Effects of noradrenaline on the transcapillary passage of albumin, fluid and CrEDTA in the perfused rat hindlimb.

Isolated rat hindlimbs were artificially perfused with serum solutions, either during maximal vasodilatation induced by papaverine or during infusions of noradrenaline (NA) at concentrations of 0.5-3.5 microM. Transcapillary exchange was followed with three independent techniques: clearance of albumin, measured after tissue accumulation of radiolabelled tracer, capillary filtration capacity (CFC), determined gravimetrically, and the capillary diffusion capacity (PS) for CrEDTA, calculated from venous indicator dilution curves after single bolus injection of dyes. During maximal vasodilation, when resistance to flow was 2.3 +/- 0.1 PRU100, albumin clearance was 0.0241 +/- 0.0012 ml min-1 100 g-1, CFC was 0.0367 +/- 0.0009 ml min-1 mmHg-1 100 g-1 and PS for CrEDTA was 8.2 +/- 0.4 ml min-1 100 g-1. Noradrenaline given before radiolabelled albumin increased vascular tone and reduced all three capillary exchange parameters in parallel. Albumin clearance was reduced also when NA was given after the introduction of radiolabelled albumin. When NA raised vascular tone to a level largely corresponding to that in resting skeletal muscle (PRU100 = 14) albumin clearance was 0.0067 ml min-1 100 g-1, CFC was 0.0141 ml min-1 mmHg-1 100 g-1 and PS for CrEDTA was 2.4 ml min-1 100 g-1. These findings suggest that albumin is transported by convective mechanisms and that the hydrostatic pressures fall in the exchange vessels as a consequence of functional precapillary 'sphincter' activity. Further, albumin clearance seems to be the same in the artificially perfused rat hindlimbs as in intact animals, at comparable vascular resistances, suggesting that capillary permeability is not influenced by the present surgical procedure.