1. The aim of this study was to compare certain renal and erythrocyte functions in the Milan hypertensive strain (MHS) of rats and in normotensive control animals (MNS).
2. Under experimental conditions close to those in conscious unmanipulated animals, the glomerular filtration rate (GFR) per g kidney weight was higher and the kidney weight/body weight ratio lower in MHS than in MNS rats (P < 0.001). This suggests that there is increased transtubular ion transport in MHS animals.
3. The MHS rats had a higher rate constant for outward Na+-K+ cotransport than the MNS animals (P < 0.02), their intra-erythrocyte sodium concentrations were lower (P < 0.05) and their erythrocyte volume was smaller (P < 0.001). Therefore, accelerated cell membrane sodium outward cotransport seems to reset the volume and the sodium content of the erythrocytes of MHS rats at a lower level.
4. We speculate that increased transport of ions across the cell membrane may be an abnormality common to erythrocytes and renal tubular cells in MHS rats.