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M J Frazer

Publications and source records attributed to M J Frazer.

10 recordsLinked to original sources
PubMed1995

Volatile anesthetics depress Ca2+ transients and glutamate release in isolated cerebral synaptosomes.

BACKGROUND: The current study was performed to determine whether volatile anesthetics may include as part of their action in the central nervous system the depression of presynaptic transmitter release by alteration in intrasynaptic [Ca2+] ([Ca2+]i). METHODS: Guinea pig cerebrocortical synaptosomes were studied at 37 degrees C suspended in control buffer solution containing 1.3 mM external [Ca2+] ([Ca2+]e). Spectrofluorometric assays were used to monitor [Ca2+]i with the Ca(2+)-sensitive fluorophore Fura-2 and to monitor glutamate release with an enzyme-coupled assay that produced the fluorescent product nicotinamide adenine dinucleotide phosphate. To activate the increase in [Ca2+]i and glutamate release, synaptosomes were depolarized by abruptly increasing external [K+] from 5 to 35 mM. Responses were determined in solutions equilibrated with approximately 1 or 2 minimum alveolar concentration (MAC) isoflurane, enflurane, or halothane and also in solutions with decreased [Ca2+]e (0.025, 0.05, 0.1, 0.2, 0.4, and 0.6 mM). RESULTS: Although they had no action on basal behavior, the anesthetics depressed the K(+)-depolarization-induced increase in both [Ca2+]i and glutamate release in a dose-dependent fashion. The [Ca2+]i transient was inhibited by 13-21% per MAC, and glutamate release was depressed 14-28% per MAC. The depression of both [Ca2+]i and glutamate release caused by 2.5% isoflurane, 3.4% enflurane, and 1.5% halothane could be reproduced by a reduction in [Ca2+]e to 0.2-0.4 mM. CONCLUSIONS: In this setting, isoflurane, enflurane, and halothane decrease [Ca2+]i in a manner consistent with inhibition of Ca2+ entry, possibly by specific voltage-gated neuronal Ca2+ channels. This decrease in [Ca2+]i is sufficient to account for all or most of the associated decrease in glutamate release.

Anesthetics, Inhalation↗
PubMed1994

Anesthetic alteration of ryanodine binding by cardiac calcium release channels.

Differential cardiac contractile depression by volatile anesthetics is well documented, and evidence points to differing actions on the myocardial sarcoplasmic reticulum (SR). Since the Ca(2+)-release channel (CaRC) of the SR binds ryanodine with high-affinity when opened by micromolar Ca2+ concentrations, ryanodine binding to cardiac SR membrane vesicles was employed as an assay of anesthetic modulation of CaRC activity. Canine ventricle was homogenized, centrifuged preparatively and then differentially on a sucrose gradient. A fraction enriched with CaRCs was defined by: the presence of a approximately 450 kDa protein consistent with CaRC; approximately 3-fold enhancement of vesicular 45Ca2+ uptake by ruthenium red; Ca(2+)-activated [3H]ryanodine binding. Specific binding of 10 nM ryanodine was activated by > 0.5 microM Ca2+ and was maximal at approximately 6 pmol/mg protein in > or = 20 microM Ca2+. Halothane (1.5%), but not isoflurane, shifted the Ca(2+)-dependence of ryanodine binding to lower [Ca2+]. With submaximal activation by 5 microM Ca2+, 1.5% and 0.75% halothane enhanced binding of 10-80 microM ryanodine, while 2.5% isoflurane and 3.5% enflurane did not. A plot of bound/free vs. bound ryanodine suggests that halothane causes a dose-dependent increase in ryanodine binding to a high-affinity site, while isoflurane has no such action. In intact myocardium, this effect will decrease Ca2+ retention in the SR so that less Ca2+ will be available to activate contractions, consistent with halothane's depressant action.

Anesthetics↗
PubMed1993

Barbiturate anesthetics depress the resting K+ conductance of myocardium.

The intravenous anesthestic thiopental has been previously shown to increase the incidence of ventricular arrhythmias, particularly when combined with epinephrine and halothane. Recent work based on microelectrode and tension measurements has indicated that thiopental may diminish membrane K+ permeability. Utilizing the whole-cell patch-clamp technique, we investigated the effect of thiopental on the current associated with the resting membrane conductance, the anomalous or inward rectifying K+ current (IK1). External application of 30 microM thiopental to frog atrial myocytes resulted in a 56 +/- 2% (mean +/- S.E.M.; n = 12 cells) reduction in the magnitude of IK1 elicited by a hyperpolarization to -110 mV. The outward current component through IK1 channels, evoked by depolarizing voltages above the resting potential, was decreased to same extent. The effect of thiopental on IK1 was concentration-dependent and the time courses of onset and recovery were rapid (tau = 10-14 sec). Ramp command potentials from -120 to +60 mV at a rate of 20 mV/sec revealed that 30 microM thiopental also depressed the delayed outward K+ current by 25 +/- 4% (n = 4). Examination of other barbiturates revealed that the potency in the suppression of IK1 was related to the octanol/water partition coefficient, suggesting a lipophilic site of action. Utilizing guinea pig ventricular myocytes, we observed a similar level of IK1 depression with thiopental, however the rates of onset and recovery were considerably slower than with frog atrial myocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1992

Halothane and isoflurane effects on Ca2+ fluxes of isolated myocardial sarcoplasmic reticulum.

To elucidate better the differential myocardial depressant actions of halogenated volatile anesthetics, anesthetic-induced changes in Ca2+ accumulation, release, and Ca-ATPase (Ca2+ pump) activity of isolated canine cardiac sarcoplasmic reticulum (SR) vesicles were examined. An initial crude microsomal fraction of homogenized canine ventricle was subfractionated on a discontinuous sucrose gradient after Ca2+ loading in the presence of phosphate. Junctional SR (JSR) enriched with terminal cisternae was identified by its content of an electrophoretically verified approximately 450-kDa protein, the Ca(2+)-release channel (CaRC). When the CaRC of JSR was blocked by 1 microM ruthenium red (RR), the rate of Ca2+ uptake increased 47% as measured spectrophotometrically using the Ca-sensitive dye antipyrylazo III. A second fraction was identified as primarily longitudinal SR (LSR) based on its trace content of 450-kDa protein and 11% increase of Ca2+ uptake with RR. Halothane (0.75-2.5%) or isoflurane (2.5-4%) decreased net Ca2+ accumulation rate by either LSR or JSR, and the decrease in uptake rate of JSR was only partially reversed by addition of 1 microM RR (27% increase for isoflurane, 7% increase for halothane). Both halothane and isoflurane increased JSR ATP consumption as measured by a coupled-enzyme assay. 45Ca2+ efflux from passively loaded SR vesicles was then determined to verify that the decreased net uptake rate was due to enhanced Ca2+ efflux from vesicles. Both anesthetics increased passive Ca2+ efflux from SR vesicles in which the CaRC was blocked by 10 microM RR as well as those in which Ca2+ release via the CaRC was activated by 10 microM Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Nitrous oxide effects on isolated myocardium: a reexamination in vitro.

This study examined in vitro myocardial depression by 50% N2O. Maximal isometric contractions of guinea pig right ventricular papillary muscles were studied in Tyrode's superfusate at 37 degrees C within a gas-tight chamber. Superfusate (pH at 7.45) and chamber were equilibrated with 95% O2/5% CO2. After control measurements in 95% O2, muscles were studied with 50% N2 and 50% N2O (45% O2/5% CO2) in random order with an intervening and final recovery in oxygen. Muscles were field stimulated after rest and at 0.1-3 Hz. At 37 degrees C, muscle performance deteriorated over time with exposure to reduced oxygen; therefore, identical experiments were performed at 30 degrees C in which no systematic deterioration occurred. Peak tension and maximum rate of tension development (dT/dtmax) were compared for each stimulation rate. At both temperatures, N2O caused a 10-15% depression of contractility as compared to that observed with nitrogen. In a second protocol, muscles were studied at 37 degrees C in 26 mM K+ Tyrode's solution with 0.10 microM isoproterenol to study enhanced contractions mediated by slow (Ca2(+)-channel-dependent) action potentials. Rested-state double stimulations were used (stimulus interval, 250-600 ms) resulting in a first rested-state contraction followed by a second contraction (C2) with rapid initial tension development. The muscles were exposed to nitrogen and N2O as in the force-frequency experiments and did not deteriorate over time. In this setting, N2O also caused a 10-15% depression of C2 contractility as compared with nitrogen. Another set of muscles was studied in 95% O2 to which 0.5% halothane or 1% isoflurane was added before exposure to nitrogen and N2O. The combined depressant action of N2O with either halothane or isoflurane did not differ from that predicted by the simple addition of independent effects; there was no evidence of synergism. Furthermore, N2O (50%) alone depressed dT/dtmax in a manner similar to that of 0.5% halothane and different from that of 1.0% isoflurane. Experiments conducted in iso-osmolar 40 mM Na+ Tyrode's solution, in which activator Ca2+ arose from the sarcoplasmic reticulum Ca2+, also showed greater depression by N2O than nitrogen. N2O (50%) is a myocardial depressant independent of concurrent hypoxic effects with a pattern and magnitude of contractile depression similar to that of 0.5% halothane.

Action Potentials↗
PubMed1989

Depressant effects of volatile anesthetics upon rat and amphibian ventricular myocardium: insights into anesthetic mechanisms of action.

To clarify the mechanisms by which volatile anesthetics may depress myocardial contractility, the depressant effects of equivalent concentrations of isoflurane, enflurane and halothane were compared in rat and frog ventricular myocardium, preparations which differ markedly in excitation-contraction coupling. In Tyrode solution, right ventricular papillary muscles from rat exhibited very large, rapidly developing contractions after rest, with a subsequent negative force-frequency relation as the stimulation rate was increased to 0.1, 0.25, 0.5, 1, 2, and 3 Hz. The large contractions after rest and at 0.1 Hz were depressed by 0.75% halothane and 1.7% enflurane to about 60% of control, but less so by 1.3% isoflurane (approximately 0.8 MAC). Halothane at 1.5% was more depressant than 2.5% isoflurane at all stimulation rates, while 3.5% enflurane caused intermediate depression (approximately 1.6 MAC). Contractions in frog ventricular strips were studied in Ringer solution following rest and at stimulation rates of 0.1, 0.25, 0.5, and 1 Hz, in the absence and presence of equivalent anesthetic concentrations. At 0.1 to 1 Hz, isoflurane was less depressant than equivalent concentrations of halothane. Enflurane (1.7%) was less depressant than 0.75% halothane at 0.1 and 0.25 Hz; 3.5% enflurane was more depressant than 2.5% isoflurane at 1 Hz. Anesthetic effects on sustained contractures were also studied in frog ventricular strips that were superfused for 4-5 min with 40, 60, 80, and 100 mM K Ringer solution. Contractures induced by 80 and 100 mM K solution were depressed more by halothane (to 60% of control) than by isoflurane or enflurane (approximately 85% of control). However, only enflurane depressed the contractions at 1 Hz more than the sustained contractures in 100 mM K Ringer. The Ca2+ for activating contractions in rat ventricle is derived largely from the sarcoplasmic reticulum, the intracellular Ca2+ accumulation and release organelle. In contrast, Ca2+ for activating contractions in the frog ventricle originates primarily from the external medium. These results suggest that halothane is more potent than isoflurane in reducing the amount of Ca2+ rapidly released from the sarcoplasmic reticulum (as observed in rat), as well as in depressing entry of extracellular Ca2+ to activate myofibrils (as in frog). Enflurane appears to have intermediate potency with actions distinct from halothane and isoflurane. The greater potency of halothane may also be due in part to greater direct depression of actin-myosin ATPase.

Anesthetics↗