Elvino J. G. Barros

Molecular Cloning and Characterization of NKT, a Gene Product Related to the Organic Cation Transporter Family That Is Almost Exclusively Expressed in the Kidney

We have identified a gene product (NKT) encoding an apparently novel transcript that appears to be related to the organic ion transporter family and is expressed almost exclusively in the kidney. Analysis of the deduced 546-amino acid protein sequence indicates that NKT is a unique gene product which shares a similar transmembrane domain hydropathy profile as well as transporter-specific amino acid motifs with a variety of bacterial and mammalian nutrient transporters. Nevertheless, the overall homology of NKT to two recently cloned organic ion transport proteins (NLT and OCT-1) is significantly greater; together these three gene products may represent a new subgroup of transporters. The NKT was characterized further with respect to its tissue distribution and its expression during kidney development. A 2.5-kilobase transcript was found in kidney and at much lower levels in brain, but not in a number of other tissues. Studies on the embryonic kidney indicate that the NKT transcript is developmentally regulated with significant expression beginning at mouse gestational day 18 and rising just before birth, consistent with a role in differentiated kidney function. Moreover, in situ hybridization detected specific signals in mouse renal proximal tubules. NKT was mapped by linkage disequilibrium to mouse chromosome 19, the same site to which several mouse mutations localize, including that for osteochondrodystrophy (ocd). Although initial experiments in a Xenopus oocyte expression system failed to demonstrate transport of known substrates for OCT-1, the homology to OCT-1 and other transporters, along with the proximal tubule localization, raise the possibility that this gene may play a role in organic solute transport or drug elimination by the kidney.

Nephrotoxicity of cyclosporine: The role of platelet-activating factor and thromboxane

Cyclosporine (CsA), an immunosuppressive agent, is potentially nephrotoxic. We had previously observed that acute administration of CsA to Munich-Wistar rats induced a decrease in single nephron glomerular filtration rate, due to a decline in glomerular plasma flow, and in the glomerular ultrafiltration coefficient. Moreover, these alterations were prevented when an antagonist of platelet-activating factor (PAF) was administered. In the present study we examined whether the protective effect of the PAF blocker in CsA nephrotoxicity could have been mediated by thromboxane (TxA2). Our data show that the PAF effects were not mediated by TxA2, since administration of dazmegrel, a thromboxane synthetase inhibitor, did not ameliorate the acute renal failure caused by CsA. Thus, PAF appears to be a direct mediator of acute CsA nephrotoxicity, while TxA2 is not significantly involved in this process.

Glomerular Hemodynamic Effects of Cyclosporine

We evaluated the effects of acute and chronic Cyclosporine (CsA) administration on renal microcirculation, as well as the role of hormonal systems in this nephrotoxicity. Studies were performed in euvolemic Munich-Wistar rats by using a micropuncture technique. Acute CsA infusion caused a decline in single nephron (SN) glomerular filtration rate (GFR) due to a decrease in glomerular plasma flow (QA) and a decrease in the glomerular ultrafiltration coefficient (Kf), despite an increase in transcapillary hydraulic pressure difference (ΔP). Both captopril and verapamil partially prevented the decrease in whole kidney function altered by acute CsA infusion. Conversely, indomethacin did not modify these parameters, suggesting that the renin-angiotensin system and calcium play a role in CsA nephrotoxicity, while the prostaglandin system participated in this event by a direct blocking of CsA. The platelet activating factor (PAF) antagonist, (BN 52 021), blunted CsA effects on superficial nephron function. Since dazmegrel, a thromboxane synthetase inhibitor, did not change the SNGFR already altered by CsA, the beneficial effects of BN 52 021 would appear to be related to a direct blocking of PAF actions. Acute CsA nephrotoxicity was not fully demonstrated in Brattleboro rats, who present a hereditary absence of antidiuretic hormone (ADH), thus suggesting that the absence of ADH can be a protective factor in CsA nephrotoxicity. Short-term chronic infusion of CsA provoked a decline in SNGFR due to a decrease in QA and ΔP, while Kf remained unaltered. Glomerular hemodynamic patterns in acute and chronic CsA administration were different, suggesting that distinct pathophysiological mechanisms were involved and that distinct pathophysiological approaches would be required to improve knowledge of CsA nephrotoxicity.

Effects of Acute and Chronic Cyclosporine Administration on Glomerular Haemodynamics in Rats

Cyclosporine (CsA) a potent immunosuppressive drug which has afforded benefit in transplantation and in the treatment of several immune-mediated diseases (1–3). However, a number of side effects have been observed, with nephrotoxicity being the most common and important (4–6). The impairment of glomerular filtration rate (GFR) and thus, acute renal failure (ARF) observed with CsA appears to be related with renal haemodynamic alterations (7,8) . No significant glomerular lesions have been found (4).

Renal Toxicity of Cis-Dichlorodiammine Platinum in Rats

Cis-Dichlorodiammine Platinum (DDP) is an effective anticancer agent that changes the natural history of many tumours, especially testicular carcinoma. However, its use in the treatment of several kinds of solid tumours has been restricted by a high renal toxicity incidence (1–3) . Acute or chronic DDP administration in experimental models have shown an important reduction on glomerular filtration rate but, the pathophysiological mechanism of this acute renal failure (ARF) is unclear.