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Studies on calcium uptake by myometrial microsomes with particular reference to the dependence on inorganic phosphate and oxalate.

Ca uptake by microsomes isolated from non-pregnant rabbit myometrium was potentiated by both inorganic phosphate (Pi) and oxalate anions. Both Pi and oxalate had little effect on the initial rate of uptake but a pronounced effect on the capacity of Ca uptake measured after 20 min which was greater in the presence of oxalate than that of Pi (5 mM each). The presence or absence of sucrose in the uptake medium had a significant effect on oxalate-induced potentiation of Ca uptake but not on that potentiated by Pi or that measured in the absence of either potentiating anion. A part of Ca accumulated additionally under the influence of sucrose could be removed by washing microsomes with KCl. Another significant difference between the pontentiating effect of oxalate and Pi was observed when the pH of the incubation medium was varied. In the presence of oxalate the pH optimum was between 6.4--6.8, whereas that in its absence or in the presence of Pi the optimal pH was around 7.2. Reduction in pH from 7.2 to 6.8 along with the substitution of KCl by sucrose resulted in 3-fold increase in Ca uptake when oxalate was used as the potentiating anion. The results suggest that Ca is taken up by a different mechanism in the presence of oxalate than that in its absence or when oxalate anion is substituted with inorganic phosphate.