ca current was reduced by 28 +/- 3% (P < 0.05) and 33 +/- 1% (P < 0.05) by 0.35 and 0.7 mm sevoflurane, respectively. CONCLUSIONS: Action potential prolongation by clinically relevant concentrations of sevoflurane is due to the suppression of transient outward K current in rat ventricular myocytes.     

Mechanisms Underlying the QT Interval-prolonging Effects of Sevoflurane and Its Interactions with Other QT-prolonging Drugs

Background: Sevoflurane prolongs ventricular repolarization in patients, but the mechanisms are not fully characterized. The effects of sevoflurane on many cloned human cardiac ion channels have not been studied, and the interactions between sevoflurane and other drugs that prolong cardiac repolarization have not been detailed.

Methods: The effects of sevoflurane on action potentials and L-type Ca2+ channels in guinea pig myocytes were examined. Sevoflurane's effects on cloned human cardiac K+ channels and the cloned human cardiac Na+ channel were studied. The consequences of combining sevoflurane and the class III antiarrhythmic drugs sotalol or dofetilide on action potential duration were also examined.

Results: Sevoflurane produced an increase in action potential duration at concentrations of 0.3-1 mm. Contrary to most drugs that delay ventricular repolarization, sevoflurane was without effect on the human ether-a-go-go-related gene cardiac potassium channel but instead produced a reduction in KvLQT1/minK K+ channel currents and inhibited the Kv4.3 K+ channel by speeding its apparent rate of inactivation. Sevoflurane had little effect on Na+ and Ca2+ channel currents at concentrations of 1 mm or less. When the authors coadministered sevoflurane with sotalol or dofetilide, synergistic effects on repolarization were observed, resulting in large increases in action potential duration (up to 66%).     

In vitro electrophysiologic effects of morphine in rabbit ventricular myocytes

BACKGROUND: Morphine is widely used in patients undergoing surgical operations and is also reported to mediate cardioprotection of preconditioning. The current study determined effects of morphine at therapeutic to pharmacologic concentrations on cardiac action potential, L-type Ca2+ current (ICa.L), delayed rectifier K+ current (IK), and inward rectifier K+ current (IK1) in isolated rabbit ventricular myocytes. METHODS: Ventricular myocytes were enzymatically isolated from rabbit hearts. Action potential and membrane currents were recorded in current and voltage clamp modes. RESULTS: Morphine at concentrations from 0.01 to 1 microM significantly prolonged cardiac action potential, and at 0.1 and 1 microM slightly but significantly hyperpolarized the resting membrane potential. In addition, morphine at 0.1 microM significantly augmented ICa.L (at +10 mV) from 5.9 +/- 1.9 to 7.3 +/- 1.7 pA/pF (by 23%; P < 0.05 vs. control) and increased IK1 (at -60 mV) from 2.8 +/- 1.0 to 3.5 +/- 0.9 pA/pF (by 27%; P < 0.05 vs. control). Five microM naltrindole (a selective delta-opioid receptor antagonist) or 5 microM norbinaltorphimine (a selective kappa-opioid receptor antagonist) prevented the increase in ICa.L induced by morphine, but 5 microM CTOP (a selective mu-opioid receptor antagonist) did not. The three types of opioid antagonists did not affect the augmentation of IK1 by morphine. Morphine had no effect on IK. CONCLUSIONS: These results indicate that morphine prolongs action potential duration by increasing ICa.L, an effect mediated by delta- and kappa-opioid receptors. It also hyperpolarizes cardiac resting membrane potential by increasing IK1, which is not mediated by opioid receptors.     

In Vitro Electrophysiologic Effects of Morphine in Rabbit Ventricular Myocytes

Background: Morphine is widely used in patients undergoing surgical operations and is also reported to mediate cardioprotection of preconditioning. The current study determined effects of morphine at therapeutic to pharmacologic concentrations on cardiac action potential, L-type Ca2+ current (ICa.L), delayed rectifier K+ current (IK), and inward rectifier K+ current (IK1) in isolated rabbit ventricular myocytes.

Methods: Ventricular myocytes were enzymatically isolated from rabbit hearts. Action potential and membrane currents were recorded in current and voltage clamp modes.

Results: Morphine at concentrations from 0.01 to 1 [mu]m significantly prolonged cardiac action potential, and at 0.1 and 1 [mu]m slightly but significantly hyperpolarized the resting membrane potential. In addition, morphine at 0.1 [mu]m significantly augmented ICa.L (at +10 mV) from 5.9 +/- 1.9 to 7.3 +/- 1.7 pA/pF (by 23%; P < 0.05 vs. control) and increased IK1 (at -60 mV) from 2.8 +/- 1.0 to 3.5 +/- 0.9 pA/pF (by 27%; P < 0.05 vs. control). Five [mu]m naltrindole (a selective [delta]-opioid receptor antagonist) or 5 [mu]m norbinaltorphimine (a selective [kappa]-opioid receptor antagonist) prevented the increase in ICa.L induced by morphine, but 5 [mu]m CTOP (a selective [mu]-opioid receptor antagonist) did not. The three types of opioid antagonists did not affect the augmentation of IK1 by morphine. Morphine had no effect on IK.     

The effects of sevoflurane and propofol on QT interval and heterologously expressed human ether-a-go-go related gene currents in Xenopus oocytes

Sevoflurane can induce prolongation of the cardiac QT interval by inhibiting the repolarization phase of the action potential. This may occur as a result of inhibition of the human ether-a-go-go related gene (HERG) channel. To clarify the mechanisms of anesthetics on HERG channels, we monitored the electrocardiogram and measured QT intervals in the guinea pig in the presence of sevoflurane and propofol. Sevoflurane (1%-4%) prolonged QTc dose-dependently (7.5%-21.2%), but propofol did not affect it. Furthermore, HERG channels were expressed in Xenopus oocytes and outward HERG currents were obtained on step depolarization from a holding potential of -70 mV. Repolarization to -70 mV from positive test potentials resulted in large outward tail currents. Sevoflurane (1%-4%), in a dose-dependent manner, inhibited the HERG outward tail currents (9.7%-26.6%), whereas steady-state currents were inhibited only at large concentrations. The time constant of the converging current was decreased in the presence of sevoflurane, but the inactivation and activation curves were not shifted. Propofol did not affect these currents within the clinically relevant concentration. In conclusion, compared with steady-state currents, sevoflurane was more potent in inhibiting the outward tail currents, suggesting that sevoflurane may modulate the HERG channel kinetics in its inactivated state.     

Effects of Ropivacaine on Action Potential Configuration and Ion Currents in Isolated Canine Ventricular Cardiomyocytes

Background: Despite the widespread clinical application of ropivacaine, there is little information on the cellular cardiac effects of the drug. In the current study, therefore, the concentration-dependent effects of ropivacaine on action potential morphology and the underlying ion currents were studied and compared with those of bupivacaine in isolated canine ventricular cardiomyocytes.

Methods: Action potentials were recorded from the enzymatically dispersed cells using sharp microelectrodes. Conventional patch clamp and action potential voltage clamp arrangements were used to study the effects of ropivacaine on transmembrane ion currents.

Results: Ropivacaine induced concentration- and frequency-dependent changes in action potential configuration, including shortening of the action potentials, reduction of their amplitude and maximum velocity of depolarization, suppression of early repolarization, and depression of plateau. Reduction in maximum velocity of depolarization was characterized with an EC50 value of 81 +/- 7 [mu]m at 1 Hz. Qualitatively similar results were obtained with bupivacaine (EC50 = 47 +/- 3 [mu]m). Under voltage clamp conditions, a variety of ion currents were blocked by ropivacaine: L-type calcium current (EC50 = 263 +/- 67 [mu]m), transient outward current (EC50 = 384 +/- 75 [mu]m), inward rectifier potassium current (EC50 = 372 +/- 35 [mu]m), rapid delayed rectifier potassium current (EC50 = 303 +/- 47 [mu]m), and slow delayed rectifier potassium current (EC50 = 106 +/- 18 [mu]m).     

An altered repolarizing potassium current in rat cardiac myocytes after subtotal nephrectomy

Renal failure in humans is associated with electrocardiographic changes including altered QT interval dispersion, which suggests that cardiac myocyte repolarization is abnormal and which appears to correlate with cardiac prognosis. In this study, cardiac myocyte repolarizing currents have been studied in isolated cells from rats 8 wk after subtotal nephrectomy (SNx), using sham-operated animals as controls. In addition, monophasic cardiac action potentials were recorded from the epicardial surface of the left ventricle (LV) apex, LV base, and the right ventricle of isolated perfused hearts paced at 320/min. SNx was associated with cardiac hypertrophy and histologic evidence of myocardial fibrosis, but SNx rats were not hypertensive. Repolarizing K(+) currents were measured using whole-cell patch-clamp, and 4-aminopyridine (4-AP)-sensitive transient outward (I(to)) and 4-AP-insensitive sustained outward (I(so)) components were quantified. After SNx, I(to) was increased by two to threefold at voltages from -30 to +60 mV and showed increased heterogeneity. For example, at 0 mV voltage clamp pulse, the median I(to) was increased from 3.23 pA/pF in control myocytes (interquartile range 3.20 pA/pF, n = 24) to 5.86 pA/pF in SNx myocytes (interquartile range 7.32 pA/pF, n = 21, P: < 0.005). The kinetics of inactivation of I(to) were altered after SNx with slowing both of the onset and the recovery from inactivation. The mean time constant of inactivation at +30 mV after SNx was 14.2 +/- 1.6 ms (n = 20) compared with control values of 9.8 +/- 0.6 ms (n = 23, P: < 0.05). Neither I(so) nor inward rectifier K(+) currents were altered after SNx. The action potential duration (APD(50)) at the left ventricular base was approximately 20% shorter (P: < 0.02) in hearts from SNx rats compared with controls. 4-AP (2 mM) prolonged the APD(50) in all regions in hearts from both SNx and control rats and abolished the APD(50) shortening in SNx. These results indicate that abnormalities of the cardiac transient outward K(+) current contribute to alterations in the cardiac action potential in renal failure and warrant further investigation because they may contribute to altered repolarization and arrythmogenesis.     

The effects of halothane, isoflurane, and sevoflurane on Ca2+ current and transient outward K+ current in subendocardial and subepicardial myocytes from the rat left ventricle

Halothane, isoflurane, and sevoflurane abbreviate ventricular action potential duration (APD), and for halothane this effect is greater in the subendocardium than in the subepicardium. In this study we investigated mechanisms underlying the regional effects of these anesthetics on APD. The effect of 0.6 mM halothane, isoflurane, and sevoflurane on the action potential, L-type Ca(2+) current, transient outward K(+) current (I(to)), and steady-state current was recorded in rat left ventricular subendocardial and subepicardial myocytes. Halothane and isoflurane (but not sevoflurane) reduced APD significantly (P < 0.05), more in subendocardial than subepicardial myocytes. Peak L-type Ca(2+) current did not differ between regions and, compared with control, was reduced significantly in both regions by 40% (P < 0.001), 20% (P < 0.001), and 12% (P < 0.01) by halothane, isoflurane, and sevoflurane, respectively. I(to) was greater in subepicardial (3.95 +/- 0.29 nA) than subendocardial (1.12 +/- 0.05 nA) myocytes. In subepicardial myocytes, peak I(to) was reduced significantly by halothane (P < 0.01) and isoflurane (P < 0.05) (by 8% and 7%, respectively) but was unaffected by sevoflurane. No significant reduction of I(to) was observed in subendocardial myocytes with the three anesthetics. The steady-state current was increased significantly (P < 0.05), but the extent of this increase did not differ between the two regions or among the three anesthetics. Therefore, greater inhibition of I(to) in subepicardial than subendocardial myocytes by halothane and isoflurane could underlie their transmural effects on APD.     

Myocardial Depressant Effects of Desflurane: Mechanical and Electrophysiologic Actions In Vitro

Background: The authors determined whether desflurane altered myocardial excitation-contraction coupling and electrophysiologic behavior in the same manner as isoflurane and sevoflurane.

Methods: The effects of desflurane on isometric force in guinea pig ventricular papillary muscles were studied in modified standard and in 26 mm K+ Tyrode solution with 0.1 [mu]m isoproterenol. Desflurane effects on sarcoplasmic reticulum Ca2+ release were also determined by examining its actions on rat papillary muscles, guinea pig papillary muscles in low-Na+ Tyrode solution, and rapid cooling contractures. Normal and slow action potentials were recorded using a conventional microelectrode technique. Ca2+ and K+ currents of guinea pig ventricular myocytes were examined.

Results: Desflurane (5.3% and 11.6%) decreased peak force to approximately 70% and 40% of the baseline, respectively, similar to the effects of equianesthetic isoflurane concentrations. With isoproterenol in 26 mm K+ Tyrode solution, desflurane markedly depressed late peaking force and modestly depressed early peak force. The rested state contractions of rat myocardium or guinea pig myocardium in low-Na+ Tyrode solution were modestly depressed, whereas rapid cooling contractures were virtually abolished after desflurane administration. Desflurane significantly prolonged the action potential duration. Desflurane reduced L-type Ca2+ current and the delayed outward K+ current but did not alter the inward rectifier K+ current.     

Inward calcium currents in cultured and freshly isolated detrusor muscle cells: evidence of a T-type calcium current

PURPOSE: We carefully examined the possible routes of Ca2+ influx, and determined whether cultured cells retain Ca2+ channels and whether the culturing process changes their properties. MATERIALS AND METHODS: Inward currents were measured under voltage clamp in freshly isolated cells and myocytes from confluent cell cultures of detrusor smooth muscle. RESULTS: In guinea pig and human cells mean peak inward current density plus or minus standard deviation decreased significantly in cell culture (2.0 +/- 0.9 versus 4.5 +/- 2.2 pA.pF.(-1)) but there was no species variation. In primary cultured and passaged guinea pig cells an inward current was identified as L-type Ca2+ current. In freshly isolated cells another component to the inward current was identified that was insensitive to 20 micromol. l(-1) verapamil and 20 to 50 micromol. l(-1) cadmium chloride but abolished by 100 micromol. l(-1) nickel chloride and identified as T-type Ca2+ current. In addition, total inward current was greater at a holding potential of -100 than -40 mV., also indicating a component of current activated at negative voltage. Steady state activation and inactivation curves of the net inward current were also compatible with a single component in cultured cells but a dual component in freshly isolated cells. The action potential was completely abolished in cultured cells by L-type Ca2+ channel blockers but incompletely so in freshly isolated cells. Outward current depended strongly on previous inward current, suggesting a predominant Ca2+ dependent outward current. CONCLUSIONS: In freshly isolated guinea pig cells T and L-type Ca2+ current is present but T-type current is absent in confluent cultures.     

Mechanisms underlying the QT interval-prolonging effects of sevoflurane and its interactions with other QT-prolonging drugs

BACKGROUND: Sevoflurane prolongs ventricular repolarization in patients, but the mechanisms are not fully characterized. The effects of sevoflurane on many cloned human cardiac ion channels have not been studied, and the interactions between sevoflurane and other drugs that prolong cardiac repolarization have not been detailed. METHODS: The effects of sevoflurane on action potentials and L-type Ca channels in guinea pig myocytes were examined. Sevoflurane's effects on cloned human cardiac K channels and the cloned human cardiac Na channel were studied. The consequences of combining sevoflurane and the class III antiarrhythmic drugs sotalol or dofetilide on action potential duration were also examined. RESULTS: Sevoflurane produced an increase in action potential duration at concentrations of 0.3-1 mm. Contrary to most drugs that delay ventricular repolarization, sevoflurane was without effect on the human ether-a-go-go-related gene cardiac potassium channel but instead produced a reduction in KvLQT1/minK K channel currents and inhibited the Kv4.3 K channel by speeding its apparent rate of inactivation. Sevoflurane had little effect on Na and Ca channel currents at concentrations of 1 mm or less. When the authors coadministered sevoflurane with sotalol or dofetilide, synergistic effects on repolarization were observed, resulting in large increases in action potential duration (up to 66%). CONCLUSION: Prolonged ventricular repolarization observed with administration of sevoflurane results from inhibition of KvLQT1/minK and Kv4.3 cardiac K channels. Combining sevoflurane with class III antiarrhythmic drugs results in supra-additive effects on action potential duration. The results indicate that sevoflurane, when administered with this class of drug, could result in excessive delays in ventricular repolarization. The results suggest the need for further clinical studies.     

Effects of halothane and isoflurane on calcium and potassium channel currents in canine coronary arterial cells.

The effects of halothane (0.75% and 1.5%) and isoflurane (2.6%) on macroscopic Ca2+ and K+ channel currents (ICa and IK, respectively) were investigated in voltage-clamped vascular muscle cells from the canine coronary artery. Single coronary arterial cells were dialyzed with K+ glutamate solution and superfused with Tyrode's solution for measurement of IK (n = 45). Stepwise depolarization from a holding potential of -60 mV to beyond -30 mV elicited an outward, slowly inactivating IK that had a macroscopic slope conductance of 18 nS. IK was reduced 75% by 10 mM 4-aminopyridine, a K+ channel antagonist. Compared to 4-aminopyridine, halothane at 0.75% and 1.5% reduced peak IK amplitude only by 14 +/- 2% and 36 +/- 3%, respectively. At approximately equianesthetic concentrations, 2.6% isoflurane suppressed IK less than did 1.5% halothane, reducing peak amplitude by 15 +/- 3%. In other sets of experiments, cells were dialyzed with 120 Cs(+)-glutamate solution and superfused with 10 mM BaCl2 or CaCl2 solutions to isolate ICa (n = 39) pharmacologically. Under these conditions, progressive depolarizing steps from -60 mV elicited a small inward current, which was potentiated 3.4-fold by equimolar substitution of Ba2+ for Ca2+ in the external solution and was blocked by 1 microM nifedipine. This inward current, which resembled L-type ICa, was blocked 37 +/- 4% and 70 +/- 4% in the presence of 0.75% and 1.5% halothane, respectively. Isoflurane (2.6%) also decreased ICa by 55 +/- 5%. It appears that while halothane and isoflurane suppress both IK and ICa, these anesthetics preferentially reduce ICa.(ABSTRACT TRUNCATED AT 250 WORDS)     

Myocardial Depressant Effects of Sevoflurane: Mechanical and Electrophysiologic Actions In Vitro

Background: The effects of anesthetic concentrations of sevoflurane were studied in isolated myocardial tissue to delineate the mechanisms by which cardiac function is altered.

Methods: Isometric force of isolated guinea pig ventricular papillary muscle was studied at 37 degrees C in normal and 26 mM Potassium sup + Tyrode's solution at various stimulation rates. Normal and slow action potentials were evaluated using conventional microelectrodes. Effects of sevoflurane on sarcoplasmic reticulum function in situ were also evaluated by its effect on rapid cooling contractures, which are known to activate Calcium2+ release from the sarcoplasmic reticulum, and on contractions of rat papillary muscle. Finally, Calcium2+ and Potassium sup + currents of isolated guinea pig ventricular myocytes were examined using the whole-cell patch clamp technique.

Results: Sevoflurane equivalent to 1.4% and 2.8% depressed guinea pig myocardial contractions to approximately 85 and approximately 65% of control, respectively, although the maximum rate of force development at 2 or 3 Hz and force in rat myocardium after rest showed less depression. In the partially depolarized, beta-adrenergically stimulated myocardium, sevoflurane selectively depressed late peak force without changing early peak force, whereas it virtually abolished rapid cooling contractures. Sevoflurane did not alter the peak amplitude or maximum depolarization rate of normal and slow action potentials, but action potential duration was significantly prolonged. In isolated guinea pig myocytes at room temperature, 0.7 mM sevoflurane (equivalent to 3.4%) depressed peak Calcium2+ current by approximately 25% and increased the apparent rate of inactivation. The delayed outward Potassium sup + current was markedly depressed, but the inwardly rectifying Potassium sup + current was only slightly affected by 0.35 mM sevoflurane.     

Ionic Mechanisms Mediating the Differential Effects of Methohexital and Thiopental on Action Potential Duration in Guinea Pig and Rabbit Isolated Ventricular Myocytes

Background: Commonly used barbiturate anesthetics may significantly influence cardiac electrophysiologic characteristics. The authors evaluated thiopental (a thiobarbiturate) and methohexital (an oxybarbiturate), two compounds with similar physicochemical properties but different structures, to determine whether they have distinct effects on the major ionic currents that determine action potential duration (APD) in ventricular myocytes.

Methods: The effects of thiopental and methohexital (50 [micro sign]M) on APD at 50% (APD50) and 90% (APD90) repolarization were studied in guinea pig and rabbit single ventricular myocytes using the patch-clamp technique in a whole-cell configuration. The ionic mechanisms underlying the APD changes were evaluated by measuring the anesthetics' effects on the L-type calcium inward current, the inward rectifier potassium current, and the delayed rectifier potassium current in guinea pig cells and on the transient outward potassium current in rabbit cells.

Results: Thiopental and methohexital caused opposite effects on APD. Whereas thiopental prolonged APD50 and APD90 in guinea pig and rabbit ventricular myocytes, methohexital shortened them. Thiopental markedly depressed both the inward and outward components of the inward rectifier potassium current, whereas methohexital caused minimal inhibition of the inward component and no change in the outward component. The delayed rectifier potassium current was inhibited by thiopental but significantly potentiated by methohexital. Neither thiopental nor methohexital significantly affected the transient outward potassium current or the L-type calcium inward current.     

Calcium currents in interstitial cells from the guinea-pig bladder

OBJECTIVE: To determine whether there are inward currents in interstitial cells (IC) isolated from the guinea-pig detrusor and if so, to characterise them using the patch-clamp technique and pharmacological agents. MATERIALS AND METHODS: Using the whole-cell patch-clamp technique, inward currents were studied in IC enzymatically isolated from the detrusor of the guinea-pig bladder. Currents were evoked by stepping positively from a holding potential of - 80 mV. RESULTS: Outward K+ currents were blocked by Cs+ internal solution to reveal inward currents, which activated at voltages more positive than - 50 mV, peaked at 0 mV, reversed near + 50 mV and were half-maximally activated at - 27 mV. The inward currents showed voltage-dependent inactivation and were half-maximally inactivated at - 36 mV. Fitting the activation and inactivation data with a Boltzmann function revealed a window current between - 40 mV and + 20 mV. The decay of the current evoked at 0 mV could be fitted with a single exponential with a mean time-constant of 88 ms. Replacing external Ca2+ with Ba2+ significantly increased this to 344 ms. The current amplitude was augmented by Ba2+, and by Bay K 8644. Inward currents were significantly reduced by 1 microm nifedipine, across the voltage range, but the blockade was more effective on the current evoked at 0 mV than that evoked by a step to - 20 mV, perhaps indicating voltage-dependence of the action of nifedipine or another component of inward current. Increasing the concentration of the drug to 10 microm caused no further significant reduction either at 0 mV or at -20 mV. However, in the presence of 1 microm nifedipine the latter current was significantly reduced by 100 microm Ni2+. Both currents were significantly reduced in Ca2+-free solution. CONCLUSIONS: IC from the guinea-pig detrusor possess inward currents with typical characteristics of L-type Ca2+ current. They also have a component of inward Ca2+ current, which was resistant to nifedipine, but sensitive to Ni2+. Further work is needed to characterise the latter conductance.     

Halothane Inhibits Contraction and Action Potential Duration to a Greater Extent in Subendocardial than Subepicardial Myocytes from the Rat Left Ventricle

Background : Halothane inhibits the 4-aminopyridine-sensitive transient outward K+ current (Ito), which in many species, including humans, plays an important role in determining action potential duration. As Ito is greater in the ventricular subepicardium than subendocardium, halothane may have differential effects on action potential duration and, therefore, contraction in cells isolated from these two regions.

Methods : Myocytes were isolated from the subendocardium and subepicardium of the rat left ventricle. Myocytes from each region were electrically stimulated at 1 Hz to measure contractions and action potentials and exposed to 0.6 mm halothane (approximately 2 x minimum alveolar concentration50 for the rat) for 1 min. The time from the peak of the action potential to repolarization at 0 and -50 mV was measured to assess the effects of halothane on action potential duration.

Results : Halothane inhibited contraction to a significantly (P = 0.002) greater extent in subendocardial myocytes than in subepicardial myocytes: the amplitude of contraction during control conditions was 3.6 +/- 0.4 [mu]m and 3.2 +/- 0.7 [mu]m in subendocardial and subepicardial cells, respectively, and this was reduced to 1.1 +/- 0.2 [mu]m (29 +/- 2% of control, P < 0.0001, n = 10) and 1.4 +/- 0.3 [mu]m (46 +/- 3% of control, P = 0.007, n = 7), respectively, after a 1-min exposure to 0.6 mm halothane. Control action potential duration (at -50 mV) was 67 +/- 10 and 28 +/- 4 ms in subendocardial and subepicardial myocytes, respectively, and these values were reduced to 39 +/- 6 ms (58 +/- 3% of control, P < 0.001) and 20 +/- 3 ms (73 +/- 5% of control, P = 0.009) by halothane, respectively.     

Effects of isoflurane on K+ and Ca2+ conductance in isolated smooth muscle cells of canine cerebral arteries.

Although isoflurane is a known cerebral vasodilator, the mechanism of isoflurane-induced vasodilation is not clear. The purpose of this study was to investigate the effects of 2.6% isoflurane (1.2 mM) on macroscopic calcium and potassium channel currents in voltage-clamped canine middle cerebral artery cells. Cells were dialyzed with K(+)-glutamate solution and superfused with Tyrode's solution for measurement of potassium current (n = 20). Stepwise depolarization from a holding potential of -60 mV to beyond -30 mV elicited an outward, slowly inactivating potassium current that was reduced 50% +/- 2% and 81% +/- 3% (mean +/- SEM) in the presence of 1 mM 4-aminopyridine and 30 mM tetraethylammonium, respectively. Calcium ionophore (A23187, 10 microM) increased the potassium current by 76% +/- 3%, suggesting calcium dependency. Isoflurane reduced the amplitude of the potassium current by 35% +/- 4%. Calcium current was measured in cells dialyzed with solution containing 130 mM Cs(+)-glutamate and superfused with solution containing 10 mM BaCl2 and 135 mM tetraethylammonium to pharmacologically isolate the calcium current (n = 13). Under these conditions, progressive depolarizing steps from -60 mV elicited an inward current that was maximally activated at +20 mV and essentially eliminated by 1 microM nifedipine. This current, resembling a long-lasting (L-type) Ca2+ channel current, was reduced 40% +/- 4% by isoflurane. The results of this study suggest that isoflurane acts directly at the vascular muscle membrane to suppress transmembrane calcium and potassium currents. The decrease in calcium current would cause vasodilation; however, the concomitant decrease in potassium current may partially antagonize the depressant effect of isoflurane mediated through calcium current reduction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Kinetic Modulation of HERG Potassium Channels by the Volatile Anesthetic Halothane

Background: HERG (human ether-a-gogo related gene) encodes the cardiac rapidly activating delayed rectifier potassium currents (Ikr), which play an important role in cardiac action potential repolarization. General anesthetics, like halothane, can prolong Q-T interval, suggesting that they act on myocellular repolarization, possibly involving HERG channels. Evidence for direct modulation of HERG channels by halothane is still lacking. To gain insight on HERG channel modulation by halothane the authors recorded macroscopic currents expressed in Xenopus oocytes and conducted non-stationary noise analysis to evaluate single channel parameters modified by the anesthetic.

Methods: Macroscopic currents were recorded in 120 mm K+ internal-5 mm K+ external solutions with the cut open oocyte technique. Macropatch recordings for non-stationary noise analysis of HERG tail currents were made in symmetrical 120 mm K+ solutions. Pulse protocols designed for HERG current recording were elicited from a holding potential of -80 mV. Halothane was delivered via gravity-fed perfusion.

Results: Halothane (0.7%, 1.5%, and 3%) decreased macroscopic HERG currents in a concentration-dependent manner (average reduction by 14%, 22%, and 35% in the range of -40 mV to 40 mV) irrespective of potential. HERG currents had slower activation and accelerated deactivation and inactivation. Non-stationary noise analysis revealed that halothane, 1.5%, decreased channel Po by 27%, whereas single-channel current amplitudes and number of channels in the patch remained unchanged.     

Effects of sevoflurane,propofol, and adjunct nitrous oxide on regional cerebral blood flow,oxygen consumption,and blood volume in humans

BACKGROUND: Anesthetic agents, especially volatile anesthetics and nitrous oxide (N2O), are suspected to perturb cerebral homeostasis and vascular reactivity. The authors quantified the effects of sevoflurane and propofol as sole anesthetics and in combination with N2O on regional cerebral blood flow (rCBF), metabolic rate of oxygen (rCMRO2), and blood volume (rCBV) in the living human brain using positron emission tomography. METHODS: 15O-labeled water, oxygen, and carbon monoxide were used as positron emission tomography tracers to determine rCBF, rCMRO2 and rCBV, respectively, in eight healthy male subjects during the awake state (baseline) and at four different anesthetic regimens: (1) sevoflurane alone, (2) sevoflurane plus 70% N2O (S+N), (3) propofol alone, and (4) propofol plus 70% N2O (P+N). Sevoflurane and propofol were titrated to keep a constant hypnotic depth (Bispectral Index 40) throughout anesthesia. End-tidal carbon dioxide was strictly kept at preinduction level. RESULTS: The mean +/- SD end-tidal concentration of sevoflurane was 1.5 +/- 0.3% during sevoflurane alone and 1.2 +/- 0.3% during S+N (P < 0.001). The measured propofol concentration was 3.7 +/- 0.7 microg/ml during propofol alone and 3.5 +/- 0.7 microg/ml during P+N (not significant). Sevoflurane alone decreased rCBF in some (to 73-80% of baseline, P < 0.01), and propofol in all brain structures (to 53-70%, P < 0.001). Only propofol reduced also rCBV (in the cortex and cerebellum to 83-86% of baseline, P < 0.05). Both sevoflurane and propofol similarly reduced rCMRO2 in all brain areas to 56-70% and 50-68% of baseline, respectively (P < 0.05). The adjunct N2O counteracted some of the rCMRO2 and rCBF reductions caused by drugs alone, and especially during S+N, a widespread reduction (P < 0.05 for all cortex and cerebellum vs. awake) in the oxygen extraction fraction was seen. Adding of N2O did not alter the rCBV effects of sevoflurane and propofol alone. CONCLUSIONS: Propofol reduced rCBF and rCMRO2 comparably. Sevoflurane reduced rCBF less than propofol but rCMRO2 to an extent similar to propofol. These reductions in flow and metabolism were partly attenuated by adjunct N2O. S+N especially reduced the oxygen extraction fraction, suggesting disturbed flow-activity coupling in humans at a moderate depth of anesthesia.     

Sevoflurane depolarizes pre-synaptic mitochondria in the central nervous system

BACKGROUND: Volatile anaesthetics protect the heart from ischaemic injury by activating mitochondrial signalling pathways. The aim of this study was to test whether sevoflurane, which is increasingly used in neuroanaesthesia, affects mitochondrial function in the central nervous system by altering the mitochondrial membrane potential (DeltaPsi(m)). METHODS: In order to correlate free cytosolic Ca(2+) ([Ca(2+)](i)) and DeltaPsi(m), rat neural presynaptic terminals (synaptosomes) were loaded with the fluorescent probes fura-2 and JC-1. During sevoflurane exposure, 4-aminopyridine (4-AP) 500 micro M to induce pre-synaptic membrane depolarization or carbonylcyanide-p-(trifluoromethoxy)-phenylhydrazone (FCCP) 1 micro M to induce maximum mitochondrial depolarization was added. In order to block mitochondrial ATP-regulated K(+)-channels (mitoK(ATP)), the antagonist 5-hydroxydecanoate (5-HD) 500 micro M was added. RESULTS: In Ca(2+)-containing medium, both sevoflurane 1 and 2 MAC gradually decreased the normalized JC-1 ratio from 0.96 +/- 0.01 in control to 0.92 +/- 0.01 and 0.89 +/- 0.01, representing a depolarization of DeltaPsi(m) (n = 9, P < 0.05). Sevoflurane 2 MAC increased [Ca(2+)](i). In Ca(2+)-depleted medium, sevoflurane 1 and 2 MAC depolarized DeltaPsi(m), while [Ca(2+)](i) remained unaltered. Sevoflurane 2 MAC attenuated the 4-AP-induced depolarization of DeltaPsi(m). When mitoK(ATP) was blocked, the sevoflurane-induced depolarization of DeltaPsi(m) was attenuated, but not blocked. The depolarizing effect of sevoflurane on DeltaPsi(m) compared with FCCP was calculated to 13.2 +/- 1.3% in Ca(2+)-containing and 15.1 +/- 1.2% in Ca(2+)-depleted medium (n = 7). CONCLUSIONS: Sevoflurane depolarizes DeltaPsi(m) in rat synaptosomes, and the effect is not dependent on Ca(2+)-influx to the cytosol. Opening of mitoK(ATP) is partly responsible for the depolarizing effect of sevoflurane.