Relation between capillary pressure and vascular tone over the range from maximum dilatation to maximum constriction in cat skeletal muscle.

An attempt was made to assess, from a large sample (n = 567), the normal level of hydrostatic capillary pressure (Pc) in resting skeletal muscle and the extent of Pc regulation as effected by strictly graded activation of metabolic and adrenergic control mechanisms over the entire physiological range of vascular tone. With the use of a new whole-organ technique, Pc towards the venous end of the capillary was continuously recorded at constant arterial pressure (100 mmHg) and under simultaneous observations of total regional vascular resistance (RT), precapillary resistance (Ra) and post-capillary resistance (RV). In the control state with a Starling fluid equilibrium, a venous pressure of 7 mmHg and normal vascular tone (RT = 19.1 +/- 0.3 PRU), Pc averaged 16.7 +/- 0.3 mmHg. Graded metabolic dilatation (muscle exercise), decreasing RT to a minimum value of 1.7 PRU, caused progressive increase in Pc up to 32 mmHg and consequent fluid filtration. Conversely, graded adrenergic constriction, increasing RT to a maximum of 100 PRU, caused a progressive decrease in Pc down to 10 mmHg and consequent fluid absorption. The relation between Pc and RT was highly non-linear, Pc increasing more steeply the more RT approached low values, and was described by the power function: Pc = 36.43 x RT-0.27 (r = -0.79, P less than 0.001). The resistance ratio, Rv/Ra (the main determinant of Pc), and vascular tone (RT) showed a similar non-linear relation. Regulatory change of Rv/Ra was mainly accomplished by active change of Ra, but a pronounced Rv decrease (venodilatation) occurred in the lowest RT range, exerting a protective function against excessive increase in Pc and detrimental plasma fluid loss.