The effect of antidiuretic hormone on solute flows in mammalian collecting tubules

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Abstract
These experiments were intended to evaluate the antidiuretic hormone (ADH)-dependent reflection coefficients of urea, sucrose, and NaCl in cortical and outer medullary collecting tubules isolated from mammalian kidney. In one group of experiments, the ADH-dependent osmotic water flows, when the perfusing solutions contained hypotonic NaCl solutions, were indistinguishable from control observations when either urea or sucrose replaced, in part, NaCl in isotonic bathing solutions (cortical collecting tubules). Similarly, both in cortical and outer medullary collecting tubules exposed to ADH, there was zero net osmotic volume flow when a portion of the NaCl in the bathing and/or perfusing solutions was replaced by either sucrose or urea, so long as the perfusing and bathing solutions were isosmolal. Taken together, these observations suggest that the ADH-dependent reflection coefficients of NaCl, urea, and sucrose, in these tubules, were identical. Since the effective hydrodynamic radii of urea and sucrose are, respectively, 1.8 and 5.2 A, it is likely that σi, for urea, sucrose, and NaCl, was unity. In support of this, the diffusion permeability coefficient (PDi cm sec-1) of urea was indistinguishable from zero. Since the limiting sites for urea penetration were the luminal interfaces of the tubules, these data are consistent with the view that ADH increases diffusional water flow across such interfaces.