Role of ATP and Na+ in the Regulation of Cytosolic Free Calcium in Kidney Cells
Cytosolic free calcium (Cai 2+) is controlled by several calcium transporters located at the plasma membrane, the inner membrane of the mitochondria and in the endoplasmic reticulum. Fig. 1 shows the various ATP-dependent pumps and antiporters involved in the control of Cai 2+ and Nai + (In this figure, the plasmalemmal Na+-Ca2+ antiporter is conventionally assumed to operate in the forward mode i.e. Ca2+ efflux energized by Na+ influx). Since Ca2+ transport out of the cell can be energized by ATP or by the Na+ electrochemical potential ΔμNa+, a fall in cellular ATP or a drop in ΔμNa+ would be expected to depress Ca2+ efflux, raise Cai 2+ and increase the cell Ca2+ content. In recent experiments (1,2) performed with cultured monkey kidney cells (LLC-MK2), my collaborators Dr. K.W. Snowdowne, C.C. Freudenrich and I found that only one of these three predictions was proved correct.
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