Effect of steviol on para-aminohippurate transport by isolated perfused rabbit renal proximal tubule

An inhibitory effect of steviol, metabolite of the natural sweetener stevioside, on transepithelial transport of p-aminohippurate (J(PAH)) was observed in isolated S(2) segments of rabbit renal proximal tubules using in vitro microperfusion. Addition of steviol (0.01--0.25 mM) to the bathing medium significantly depressed J(PAH) (approximately 50--90%). This inhibitory effect was dose-dependent and was maximum at a concentration of 0.05 mM. To further examine this effect, a steviol concentration (0.01 mM) that produced approximately 50% inhibition of J(PAH), was chosen. Addition of 0.01 mM steviol to the bathing medium significantly depressed J(PAH) by about 50 to 60%. Steviol at the same concentration (0.01 mM), when present in the tubule lumen, had no significant effect on J(PAH). Addition of 0.01 mM steviol to lumen and bath simultaneously, produced a slightly greater inhibitory effect compared with addition to bath alone (60 versus 70%). A higher concentration of steviol, 0.05 mM (which maximally inhibited J(PAH) when on the basolateral side), was required on the luminal side than on the basolateral side before an inhibitory effect was observed. To further examine the mechanism by which steviol inhibited J(PAH), its effect on Na(+)-K(+) ATPase activity and ATP content was determined. Steviol at concentrations of 0.01 and 0.05 mM had no effect on Na(+)-K(+) ATPase activity or cell ATP content. Kinetic analyses indicated that steviol can competitively inhibit PAH transport at the basolateral membrane. The present study clearly showed that steviol can have a direct inhibitory effect on renal tubular transport by competitive binding with organic anion transporter.