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The Mechanism of the Negative Inotropic Effect of Methoxyflurane on Isolated Rat Atria

The force of contraction of isolated rat atria suspended in Krebs-Ringer bicarbonate medium containing 5.5 mM glucose was depressed 55 per cent by approximately 9 mg/100 ml of methoxyflurane in the medium bathing the atria. This corresponds to a partial pressure of 1.84 torr (0.24 vol per cent). Addition of fructose (30 mM) or pyruvate (5 mM) to atria depressed by methoxyflurane resulted in a marked positive inotropic action. Additional glucose (20 mM) was relatively ineffective in atria similarly treated. These results suggest an interference with glucose utilization at an early step in glycolysis above the phosphofructokinase step. Methoxyflurane had its usual depressant action in the absence of exogenous glucose (substrate-free medium). Thus, glucose uptake and phosphorylation were ruled out as the site of action of methoxyflurane. These results suggest that methoxyflurane's depressant action results from a block at the glucose phosphate isomerase step, the step responsible for converting glucose-6-phosphate to fructose-6-phosphate. Multiple sites of action are not ruled out by these results.