Summary. The present studies were designed to assess the ways in which antidiuretic hormone (ADH) alters water and solute permeation across isolated, rabbit cortical collecting tubules. In earlier work, it was observed: that ADH produced a tenfold increment in P/(cm per sec), the osmotic water permeability coefficient, and a fourfold increment in PD~ (cm per sec), the diffusional water permeability coefficient; that small hydrophilic solutes such as urea, thiourea and acetamide (each having oil/water partition coefficients __< 0.0008) had vanishingly low permeation coefficients and unity reflection coefficients, even in the presence of ADH; that lumen to bath osmosis involved a transcellular route; and, that the disparity between Ps and Po~, either with or without ADH, could be rationalized in terms of cellular diffusion constraints, i.e., that water transport across luminal membranes was diffusionat.The present experiments evaluated the effects of ADH on diffusion of moderately lipophilic solutes (e.g., butyramide, isobutyramide, and antipyrine, each solute having an oil/water partition > 0.0008) across luminal membranes of rabbit cortical collecting tubules, and the effects of ADH on the apparent activation energies (EA, kcal per mole) for water and solute permeation across these tubules. Three major results were obtained: (1) ADH produced a 60-100 ~ increase in the permeation rates for these solutes. (2) The ADH-dependent apparent E A for water permeation was 9.35_+0.92 kcal per mole, and the ADH-dependent apparent E A for permeation of moderately lipophilic solutes was in the range 15.8-19.6 kcal per mole. (3) The ADH-independent E A values for these transport processes were statistically indistinguishable from the ADH-dependent E A values.When viewed in the context of transport mechanisms for water and solute permeation across synthetic lipid bilayer membrane systems, these results are consistent with the possibility that diffusion of water and moderately lipophilic solutes across mammalian collecting tubules may involve parallel sites in luminal plasma membranes: routes for water diffusion

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Cycloheximide depresses maximum rate of change in membrane potential observed during the rising phase of the action potential in single medullated axons of Xenopus. To,e course of depression is independent of cycloheximide concentration over a range that almost completely inhibits leucine incorporation into axonal proteins.