High-affinity pH-dependent passive Ca binding by myometrial plasma membrane vesicles.
Rat myometrium plasma membrane-(PM) enriched fraction N1 binds calcium passively in a pH-dependent manner at a Ca2+ concentration of 1 microM. The Ca binding increases with increasing pH from 6.27 to 7.47 with a half maximum near 6.8. The difference between binding at 6.27 and 7.07 (the pH-dependent Ca binding) depends on the pH of the reaction medium rather than the pH of the medium in which the membranes had previously been suspended. The pH-dependent Ca binding is not an artifact due to EGTA, the pH buffer used, or soluble protein trapped inside the membrane vesicles. The pH-dependent Ca binding occurs with a dissociation constant value of 0.28 microM and Hill coefficient of 2.37 for Ca2+. The high affinity pH-dependent Ca uptake and the release of Ca2+ from the membranes is virtually complete in 10 s in the presence of 1 microM A23187 but not in its absence. The distribution of the pH-dependent Ca binding in the various rat myometrium subcellular fractions parallels the activity of 5'-nucleotidase in these fractions and not the activities of NADPH-dependent or succinate-dependent cytochrome c reductases. The high affinity and rapid binding and release of Ca at the pH-dependent Ca binding sites in the PM-enriched fraction suggests that the binding and release from these sites may play a key role in excitation-contraction coupling of the smooth muscle.
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D005260:Female; D006863:Hydrogen-Ion Concentration; D007093:Imidazoles / Q000494:pharmacology; D009215:Myometrium / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013347:Subcellular Fractions / Q000378:metabolism; D014475:Uncoupling Agents / Q000494:pharmacology; D014599:Uterus / Q000378:metabolism*
D007093:Imidazoles; D014475:Uncoupling Agents; D000001:Calcimycin; D004533:Egtazic Acid; D002118:Calcium