Influence of hydrostatic pressure gradients on net transfer of sodium and water across isolated rat colonic mucosa.

1. The dependence of net transfer of water and sodium on hydrostatic pressure gradients from the serosal to the mucosal side was investigated in everted sacs of the stripped mucosa of the rat colon. 2. In the range of 3-20 cm H2O both, net sodium and water transfer were linearly dependent on hydrostatic pressure. The hydraulic permeability coefficient was 1.1 ml per gram dry weight, hour and cm H2O. At a pressure gradient of 5.8 cm H2O the net movement of water from the mucosal to the serosal side ceased. Above this pressure a net movement in the opposite direction occurs. Sodium net movement from the mucosal to the serosal side ceased at 11 cm H2O. The fluid, which appears-driven by higher values of hydrostatic pressure-on the mucosal side, is hypotonic. 3. Oxyphenisation increases the hydraulic permeability of the colonic epithelium. The fluid, which appears--driven by the hydrostatic pressure gradient--on the mucosal side, is isotonic.