We assessed the effects of antidiuretic hormone and cyclic adenosine monophosphate (cAMP) analogues on transepithelial voltage, Ve, and/or net chloride absorption in isolated mouse medullary (mTALH) and cortical (cTALH) thick ascending limbs of Henle; the passive NaCl permeability characteristics and electrical properties of the mTALH; and the effects of anion and cation substitutions and transport inhibitors on both basal and ADH-stimulated Ve and/or net chloride absorption in the mTALH. The data demonstrate that these two segments are functionally heterogeneous: ADH, at concentrations comparable to plasma levels seen in mammalian species during ordinary antidiuresis, and/or cAMP increase three- to fourfold the rate of NaCl absorption in the mTALH but not in the cTALH. The ion substitution and inhibitor data are consistent with the view that NaCl absorption in the mTALH depends on a secondary active transport process: NaCl entry across luminal membranes is a coupled process of indeterminate stoichiometry that is driven by the transmembrane electrochemical gradient for Na+, which is maintained by Na+-K+-ATPase. Finally, the data demonstrate that the mTALH is electrically leaky whether measured electrically, 11 omega . cm2, or isotopically, 50 omega . cm2, but essentially water impermeable; and that the mTALH is perm-selective for Na+ with respect to Cl-. The disparity between electrical resistances measured directly with respect to those calculated from tracer fluxes, together with the hybrid characteristics of mTALH junctional complexes (leaky to Na+ and Cl-; tight to water), may be reconciled by assuming that mTALH junctional complexes contain passive ion permeation pathways composed of narrow channels through which ions pass in single-file fashion.

The mammalian renal thick ascending limb of Henle serves to dilute the urine and supply energy for countercurrent multiplication as a consequence of its ability to absorb NaCl in excess of water. During the past few years our notions of the mechanism of and the factors that control NaCl absorption by the thick ascending limb of Henle have been altered substantially. This review presents evidence for a model of NaCl absorption in the thick ascending limb in which net Cl- absorption is rheogenic and involves a secondary active transport process. According to this model, net Cl- absorption occurs via a furosemide-sensitive coupled electroneutral (1Na+, 1K+, 2Cl-) apical Cl- entry mechanism in parallel with a large K+ conductance in apical plasma membranes and a conductive Cl- exit mechanism in basolateral plasma membranes. The lumen-positive voltage and high paracellular conductance in the thick ascending limb provides a means of driving 50% of net Na+ absorption through the paracellular route and reduces, with respect to exclusively transcellular active Na+ absorption, the metabolic energy expenditure for net Na+ absorption. In some mammalian species, ADH may enhance the rate of NaCl absorption in the medullary thick ascending limb by directly increasing the functional number of (1Na+, 1K+, 2Cl-) cotransport units as well as the K+ conductance unit in apical plasma membranes and by indirectly increasing basolateral membrane Cl- conductance. This latter effect on basolateral Cl- conductance may be secondary to a hormone-induced increase in cell Cl- activity. The ADH-dependent increase in NaCl absorption can be modified directly by both prostaglandin E2 and peritubular hypertonicity. During antidiuresis these hormonal and nonhormonal modulations of NaCl absorption may provide a number of feedback systems that control the level of interstitial osmolality and, as a consequence, concentrating power without affecting net NaCl absorption.

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This paper reports experiments designed to assess the relations between net salt absorption and transcellular routes for ion conductance in single mouse medullary thick ascending limbs of Henle microperfused in vitro. The experimental data indicate that ADH significantly increased the transepithelial electrical conductance, and that this conductance increase could be rationalized in terms of transcellular conductance changes. A minimal estimate (Gminc) of the transcellular conductance, estimated from Ba++ blockade of apical membrane K+ channels, indicated that Gminc was approximately 30-40% of the measured transepithelial conductance. In apical membranes, K+ was the major conductive species; and ADH increased the magnitude of a Ba++-sensitive K+ conductance under conditions where net Cl- absorption was nearly abolished. In basolateral membranes, ADH increased the magnitude of a Cl- conductance; this ADH-dependent increase in basal Cl- conductance depended on a simultaneous hormone-dependent increase in the rate of the net Cl- absorption. Cl- removal from luminal solutions had no detectable effect on Ge, and net Cl- absorption was reduced at luminal K+ concentrations less than 5mM; thus apical Cl- entry may have been a Na+, K+, 2Cl- cotransport process having a negligible conductance. The net rate of K+ secretion was approximately 10% of the net rate of Cl- absorption, while the chemical rate of net Cl- absorption was virtually equal to the equivalent short-circuit current. Thus net Cl- absorption was rheogenic; and approximately half of net Na+ absorption could be rationalized in terms of dissipative flux through the paracellular pathway. These findings, coupled with the observation that K+ was the principal conductive species in apical plasma membranes, support the view that the majority of K+ efflux from cell to lumen through the Ba++-sensitive apical K+ conductance pathway was recycled into cells by Na+, K+,2Cl- cotransport.

A B S T R A C T These experiments were intended to evaluate the antidiur-etic 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 ai, for urea, sucrose, and NaCl, was unity. In support of this, the diffusion permeability coefficient (PDi, cm sec') 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.