Osmotic hypertonicity of the renal medulla during changes in renal perfusion pressure in the rat.

1. The relationship between renal perfusion pressure (RPP) and ion concentration in renal medulla was studied in anaesthetized rats. RPP was changed in steps within the pressure range 130-80 mmHg, while tissue electrical admittance (Y, index of interstitial ion concentration) and medullary and cortical blood flow (MBF and CBF; laser Doppler flowmetry) were measured, along with glomerular filtration rate (C in) and renal excretion. 2. With a RPP reduction from 130 to 120 mmHg, tissue Y remained stable; at 100 and 80 mmHg, Y was 5 and 17 % lower, respectively, than at 120 mmHg. 3. CBF fell less than RPP (partial autoregulation) in the range 130-100 mmHg only. MBF was autoregulated within 120-100 mmHg, but not above or below this range. 4. Each step of RPP reduction was followed by a decrease in sodium and water excretion (UNaV and V). The osmolality of excised inner medulla fragments was similar at 120 and 105 mmHg (586 +/- 45 and 618 +/- 35 mosmol (kg H2O)-1, respectively) but lower at 80 mmHg (434 +/- 31 mosmol (kg H2O)-1, P < 0.01); the ion concentration changed in parallel. 5. The data show that medullary hypertonicity was well preserved during RPP fluctuations within 130-100 mmHg, but not below this range. RPP-dependent changes of UNaV and V were not clearly associated with changes in solute concentration in medullary tissue.