Effect of prostaglandins E2 and F2α on membrane calcium binding, Ca2+/Mg2+-ATPase activity and membrane fluidity in rat myometrial plasma membranes

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Abstract
Myometrial plasma membrane (MPM) preparations from rats treated with oestradiol were obtained by discontinuous sucrose-gradient centrifugation. The preparations contained calcium-stimulated and magnesium-dependent ATPase (Ca2+/Mg2+-ATPase). A dramatic decrease in the activity of Ca2+/Mg2+-ATPase was observed when preparations were treated with 0·025–10μmol protaglandins E2 and F2α (PGE2 and PGF2α)/l. In contrast, there was a marked increase in MPM-bound 5′-nucleotidase activity at low concentrations (up to 2 μmol/l) of PGE2 and PGF2α; higher concentrations (up to 10 μmol/l), however, led to a progressive inhibition of enzyme activity. Association (specific and non-specific binding) of PGE2 and PGF2α with MPM at pH 7 was found to require Ca2+ (half-maximal concentration approximately 0·7 mmol/l). Changes in the allosteric properties of MPM-bound 5′-nucleotidase by concanavalin A (as reflected by changes in the Hill coefficient) indicated a fluidization of the membrane induced by PGE2 and PGF2α. The steady-state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene-labelled MPM decreased in PGE2- and PGF2α-treated MPM from 1·24 ± 0·04 (s.d.) to 0·66 ± 0·01 and 0·74 ± 0·01 respectively, which is consistent with a general increase in membrane fluidity. It is suggested that PGE2 and PGF2α promote changes in the physical properties of MPM which may be relevant to the induction of uterine contractions by enzymatic regulation of intracellular calcium concentrations. J. Endocr. (1986) 110, 395–404