Separate entry pathways for phosphate and oxalate in rat brain microsomes

ATP-dependent Ca-45 uptake in rat brain microsomes was measured in intracel lular-like media containing different concentrations of PO4 and oxalate. In the absence of divalent anions, there was a transient Ca-45 accumulation, lasting only a few minutes. Addition of PO4 did not change the initial accu mulation but added a second stage that increased with PO4 concentration. Ac cumulation during the second stage was inhibited by the following anion tra nsport inhibitors: niflumic acid (50 mu M), 4,4'-dinitrostilbene-2,2'-disul fonic acid (DNDS; 250 mu M), and DIDS (3-5 mu M); accumulation during the i nitial stage was unaffected. Higher concentrations of DIDS (100 mu M), howe ver, inhibited the initial stage as well. Uptake was unaffected by 20 mM Na , an activator, or 1 mM arsenate, an inhibitor of Na-PO4 cotransport. An ox alate-supported Ca-45 uptake was larger, less sensitive to DIDS, and enhanc ed by the catalytic subunit of protein kinase A (40 U/ml). Combinations of PO4 and oxalate had activating and inhibitory effects that could be explain ed by PO4 inhibition of an oxalate-dependent pathway, but not vice versa. T hese results support the existence of separate transport pathways for oxala te and PO4 in brain endoplasmic reticulum.