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 Staphylococcu s aureus. This treatment rendered the surface membrane permeable to sm all molecular weight substances. Simultaneous measurements of tension and calcium concentration ([Ca2+]) (using indo-1 fluorescence) were us ed to investigate the effects of pH and inorganic phosphate concentrat ion ([P(i)]) on Ca2+-activated force generated by the contractile prot eins. 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 m m. 3. Further experiments at a submaximal [Ca2+] showed that Ca 2+ -ac tivated force was enhanced by raising [P(i)] from 6 to 11 mm suggestin g that P(i) increased the Ca2+ sensitivity of tension production. Base d on these results, calculations indicate that the apparent affinity c onstant 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 forc e with a small depression in the apparent Ca2+ sensitivity of tension production. This effect of pH on maximum (100 mum Ca2+ ) and submaximu m (0.3 mum Ca 2+) Ca 2+-activated force was observed over a range of a cidic 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 myom etrium are abolished under hypoxic conditions. Under these same condit ions intracellular [P(i)] rises and pH falls. The results of this stud y 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 contrac tile proteins can account for the effect of hypoxia on spontaneous con tractions.