Ca2+ transport studied with arsenazo III in Tetrahymena microsomes. Effects of calcium ionophore A23187 and trifluoperazine.

Transport of Ca2+ in microsomal membrane vesicles of the Tetrahymena has been investigated using arsenazo III as a Ca2+ indicator. The microsomes previously shown to carry a Mg2+-dependent, Ca2+-stimulated ATPase (Muto, Y. and Nozawa, Y. (1984) Biochim. Biophys. Acta 777, 67-74) accumulated calcium upon addition of ATP and Ca2+ sequestered into microsomal vesicles was rapidly discharged by the Ca2+ ionophore A23187. Kinetic studies indicated that the apparent Km for free Ca2+ and ATP are 0.4 and 59 microM, respectively. The Vmax was about 40 nmol/mg protein per min at 37 degrees C. The calcium accumulated during ATP-dependent uptake was released after depletion of ATP in the incubation medium. Furthermore, addition of trifluoperazine which inhibited both (Ca2+ + Mg2+)-ATPase and ATP-dependent Ca2+ uptake rapidly released the calcium accumulated in the microsomal vesicles. These observations suggest that Tetrahymena microsome contains both abilities to take up and to release calcium and may act as a Ca2+-regulating site in this organism.