Methods
In order to separate cooling sensitive transcytosis from passive transport processes, which should be much less cooling sensitive, we induced hypothermia in rats (~300g), reducing their body temperature to 19^oC. Control rats were kept at 37^oC. Radiolabeled human albumin or LDL was given intraarterially and ⁵¹Cr-EDTA infused intravenously. During tracer administration, a 2h peritoneal dialysis (PD) dwell was performed using 16 ml of conventional dialysis fluid (essentially a lactated Ringer's solution containing 1.36% glucose as osmotic agent). Heart rate (HR) and blood pressure (BP) were continuously monitored. Clearance (Cl) of LDL and albumin to the dialysate, and the peritoneal permeability - surface area product (PS) for ⁵¹Cr-EDTA, were assessed.

Results
During cooling to 19^oC, clearance of LDL to the dialysate fell from 3.04±0.27 (SE) to 0.55±0.06 ml/min (p<0.05; n=7), i.e. by ~80%, and Cl of albumin from 8.68±1.48 to 3.31±0.47 ml/min (p<0.05; n=6), i.e. by 62%, while the concomitant reduction in PS for Cr-EDTA (from 0.43±0.09 ml/min to 0.08±0.01 ml/min, p<0.05; n=9) occurred by ~80%. The increase in plasma viscosity was ~50%, while HR and BP were reduced by 72% and 60%, respectively, reflecting a reduced cardiac output (CO) at 19^oC.

Conclusions
In conclusion, a temperature reduction from 37^oC to 19^oC reduced transperitoneal transport of albumin, LDL and Cr-EDTA by approximately 65-80%, which exceeds the cooling induced effects on plasma viscosity (35%), most probably as a result of concomitant reductions in microvascular hydrostatic pressure (albumin, LDL) and marked reductions in tissue perfusion (Cr-EDTA). The reduction in LDL transport in excess of that of albumin suggests that transendothelial LDL passage in vivo partly occurs by active transcytosis.