Halothane

Abstract: Volatile anesthetics produce safe, reversible unconsciousness, amnesia and analgesia via hyperpolarization of mammalian neurons. In molluscan pacemaker neurons, they activate an inhibitory synaptic K+ current (IKAn), proposed to be important in general anesthesia. Here we show that TASK and TREK-1, two recently cloned mammalian two-P-domain K+ channels similar to IKAn in biophysical properties, are activated by volatile general anesthetics. Chloroform, diethyl ether, halothane and isoflurane activated TREK-1, whereas only halothane and isoflurane activated TASK. Carboxy (C)-terminal regions were critical for anesthetic activation in both channels. Thus both TREK-1 and TASK are possibly important target sites for these agents.

Abstract: General anesthetics are widely used in clinical practice. On the molecular level, these compounds have been shown to modulate the activity of various neuronal ion channels. However, the functional relevance of identified sites in mediating essential components of the general anesthetic state, such as immobility and hypnosis, is still unknown. Using gene-targeting technology, we generated mice harboring a subtle point mutation (N265M) in the second transmembrane region of the beta3 subunit of the GABA(A) receptor. In these mice, the suppression of noxious-evoked movements in response to the intravenous anesthetics etomidate and propofol is completely abolished, while only slightly decreased with the volatile anesthetics enflurane and halothane. beta3(N265M) mice also display a profound reduction in the loss of righting reflex duration in response to intravenous but not volatile anesthetics. In addition, electrophysiological recordings revealed that anesthetic agents were significantly less effective in enhancing GABA(A) receptor-mediated currents, and in decreasing spontaneous action potential firing in cortical brain slices derived from mutant mice. Taken together, our results demonstrate that a single molecular target, and indeed a specific residue (N265) located within the GABA(A) receptor beta3 subunit, is a major determinant of behavioral responses evoked by the intravenous anesthetics etomidate and propofol, whereas volatile anesthetics appear to act via a broader spectrum of molecular targets.

Abstract: 1. The optical isomers of propranolol have been compared for their β-blocking and antiarrhythmic activities. 2. In blocking the positive inotropic and chronotropic responses to isoprenaline, (+)-propranolol had less than one hundredth the potency of (-)-propranolol. At dose levels of (+)-propranolol which attenuated the responses to isoprenaline, there was a significant prolongation of the PR interval of the electrocardiogram. 3. The metabolic responses to isoprenaline in dogs (an increase in circulating glucose, lactate and free fatty acids) were all blocked by (-)-propranolol. (+)-Propranolol had no effect on fatty acid mobilization but significantly reduced the increments in both lactate and glucose. 4. Both isomers of propranolol possessed similar depressant potency on isolated atrial muscle taken from guinea-pigs. 5. The isomers of propranolol exhibited similar local anaesthetic potencies on an isolated frog nerve preparation at a level approximately three times that of procaine. The racemic compound was significantly less potent than either isomer. 6. Both isomers of propranolol were capable of preventing adrenaline-induced cardiac arrhythmias in cats anaesthetized with halothane, but the mean dose of (-)-propranolol was 0.09±0.02 mg/kg whereas that of (+)-propranolol was 4.2±1.2 mg/kg. At the effective dose level of (+)-propranolol there was a significant prolongation of the PR interval of the electrocardiogram. Blockade of arrhythmias with both isomers was surmountable by increasing the dose of adrenaline. 7. Both isomers of propranolol were also capable of reversing ventricular tachycardia caused by ouabain in anaesthetized cats and dogs. The dose of (-)-propranolol was significantly smaller than that of (+)-propranolol in both species but much higher than that required to produce evidence of β-blockade. 8. The implications of these results are discussed.