Interactions of temperature and ADH on transport processes in cortical collecting tubules

We evaluated the temperature dependence of the permeability coefficients for ADH-independent and ADH-dependent zero volume flow diffusion of THO (PDw, micron/s), for ADH-dependent zero volume flow diffusion of the highly lipophilic solute n-[3H]butanol (PDb, micron/s), and for ADH-dependent lumen-to-bath osmosis (Pf, micron/s) in rabbit isolated cortical collecting tubules. The ADH-dependnet Pf and PDw data are consistent with the hypothesis that water crosses the apical plasma membranes of these tubules through narrow aqueous channels by single-file diffusion; and using the raw temperature-dependent data, we calculate that these channels contain congruent to six H2O molecules per channel, both in the presence and in the absence of ADH. Apparent activation energies (EA, kcal/mol), as an upper estimate of the true activation energies, were calculated for these transport processes. The EA for osmosis for 6-23 degrees C, 9.7 +/- 1.1 kcal/mol, was indistinguishable from that reported previously for 23-37 degrees C either in the presence of absence of ADH, and from the ADH-independent EA for THO diffusion, 10.11 +/- 0.68 kcal/mol, measured in the range 15-37 degrees C. We interpret these data to indicate that in the absence of ADH THO diffusion at zero volume flow is hindered primarily by the same sites, narrow channels in apical membranes. The apparent EA for ADH-dependent zero volume flow THO diffusion, 5.11 +/- 0.40 kcal/mol, measured over the temperature range 6-38 degrees C, is approximately half the apparent EA for osmosis in the presence or absence of ADH and ADH-independent THO diffusion. The former value, when corrected for diffusion constraints in series with apical plasma membranes, becomes indistinguishable from the latter values. These date are consistent with the possibility that two factors contribute to the ADH-mediated disparity between Pf and PDw in these tubules: cytosolic diffusion constraints that impede ADH-dependent THO diffusion but not osmotic volume flow, and narrow aqueous channels as the primary route for water flux through apical plasma membranes. Finally, the EA for n-butanol diffusion shows a break in the Arrhenius plot. Above 23 degrees C, the EA is 4.77 +/- 0.77 kcal/mol and n-butanol diffusion is impeded primarily by the cytosol. Below 23 degrees C, the EA rises to approximately 9 kcal/mol because the hydrophobic regions of apical plasma membranes contribute an increasing fraction of the total resistance to n-butanol diffusion.