EFFECTS OF PH AND INORGANIC-PHOSPHATE ON FORCE PRODUCTION IN ALPHA-TOXIN-PERMEABILIZED ISOLATED RAT UTERINE SMOOTH-MUSCLE

1. Strips of longitudinal smooth muscle isolated from rat uterus were permeabilized using crude alpha-toxin from the bacterium Staphylococcus aureus. This treatment rendered the surface membrane permeable to small molecular weight substances. Simultaneous measurements of tension and calcium concentration ([Ca2+]) (using indo-1 fluorescence) were used to investigate the effects of pH and inorganic phosphate concentration ([P(i)]) on Ca2+-activated force generated by the contractile proteins. 2. Raising the [P(i)] from 1 to 1 1 mm at a pH of 7-2 depressed both maximal and submaximal Ca 2+ -activated force. This effect of P(i) was concentration dependent having the majority of its effect by 6 mm. 3. Further experiments at a submaximal [Ca2+] showed that Ca 2+ -activated force was enhanced by raising [P(i)] from 6 to 11 mm suggesting that P(i) increased the Ca2+ sensitivity of tension production. Based on these results, calculations indicate that the apparent affinity constant of Ca2+ for the contractile proteins increased from 4 x 10(6) M-1 to 6 x 10(6) M-1 on raising [P(i)] from 1 to 11 mm. 4. Lowering pH from 7.2 to 6.7 at a [P(i)] of 1 mm potentiated Ca 2+ -activated force with a small depression in the apparent Ca2+ sensitivity of tension production. This effect of pH on maximum (100 mum Ca2+ ) and submaximum (0.3 mum Ca 2+) Ca 2+-activated force was observed over a range of acidic pHs (7.0-6.7). 5. Increasing pH from 7.2 to 7.7 at a [P(i)] of 1 mm depressed Ca2+-activated force with no effect on Ca 2+ sensitivity of tension production. 6. Spontaneous contractions in intact rat myometrium are abolished under hypoxic conditions. Under these same conditions intracellular [P(i)] rises and pH falls. The results of this study suggest that taken individually neither the effect of a rise in [P(i)] nor a fall in pH on Ca 2+ -activated force generated by the contractile proteins can account for the effect of hypoxia on spontaneous contractions.