[Mg2+,ATP-dependent transport of Ca2+ in the endoplasmic reticulum of myometrial cells].

An in-depth study of the effects of Ca(2+)-precipitating anions (oxalate, phosphate), the Ca2+ ionophore A23187 and some highly effective selective inhibitors of the endo(sarco)plasmic reticulum Ca(2+)-pump (tapsigargin, cyclopiasonic acid) on ruthenium red (10 microM)-insensitive, Mg2+,ATP-dependent accumulation of Ca2+ in digitonin-permeabilized myometrium cells of nonpregnant estrogen-treated rats, has been carried out. Oxalate and phosphate stimulated Mg2+,ATP-dependent accumulation of Ca2+ in permeabilized smooth muscle cells. The Ca2+ ionophore A23187 (5 microM) added to the medium containing oxalate (10 mM) or phosphate (20 mM) prevented the energy-dependent accumulation of Ca2+. Tapsigargin (50 nM) and cyclopiasonic acid (10 microM) fully suppressed the Mg2+,ATP-dependent Ca2+ increase measured in the presence of the Ca(2+)-precipitating anions. The kinetic data indicate that the tapsigargin affinity for the energy-dependent Ca2+ transporting system is much higher than that of cyclopiasonic acid: Ki app values are 0.9 nM and 0.6 microM, respectively. Oxalate (10 mM) added to the incubation medium initially containing tapsigargin (50 nM) failed to induce the Mg2+,ATP-dependent accumulation of Ca2+ in permeabilized myocytes. The data obtained provide biochemical evidence for the existence in myometrium cells of a Ca(2+)-pump providing the Mg2+,ATP-dependent accumulation of Ca2+ in the nonmitochondrial Ca2+ depot--the endoplasmic reticulum. This reaction is stimulated by precipitating penetrating anions and suppressed by selective inhibitors.