ANP INCREASES CAPILLARY-PERMEABILITY TO PROTEIN INDEPENDENT OF PERFUSATE PROTEIN-COMPOSITION

In further studying hormonal regulation of microvascular exchange, we tested two hypotheses: 1) atrial natriuretic peptide (ANP) would incre ase capillary permeability to protein and 2) the actions of ANP on cap illary permeability would be independent of the perfusate protein comp osition. These hypotheses were tested by assessing in situ capillary t ransport coefficients: hydraulic conductivity (L(p)), apparent and dif fusive permeabilities (P-s and P-sd, respectively) to the protein alph a-lactalbumine, and convective coupling [L(p)(1 - sigma)] in mesenteri c capillaries in cerebrally pithed frogs (Rana pipiens). The transport coefficients were determined in the absence and presence of frog ANP (fANP) during perfusion with either frog plasma or bovine serum albumi n (BSA). Control L(p) was 1.8-fold greater in vessels perfused with BS A compared with plasma. In the presence of 10 or 100 nM fANP, L(p) was increased by approximate to 3.5-fold in plasma-perfused vessels and a pproximate to 2-fold in BSA-perfused vessels from their respective con trols. Control P-s was 4.2-fold higher in vessels perfused with BSA co mpared with plasma. Despite the differences in control permeability, t he increase in P-s by fANP was of similar magnitude (2- to 3-fold) for both protein perfusates. Analysis of the pressure-dependent alpha-lac talbumin flux suggested that the increase in capillary permeability in duced by fANP is consistent with fANP increasing permeability without altering the selectivity of the capillary barrier.