Influence of increased glucose concentration and temperature on contractile activity of rat papillary muscles during and after anoxia

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Abstract
Isometrically contracting rat papillary muscles were used to study the influence of an increase in glucose concentration (from 5.5 to 16.5 mmol/1) and the addition of sucrose (11 mmol/1) and insulin (500 mU/ml) to the standard buffer solution (containing 5.5 mmol/1 glucose), on mechanical performance under anoxic conditions. During anoxia, the increased glucose concentration prevented the rise in resting tension and reduced the degree of decay in active isometric tension, promoting a quicker and more complete recovery with reoxygenation. This was observed whether the additional glucose was added either from the beginning of the anoxic period, or after the initial 30 min of anoxia. The protective effect of the increased glucose concentration was observed when the experiments were performed at both 30°C and 37°C. Muscles recovered more completely at 30°C than at 37°C. Under 5.5 mmol/1 glucose concentration, resting tension remained markedly elevated after 60 min of reoxygenation at 37°C, whereas at 30°C there was a gradual disappearance of contracture with reoxygenation. Sucrose was also studied in equiosmolar concentration to that in the high glucose concentration experiments. Only a marginal beneficial action was observed with sucrose, leading to the conclusion that the protective effect of glucose is almost entirely due to its metabolic action. In this work, insulin (500 mU/ml) failed to promote any protection in terms of contractile behaviour during anoxia and subsequent reoxygenation under the influence of normal glucose concentration. The sudden increase in duration of contraction during reoxygenation was progressively reduced when the anoxic period was prolonged from 15 min to 90 min, under 5.5 mmol/l glucose concentration; increased glucose concentration resulted in persistence of markedly prolonged contraction with reoxygenation even after a 90-min anoxic period.

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