Tension maintenance, calcium content and energy production of the taenia of the guinea‐pig caecum under hypoxia.

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Abstract
When potassium (45.4 mM) was applied to isolated taenia of guinea‐pig caecum, the muscle developed a rapid phasic and sustained tonic tension during aerobic conditions bubbled with 95% O2:5% CO2. Under hypoxic conditions bubbled with 95% N2:5% CO2, the taenia lost its ability to respond to high potassium with sustained tonic contraction, although it still showed rapid phasic contraction. Raising the glucose concentration from 5.5 to 55.5 mM in the presence of high potassium during hypoxia caused development of a sustained contraction which was 50% that of the muscle in aerobic conditions. In the presence of high potassium, the ATP content of the taenia decreased in hypoxia, but increased with increasing glucose concentration. When the taenia was exposed to hypoxic conditions, the amount of lactate released from the muscle increased. Raising the glucose concentration caused a further increase in lactate release in the presence of high potassium under hypoxia. Good correlations (gamma greater than 0.9) were observed between tension development, the ATP content and lactate release of the taenia in the presence of high potassium under hypoxia when the glucose concentration was varied between 5.5 and 55.5 mM. The total calcium content was increased by the presence of high potassium under aerobic conditions and the increase was abolished when the muscle was exposed to hypoxic conditions. Under hypoxia the total calcium content was not increased by raising the glucose concentration in the presence of high potassium. The cellular calcium content of the taenia, determined by the lanthanum method, was increased in the presence of high potassium under aerobic and hypoxic conditions, but the content was smaller in hypoxic conditions than in aerobic conditions. Under hypoxic conditions, raising the glucose concentration in the presence of high potassium did not affect the cellular calcium content. These results suggest that under hypoxic conditions the potassium‐induced sustained contraction of the taenia is increased by raising the glucose concentration owing to increased ATP production through the glycolytic pathway, but not through mechanisms increasing the intracellular Ca concentration.