Acute regulation of OAT3-mediated estrone sulfate transport in isolated rabbit renal proximal tubules.

We investigated the regulation of organic anion transport driven by the organic anion transporter 3 (OAT3), a multispecific OAT localized at the basolateral membrane of the renal proximal tubule. PMA, a PKC activator, inhibited uptake of estrone sulfate (ES), a prototypic substrate for OAT3, in a dose- and time-dependent manner. This inhibition was reduced by 100 nM bisindoylmaleimide I (BIM), a specific PKC inhibitor. The alpha(1)-adrenergic receptor agonist phenylephrine also inhibited ES uptake, and this effect was reduced by BIM. These results suggest that PKC activation downregulates OAT3-mediated organic anion transport. In contrast, epidermal growth factor (EGF) increased ES uptake following activation of MAPK. Exposure to PGE(2) or dibutyryl (db)-cAMP also enhanced ES uptake. Stimulation produced by PGE(2) and db-cAMP was prevented by the PKA inhibitor H-89, indicating that this stimulation required PKA activation. In addition, inhibition of cyclooxygenase 1 (COX1) (but not COX2) inhibited ES uptake. Furthermore, the stimulatory effect of EGF was eliminated by inhibition of either COX1 or PKA. These data suggest that EGF stimulates ES uptake by a process in which MAPK activation results in increased PGE(2) production that, in turn, activates PKA and subsequently stimulates ES uptake. Interestingly, EGF did not induce upregulation immediately following phenylephrine-induced downregulation; and phenylephrine did not induce downregulation immediately after EGF-induced upregulation. These data are the first to show the regulatory response of organic anion transport driven by OAT3 in intact renal proximal tubules.

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.

Effect of stevioside on PAH transport by isolated perfused rabbit renal proximal tubule.

Stevioside, a non-caloric sweetening agent, is used as a sugar substitute. An influence of stevioside on renal function has been suggested, but little is known about its effect on tubular function. Therefore, the present study was designed to explore the direct effect of stevioside on transepithelial transport of p-aminohippurate (PAH) in isolated S2 segments of rabbit proximal renal tubules using in vitro microperfusion. Addition of stevioside at a concentration of 0.45 mM to either the tubular lumen, bathing medium, or both at the same time had no effect on transepithelial transport of PAH. Similarly, a concentration of 0.70 mM (maximum solubility in the buffer) when present in the lumen, had no effect on PAH transport. However, this concentration in the bathing medium inhibited PAH transport significantly by about 25-35%. The inhibitory effect of stevioside was gradually abolished after it was removed from the bath. Addition of 0.70 mM stevioside to both lumen and bathing medium at the same time produced no added inhibitory effect. Stevioside at this concentration has no effect on Na+/K+-ATPase activity as well as cell ATP content. These findings suggest that stevioside, at a pharmacological concentration of 0.70 mM, inhibits transepithelial transport of PAH by interfering with the basolateral entry step, the rate-limiting step for transepithelial transport. The lack of effect of stevioside on transepithelial transport of PAH on the luminal side and its reversible inhibitory effect on the basolateral side indicate that stevioside does not permanently change PAH transport and should not harm renal tubular function at normal human intake levels.

PAH-alpha-KG countertransport stimulates PAH uptake and net secretion in isolated snake renal tubules.

The possibility that p-aminohippurate (PAH)-alpha-ketoglutarate (alpha-KG) countertransport could stimulate basolateral uptake and net transepithelial secretion of PAH was examined in intact isolated nonperfused and perfused snake (Thamnophis spp.) distal proximal renal tubules. Preloading tubules with alpha-KG (100 microM) for 30 min increased [14C]PAH rate of uptake by nonperfused tubules and rate of net secretion by perfused tubules approximately three- to fivefold without increasing cellular ATP content. During stimulation of net secretion, intracellular [14C]PAH concentration increased to same extent as net secretion. Presence of Li+ (2 mM) or absence of Na+ (inhibitors of Na(+)-dicarboxylate cotransport) in bath during alpha-KG preloading eliminated stimulation of PAH transport. Addition of unlabeled alpha-KG to bathing medium stimulated efflux of [14C]PAH across basolateral membrane of tubules with oil-filled lumina, further supporting the concept of PAH-alpha-KG countertransport. Preloading with succinate (100 microM) also stimulated [14C]PAH uptake by nonperfused tubules and net secretion by perfused tubules, but stimulation was only 1.5- to 2-fold and cellular ATP content increased. Moreover, preloading with methyl succinate, a slowly metabolized derivative of succinate, did not stimulate PAH uptake by nonperfused tubules or secretion by perfused tubules. Thus succinate appears to stimulate PAH transport via metabolism, possibly by conversion to alpha-KG, not by direct countertransport. This study indicates for the first time in intact renal tubules that PAH-alpha-KG countertransport can stimulate net PAH secretion by generating an increased intracellular PAH concentration.