Lack of inhibition by inhalational anesthetics of myocardial contraction dependent on intracellular sodium activity.
BACKGROUND
When cardiac muscle is stimulated at a high frequency after rest, the peak force increases rapidly at first and then slowly. The slow phase reflects Ca2+ accumulation dependent on an increase in intracellular Na+ activity. The positive inotropic effect of high concentrations of cardiac glycosides also involves Ca2+ accumulation dependent on an increase in intracellular Na+ activity. The effect of isoflurane on the component of myocardial contraction dependent on an increase in intracellular Na+ activity is not well understood.


METHODS
The isometric force of rabbit papillary muscle contractions was measured. The authors studied the effects of isoflurane on the force increase that occurs when the muscles are stimulated after a rest and on the force increase induced by ouabain.


RESULTS
Isoflurane (1.5%, 2.4%) had no statistically significant effect on the magnitude of the slow phase of force increase when the muscles were stimulated at 2 Hz after a rest. Isoflurane (1.5%) did not decrease the magnitude of force increase induced by ouabain (1 microM).


CONCLUSIONS
Isoflurane has little effect on myocardial contractions that depend on Ca2+ accumulation after an increase in intracellular Na+ activity. This may partly account for the smaller cardiac depressant effect of isoflurane observed at high frequencies rather than at low frequencies. The results of the present study and an earlier study with halothane suggest that the lack of inhibition of contractions dependent on Ca2+ accumulation after an increase in intracellular Na+ activity may be a common property of inhalational anesthetics.
Department of Anesthesiology, University of Wisconsin, Madison 53792-3272, USA. hkomai@facstaff.wisc.edu
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D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D003593:Cytoplasm / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006321:Heart / Q000502:physiology*; D066298:In Vitro Techniques; D007530:Isoflurane / Q000494:pharmacology; D009200:Myocardial Contraction / Q000187:drug effects*; D009206:Myocardium / Q000166:cytology; D010042:Ouabain / Q000494:pharmacology; D011817:Rabbits; D012964:Sodium / Q000502:physiology*; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors
D018685:Anesthetics, Inhalation; D004791:Enzyme Inhibitors; D010042:Ouabain; D012964:Sodium; D007530:Isoflurane; D000254:Sodium-Potassium-Exchanging ATPase