Ca²⁺-dependent ATPases in the basolateral membrane of rat kidney cortex

The basolateral segment of the rat renal tubular plasma membrane possesses Ca²⁺-dependent ATPase activity which was independent of Mg²⁺. Two kinetic forms were found: one, was a high affinity (apparent K(m) for free Ca²⁺ of 172 nM) low capacity (V(max) of 144 nmol of P(i)·min^-1·mg^-1 protein) type; the other, had low affinity (apparent K(m) of 25 μM) and high capacity (896 nmol of P(i)·min^-1·mg^-1 protein). Mg²⁺ inhibited both Ca²⁺-ATPases. The high affinity enzyme exhibited positive cooperativity with respect to ATP, with a n value of 1.6. Ca²⁺-ATPase activity was not affected by calmodulin and was not inhibited by vanadate. On the other hand, both high and low affinity Ca²⁺-ATPase activities were increased when 1,25-dihydroxycholecalciferol was given to vitamin D-deficient rats. Kinetically, the enhanced activities were due to an increase in the V(max) values; the apparent affinities for free Ca²⁺ were not changed. The physiological function of the vitamin D-sensitive, Mg⁺-independent, Ca²⁺-ATPase activities remains to be established.