Separate entry pathways for phosphate and oxalate in rat brain microsomes

ATP-dependent 45Ca uptake in rat brain microsomes was measured in intracellular-like media containing different concentrations of PO4 and oxalate. In the absence of divalent anions, there was a transient 45Ca accumulation, lasting only a few minutes. Addition of PO4did not change the initial accumulation but added a second stage that increased with PO4 concentration. Accumulation during the second stage was inhibited by the following anion transport inhibitors: niflumic acid (50 μM), 4,4′-dinitrostilbene-2,2′-disulfonic acid (DNDS; 250 μM), and DIDS (3–5 μM); accumulation during the initial stage was unaffected. Higher concentrations of DIDS (100 μM), however, inhibited the initial stage as well. Uptake was unaffected by 20 mM Na, an activator, or 1 mM arsenate, an inhibitor of Na-PO4cotransport. An oxalate-supported 45Ca uptake was larger, less sensitive to DIDS, and enhanced by the catalytic subunit of protein kinase A (40 U/ml). Combinations of PO4 and oxalate had activating and inhibitory effects that could be explained by PO4 inhibition of an oxalate-dependent pathway, but not vice versa. These results support the existence of separate transport pathways for oxalate and PO4 in brain endoplasmic reticulum.