Effect of probenecid on tetraethyl ammonium (TEA) transport across basolateral membrane of rabbit proximal tubule

Characteristics of spontaneous calcium oscillations in renal tubular epithelial cells

BACKGROUND

The kidney is a major organ involved in calcium (Ca(2+)) metabolism. Ca(2+) is transported through renal tubular epithelial cells. The intracellular free calcium concentration ([Ca(2+)](i)) is tightly controlled at a low concentration, but transient increases and oscillations in [Ca(2+)](i) are induced by various conditions. In this study, we investigated the mechanisms underlying the spontaneous [Ca(2+)](i) oscillations observed in MDCK cells.

METHODS

[Ca(2+)](i) was monitored in fura-2-loaded Madin-Darby canine kidney (MDCK) cells using a calcium imaging system. We investigated the mechanism by which [Ca(2+)](i) changed by applying drugs or by changing the extracellular Ca(2+) concentration.

RESULTS

Spontaneous [Ca(2+)](i) oscillations occurred in MDCK cells. The oscillations occurred irregularly and were not transmitted to neighboring cells. Spontaneous [Ca(2+)](i) oscillations in MDCK cells were initiated by Ca(2+) release from ryanodine/IP(3)-sensitive intracellular calcium stores, and their frequency was largely unaffected by the extracellular Ca(2+) concentration. Moreover, the frequency of the oscillations was increased by extracellular nucleotide, but was decreased when the nucleotides were removed.

CONCLUSIONS

Our study suggested that [Ca(2+)](i) release from ryanodine/IP(3)-sensitive intracellular calcium stores mediates spontaneous [Ca(2+)](i) oscillations in MDCK cells. Calcium oscillations may be associated with the function of the renal tubular epithelial cells.

Probenecid interferes with renal oxidative metabolism: a potential pitfall in its use as an inhibitor of drug transport

The anionic drug probenecid has been traditionally used as an inhibitor of renal organic anion transport. More recently the drug was found to inhibit organic cation transport as well, and it is used to retain intracellularly loaded fluorophores. In these investigations it is implicitly assumed that probenecid performs its activity through competition for transport. Here we studied the possibility that probenecid provokes its effect through inhibition of cellular oxidative metabolism. Oxygen consumption was measured in isolated rat kidney cortex mitochondria. At concentrations of 1 mM or higher, probenecid increased the resting state (state 4) and decreased the ADP-stimulated respiration (state 3). A complete loss in respiratory control was observed at 10 mM probenecid. After incubating isolated rat kidney proximal tubular cells (PTC) for 30 min with probenecid a concentration-dependent reduction in ATP content was observed, which was significant at concentrations of 1 mM and higher. Using digital image fluorescence microscopy the membrane potential in PTC was measured with bisoxonol. The mitochondrial effects of probenecid were paralleled by a depolarization of the plasma membrane, immediately after drug addition. All events are likely to be a result of membrane disordering due to the lipophilic character of probenecid, and may explain, at least in part, the various inhibitory effects found for the drug. We recommend to be cautious with applying probenecid in cellular research.

Glomerular filtration and saturable absorption of iohexol in the rat isolated perfused kidney

1. The renal handling of iohexol was examined in the rat isolated perfused kidney (IPK) over a perfusate concentration range of 5-20 micrograms ml-1. 2. At a concentration of 5 micrograms ml-1, a ratio of renal clearance over clearance by glomerular filtration (ClR/GF) of 0.63 +/- 0.06 could be determined. This ratio increased until 1.02 +/- 0.06 at 20 micrograms ml-1, indicating that a saturable mechanism is involved in the luminal disappearance of the drug. 3. Pretreatment of the kidneys with polylysine, probenecid or diatrizoate resulted in a significantly enhanced clearance of iohexol, probably due to inhibition of membrane binding. Renal clearance data were fitted to a kinetic model including filtration into the primary urine followed by saturable absorption at the luminal membrane. An absorption constant, KA, of 7.3 +/- 1.3 micrograms ml-1, and a maximum rate of absorption, VA,Max, of 1.4 +/- 0.1 micrograms min-1 were determined. 4. Iohexol accumulated in kidney tissue, reaching a concentration of 2 to 7.5 times the perfusate concentration. In freshly isolated proximal tubular cells and kidney cortex mitochondria, iohexol reduced the uncoupled respiratory rate at a concentration comparable to the highest tissue concentration found in the IPK. 5. In conclusion, iohexol is not only filtered by the kidney but also reabsorbed via a saturable mechanism, which results in tubular accumulation. Intracellularly sequestered iohexol may affect mitochondrial oxidative metabolism. Our results indicate that iohexol is not a true filtration marker.