Influence of vascular muscle tone upon isogravimetric capillary pressure

Isogravimetric capillary pressure (P_ci) was determined in vitro in the perfused, skinned dog forearm preparation. Two procedures were employed. In one, arterial pressure was lowered before venous pressure was elevated (A, V sequence). In the second procedure, venous pressure was elevated before arterial pressure was lowered (V,A sequence). If the composition of the blood perfusing the preparation remains constant, the capillary pressure determined by both procedures should be equal. However, it was found that the A,V sequence resulted in a lower estimate of P_ci and a higher value for venous resistance than did the V,A sequence. A deliberate filtration of 2–5 g of fluid elevated the mean P_ci values but did not abolish the A,V and V,A difference. Addition of KCN to relax smooth muscle, yielded a single P_ci value for both the A,V and V,A sequences. This value was higher than the values obtained prior to KCN treatment. Moreover, A,V and V,A KCN isogravimetric sequences exhibited no differences in venous resistance or venous pressure at comparable levels of flow, whereas differences were present before treatment with KCN. Therefore, it was concluded that the double set of data in the untreated state was due to the presence of active and/or myogenic postcapillary smooth muscle. Because of the influence of venous smooth muscle, most estimates of P_ci underestimate plasma colloid osmotic pressure. Moreover, calculations of venous resistance employing this P_ci would be underestimated, since these calculations involve P_ci.