Cause of the reduction in renal blood flow in the hypothermic (27 degrees C) dog

Effect of nitric oxide on renal autoregulation during hypothermia in the rat

Hypothermia-induced reduction of metabolic rate is accompanied by depression of both glomerular perfusion and filtration. The present study investigated whether these changes are linked to changes in renal autoregulation and nitric oxide (NO) signalling. During hypothermia, renal blood flow (RBF) and glomerular filtration rate (GFR) were reduced and urine production was increased, and this was linked with reduced plasma cGMP levels and increased renal vascular resistance. Although stimulation of NO production decreased vascular resistance, blood pressure and urine flow, intravenous infusion of the NO precursor L-arginine or the NO donor sodium nitroprusside did not alter RBF or GFR. In contrast, inhibition of NO synthesis by N^w-nitro-L-arginine led to a further decline in both parameters. Functional renal autoregulation was apparent at both temperatures. Below the autoregulatory range, RBF in both cases increased in proportion to the perfusion ±pressure, although, the slope of the first ascending limb of the pressure-flow relationship was lower during hypothermia. The main difference was rather that the curves obtained during hypothermia levelled off already at a RBF of 3.9 ± 0.3 mL/min then remained stable throughout the autoregulatory pressure range, compared to 7.6 ± 0.3 mL/min during normothermia. This was found to be due to a threefold increase in, primarily, the afferent arteriolar resistance from 2.6 to 7.5 mmHg min mL⁻¹. Infusion of sodium nitroprusside did not significantly affect RBF during hypothermia, although a small increase at pressures below the autoregulatory range was observed. In conclusion, cold-induced rise in renal vascular resistance results from afferent arteriolar vasoconstriction by the autoregulatory mechanism, setting RBF and GFR in proportion to the metabolic rate, which cannot be explained by reduced NO production alone.

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

The effects of 1-2 h of hypothermia at 28 degrees 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 37.5 degrees C (control) to 4.0 +/- 0.47 at 28 degrees 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-1, 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 microL 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.