Abstract
Citrate inhibits calcium nephrolithiasis by complexing calcium (Ca) in a soluble form. Lowering apical extracellular Ca from 1.2mM (normal levels, Nl) to <60μM (low) results in an increase in Cit and succinate (Suc) transport in OK cells. Activation of the calcium-sensing receptor (CaSR) with spermine inhibits transport in both low and Nl Ca. But activation of PKC with PMA inhibited transport in low Ca only, implying that Ca-sensitive dicarboxylate transport is regulated by the CaSR through the Gq/PKC pathway. The purpose here is to determine the role of the Gi pathway. 14C-Suc uptakes were performed in OK cells and HRPE cells transfected with NaDC1 (CUBS). CaSRs inhibit adenylate cyclase (AC) through Gi. To mimic Gi, cells were pre-incubated with an AC inhibitor, MDL-12,330A (50μM for 30min) followed by uptake. The addition of MDL inhibited uptake in both Nl (0.09±0.01 to 0.05±0.01, p<0.01) and low Ca (0.13±0.01 to 0.08±0.01, p<0.01). In CUBS uptake was also inhibited in Nl (0.6±0.02 to 0.3±0.03, p<0.01) and low Ca (0.5±0.04 to 0.2±0.03, p<0.01). These results are similar to our studies of CaSR activation by spermine. If MDL inhibits transport via AC inhibition, 8-Br-cAMP (8-Br) should reverse MDL action. In OK cells, MDL eliminated the Ca-sensitivity; i.e. Nl (0.05±0.004) vs low Ca (0.05±0.002). In low Ca, 8-Br negated the inhibitory effects of MDL thus significantly increasing uptake (0.049±0.002 to 0.063±0.002, p<0.05). Thus stimulation of the CaSR through both Gq and Gi indicates CaSR involvement in the regulation of Ca-sensitive citrate transport. NIH/NIDDK
Published Version
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