Autoregulation of capillary pressure and filtration rate in isolated rat hindquarters

The hypothesis that skeletal muscle capillary pressure and/or capillary filtration rate are autoregulated was tested in 10 isolated rat hindquarters. Capillary pressure was directly assessed with the venous occlusion technique as abdominal aortic pressure was reduced in 25-mmHg decrements from 125 to 25 mmHg. Capillary pressure was not altered by reduction of arterial pressure from 125 to 100 mmHg, but it decreased progressively when arterial pressure was reduced from 100 to 25 mmHg. As perfusion pressure was reduced, capillary filtration rate decreased progressively, while the capillary filtration coefficient increased. The progressive decrease in capillary pressures was less than that predicted for a totally passive system, implying that capillary pressure was autoregulated to some degree. However, analysis of pre- to postcapillary resistance ratios suggested that the degree of capillary pressure autoregulation was minimal when perfusion pressures varied over a range of 100-25 mmHg. Capillary filtration rate was maintained better than would be predicted from the measured fall in capillary pressure by readjustments of interstitial Starling forces. These results indicate that capillary pressure is poorly autoregulated in rat skeletal muscle but that compensatory readjustments in interstitial Starling forces help maintain fluid balance and prevent excess dehydration of the interstitium of skeletal muscle as arterial pressure is reduced.