Capillary permeability: an albumin component attenuates active changes in Lp.

The mechanisms whereby atrial natriuretic peptide (AP) acutely and reversibly elevates hydraulic conductivity (Lp) are not known. This is the second of two studies of the influence of perfusate albumin composition on AP alterations in capillary Lp. In this study, we investigated the effect of dialysis of the perfusate albumin. A 2.2-fold increase in frog (Rana pipiens) mesenteric microvessel Lp occurred when 100 nM AP was added to the control perfusate containing dialyzed, crystallized bovine serum albumin (DXL-BSA 10 mg/ml; n = 20). By contrast, Lp was unchanged by 100 nM AP in 10 mg/ml untreated, crystallized BSA (XL-BSA; n = 8). The response to AP was unaltered at DXL-BSA contents of 10, 20, or 30 mg/ml (2.4-, 2.2-, and 3.1-fold, respectively; n = 8). Dialysis of the albumin, per se, did not influence control Lp (LpXL-BSA/LpDXL-BSA = 1.0; n = 5). The receptor-independent nitrovasodilator, sodium nitroprusside (SNP; 1 microM) elevated Lp by 1.7-fold in DXL-BSA (n = 30). The response was abolished in XL-BSA (n = 8). We conclude that small hydrophilic albumin-associated substances antagonize AP- and SNP-induced elevations of exchange microvessel hydraulic conductance without interfering with albumin's role in the maintenance of normal exchange vessel permeability.