We have demonstrated that renal apical membrane presents a 2Na⁺/1Cl^-/1creatine cotransport activity. The goal of this study was to determine whether this transporter is ontogenically regulated, and if so, to investigate the molecular mechanism(s) involved in such regulation. Na⁺/Cl^-/creatine transport activity was evaluated by measuring [14C]-creatine uptake into renal brush-border membrane vesicles (BBMV). CRT mRNA expression was measured by Northern and real-time PCR assays. E20 fetuses, newborn, suckling, weaning and adult (2- and 8-month-old) Wistar rats were used. All procedures accord with current national/local ethical guidelines.

Neither the vesicular volume (1.2 ml/mg protein), the binding of creatine to the BBMV, nor the purity of the BBMV preparations was affected by maturation. Fetal and neonatal BBMVs contained a creatine transporter that was qualitatively indistinguishable from that in the adult: it was concentrative, Na⁺ and Cl^--dependent, electrogenic and inhibited by guanidinopropionic acid. Maturation increased this transport activity by increasing the maximal rate of transport (Vmax) without significantly changing the apparent Kt. Northern analysis revealed two transcripts for CRT of 2.7 kb and 4.2 kb in all the ages tested. Northern and real-time PCR assays showed that, as seen with NaCl-dependent creatine transport activity, maturation increased CRT mRNA expression. This study reports for the first time that: i) an apical renal Na⁺/Cl^-/creatine cotransporter is already active in rat fetuses, ii) development regulates Na⁺/Cl^-/creatine cotransport activity by increasing the density and/or turnover of the transporters and iii) this regulation appears to involve transcriptional regulatory mechanism(s).

Supported by the Spanish MCyT-FEDER BFI 2003-00222