Osmosis in Cortical Collecting Tubules : ADH-Independent Osmotic Flow Rectification

The present experiments were designed to evaluate the effects of varying the osmolality of luminal solutions on the antidiuretic hormone (ADH)-independent water and solute permeability properties of isolated rabbit cortical collecting tubules. In the absence of ADH, the osmotic water permeability coefficient (cm s^–1) P_f^l→b, computed from volume flows from hypotonic lumen to isotonic bath, was 20 ± 4 x 10^–4 (SEM); the value of P_f^b→l in the absence of ADH, computed from volume flows from isotonic bath to hypertonic lumen, was 88 ± 15 x 10^–4 cm s^–1. We also measured apparent urea permeability coefficients (cm s^–1) from ¹⁴C-urea fluxes from lumen to bath (P_DDurea^l→b) and from bath to lumen (P_DDurea^b→l). For hypotonic luminal solutions and isotonic bathing solutions, P_DDurea^l→b was 0.045 ± 0.004 x 10^–4 and was unaffected by ADH. The ADH-independent values of P_DDurea^l→b and P_urea^b→l were, respectively, 0.216 ± 0.022 x 10^–4 cm s^–1 and 0.033 ± 0.002 x 10^–4 cm s^–1 for isotonic bathing solutions and luminal solutions made hypertonic with urea, i.e., there was an absolute increase in urea permeability and asymmetry of urea fluxes. Significantly, P_DDurea^l→b did not rise when luminal hypertonicity was produced by sucrose; and, bathing fluid hypertonicity did not alter tubular permeability to water or to urea. We interpret these data to indicate that luminal hypertonicity increased the leakiness of tight junctions to water and urea but not sucrose. Since the value of P_f^b→l in the absence of ADH, when tight junctions were open to urea, was approximately half of the value of P_f^l→b in the presence of ADH, when tight junctions were closed to urea, we conclude that tight junctions are negligible paracellular shunts for lumen to bath osmosis with ADH. These findings, together with those in the preceding paper, are discussed in terms of a solubility-diffusion model for water permeation in which ADH increases water solubility in luminal plasma membranes.