[Two components of sodium-azide insensitive Mg2+,ATP-dependent Ca2+ transport in ureteral smooth muscle membrane structures].

The effects of Ca(2+)-precipitating anions (oxalate and phosphate) and effective inhibitors of endo/sarcoplasmic reticulum calcium pump (thapsigargin and cyclopiazonic acid) on azide-insensitive (5 mM) Mg2+,ATP-dependent Ca2+ accumulation in microsomes of ureter smooth muscle cells were studied. Oxalate (0-20 mM) and phosphate (0-60 mM) stimulate Mg2+,ATP-dependent Ca2+ accumulation. Thapsigargin and cyclopiazonic acid at 100 nM and 20 microM, respectively, completely inhibited (i.e., down to the level in the absence of oxalate) Ca2+ accumulation activated by 10 nM oxalate. These inhibitors only partially inhibited Ca2+ accumulation activated by 40 mM phosphate. Mg2+,ATP-dependent Ca2+ accumulation in microsomes, which is inhibited by thapsigargin and cyclopiazonic acid and activated by oxalate or phosphate, can result from functioning of calcium pump in endoplasmic reticulum of ureter myocytes. The inhibition constant, Ki, was calculated by the method of Hill and it was 0.3 nM and 0.2 microM for thapsigargin or cyclopiazonic acid, respectively. Mg2+,ATP-dependent Ca2+ accumulation in microsomes, which is insensitive to thapsigargin or cyclopiazonic acid and activated by phosphate, can result from functioning of calcium pump in plasma membranes of ureter myocytes.