Abstract 067: Mechanism of Action of 8-Aminoguanine on Renal Excretory Function

Abstract

The endogenous purines 8-aminoguanosine and 8-aminoguanine (8-AG) are K + -sparing natriuretics that increase glucose excretion and attenuate salt-induced hypertension. Most effects of 8-aminoguanosine are not direct, but require conversion in the systemic circulation to 8-AG (i.e., 8-aminoguanosine is a prodrug/prohormone). However, the mechanism of action by which 8-AG affects renal excretory function is unknown and is the subject of this investigation. Because 8-AG has structural similarities with inhibitors of epithelial Na + channels (ENaC), Na + /H + exchangers (NHE), and adenosine A 1 receptors, we examined the effects of 8-AG on amiloride-sensitive ENaC activity in mouse collecting duct cells, on intracellular pH of human renal proximal tubule epithelial cells, and on in vivo (in rats) pharmacological responses to 2-chloro-N 6 -cyclopentyladenosine (A 1 -receptor agonist). 8-AG did not block ENaC, NHE, or A 1 receptors. Because Rac1 enhances activity of mineralocorticoid receptors and some guanosine analogues inhibit Rac1, we examined the effects of 8-AG on Rac1 activity in mouse collecting duct cells. Rac1 activity was significantly ( P =0.024) inhibited by 8-AG (30 μM; from 2.5 ± 0.7 to 1.7 ± 0.7 arbitrary units). Because 8-AG is an inhibitor of purine nucleoside phosphorylase (PNPase; metabolizes inosine to hypoxanthine and guanosine to guanine), we compared the renal effects (in rats) of approximately equipotent intravenous doses of 8-AG (33.5 μmol/kg) vs. 9-deazaguanine (9-DG; 67 μmol/kg; PNPase inhibitor). 8-AG and 9-DG induced similar increases in urinary Na + (μmol/30 min; 8-AG, from 4.4 ± 2.0 to 38 ± 17; 9-DG, from 5.6 ± 3.3 to 34 ± 8.6) and glucose (μg/30 min; 8-AG, from 31 ± 17 to 225 ± 77; 9-DG, from 6.9 ± 3.9 to 144 ± 81) excretion. Intravenous 8-AG increased urinary excretion of guanosine and inosine (PNPase substrates), yet decreased excretion of guanine and hypoxanthine (PNPase products). 8-AG reduced K + excretion (μmol/30 min; 8-AG, from 29 ± 6.6 to 5.4 ± 1.7) whereas 9-DG did not. However, the Rac1 inhibitor Nsc23766 (9.4 μmol/kg) mimicked the effects of 8-AG on K + excretion (μmol/30 min; from 40 ± 7.1 to 18 ± 6.5). Conclusion: Likely 8-AG increases Na + and glucose excretion by blocking PNPase and decreases K + excretion by inhibiting Rac1.