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Effects of pH and inorganic phosphate on force production in alpha‐toxin‐permeabilized isolated rat uterine smooth muscle.

1993; Wiley; Volume: 465; Issue: 1 Linguagem: Inglês

10.1113/jphysiol.1993.sp019697

1469-7793

Catherine A. Crichton, Michael J. Taggart, Susan Wray, Godfrey L. Smith,

Hemoglobin structure and function

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 ([Pi]) on Ca(2+)‐activated force generated by the contractile proteins. 2. Raising the [Pi] from 1 to 11 mM at a pH of 7.2 depressed both maximal and submaximal Ca(2+)‐activated force. This effect of Pi 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 [Pi] from 6 to 11 mM suggesting that Pi 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 [Pi] from 1 to 11 mM. 4. Lowering pH from 7.2 to 6.7 at a [Pi] 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 microM Ca2+) and submaximum (0.3 microM Ca2+) 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 [Pi] of 1 mM depressed Ca(2+)‐activated force with no effect on Ca2+ sensitivity of tension production. 6. Spontaneous contractions in intact rat myometrium are abolished under hypoxic conditions. Under these same conditions intracellular [Pi] rises and pH falls. The results of this study suggest that taken individually neither the effect of a rise in [Pi] 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.