The effects of hypothermia on renal function and haemodynamics in the rat

The effects of 1–2 h of hypothermia a t 28d?C and rewarming on renal function were investigated in anaesthetized rats, using conventional clearance methods and the micropuncture technique. Renal blood flow (RBF) decreased from 7.3 ± 0.51 mL min^-1 at 37.5d?C (control) to 4.0 ± 0.47 at 28d?C, with almost complete restoration to 6.9 ± 0.59 mL min^-1 after rewarming. Systemic blood pressure remained essentially unaltered. The RBF reduction seen during hypothermia was due to a 75% increase in vascular resistance, mainly attributable to constriction of the afferent arteriole and increased blood viscosity. This was accompanied by a decline in glomerular capillary pressure from 56.7 ± 0.6 to 46.4 ± 1.3 mmHg, overshooting to 59.0 ± 0.7 mmHg. The glomerular filtration rate (GFR) decreased from 1.1 ± 0.08 to 0.6 ± 0.04 mL min^-l, returning to 1.0 ± 0.07 after rewarming, a pattern also observed for single nephron GFR. This resulted from a decrease in net driving force for glomerular filtration, whereas the filtration coefficient was not affected. Both proximal and distal tubular fluid flow decreased, but fractional reabsorption remained unchanged. In contrast, urine flow increased from 1.8 ± 0.16 to 5.7 ± 1.08 μL min^-1, returning to 2.1 ± 0.18, the increase during hypothermia mainly resulting from a disproportionately reduced fluid reabsorption beyond the mid-distal tubule.