Effects of hypoxia on high-energy phosphagen content, energy metabolism and isometric force in guinea-pig taenia caeci.

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1. Previously, Ishida, Takagi & Urakawa (1984) reported that, in the presence of high K+ (45.4 mM) under hypoxia (95% N2-5% CO2 bubbling), tension and ATP content of the smooth muscle of the guinea-pig taenia caeci increased concomitantly when the glucose concentration was raised. Tension and energy metabolism of the taenia seemed to be closely correlated. In the present experiments, we investigated the metabolic changes during the relaxation phase after the taenia was exposed to hypoxia in the presence of high K+ by measuring the content of phosphagen, inorganic phosphate (Pi) and lactate in the tissue. Oxygen consumption and lactate release from the tissue were also determined to estimate the rate of ATP synthesis. 2. Under hypoxic conditions, high-K(+)-induced tension decreased to one-tenth of maximum in aerobic conditions (95% O2-5% CO2 bubbling); increasing the calcium concentration from 2.5 to 10 mM had no effect. To test receptor-associated stimuli, carbachol or histamine (both 5 microM) had little effect on tension in hypoxia. The calcium ionophore A23187 (10 microM) also did not produce any significant contraction in the presence of high K+ under hypoxia. 3. Hypoxia in the presence of high K+ elicited a rapid decrease in phosphocreatine, approximately in parallel with the decrease in tension. The ATP content decreased gradually while Pi content increased. Lactate content increased rapidly and then partially decreased. 4. The rate of ATP synthesis estimated from the oxygen consumption and lactate release of the taenia was linearly correlated with tension development under aerobic and hypoxic conditions. 5. These results suggest that the decrease in tension of the taenia observed under hypoxic conditions is due to an inhibition of energy metabolism, and not due to an oxygen-sensing step in excitation-contraction coupling.

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