Simultaneous measurements of capillary filtration and diffusion capacities during graded infusions of noradrenaline (NA) and 5-hydroxytryptamine (5-HT) into the rat hindquarter vascular bed

Abstract

The relationships between capillary diffusion capacity (PS) for Cr-EDTA respective capillary filtration capacity (CFC) and vascular resistance during graded intraarterial infusions of NA [norepinephrine] and 5-HT [5-hydroxytryptamine] into the artificially constant flow perfused rat hindquarter vascular bed were investigated. During maximal vasodilatation PS for Cr-EDTA was 5.5-5.7 ml/min .times. 100 g; CFC 0.04 ml/min .times. mmHg .times. 100 g; vascular resistance 2.8 mmHg/ml .times. min .times. 100 g (PRU100); and isogravimetric capillary pressure 12.8 mmHg on an average. Setting out from maximal vasodilatation, increasing doses of Na and 5-HT produced graded reductions in capillary surface area as reflected by progressive decreases in PS for Cr-EDTA and CFC. These changes occurred simultaneously with progressive increases in pre- and postcapillary resistances, causing elevations in arterial and capillary hydrostatic pressures and in capillary fluid filtration at constant flow. Capillary hydrostatic pressure increased maximally to 45 mmHg (calculated for Na) and vascular resistance to 21 mmHg/ml .times. min .times. 100 g on an average. PS for Cr-EDTA decreased maximally to 0.7-1 ml/min .times. 100 g for NA and 5-HT; the relationships between PS for Cr-EDTA and PRU100 for NA respective 5-HT were almost identical. Capillary surface area for nutritional exchange evidently is affected similarly by drugs. The CFC-PRU100 relationship was shifted towards 30-50% higher CFC values for 5-HT than for NA at almost every level of vasoconstriction. 5-HT reduces capillary surface area and induces moderate increases in capillary permeability through increases in number and/or radius of large pores (gaps). During NA-induced vasoconstriction, when virtually no changes in capillary permeability occurred, PS for Cr-EDTA was reduced to a relatively greater extent than CFC; the discrepancy was most pronounced during marked vasoconstriction. The significance of this finding is discussed.