Studies on the relationship between rat renal medullary cell volume and external anion concentration in hyperosmolal media

Hypertonicity in fused Madin-Darby canine kidney cells: transient rise in NaHCO3 followed by sustained KCl accumulation

We investigated mechanisms of regulatory volume increase in fused Madin-Darby canine kidney (MDCK) cells, a cell line originally derived from renal collecting duct. The intracellular ion concentrations as well as the concentration of the volume marker tetramethylammonium+ were measured by means of ion-selective microelectrodes. Application of hypertonic Ringer bicarbonate solution (+150 mmol/l mannitol) resulted in cell shrinkage to 84 +/- 2% of the initial cell volume (shrinkage expected for an ideal osmometer = 66%), indicating a significant regulatory volume increase. During the first 90 s of the hypertonic stress, a transient increase in intracellular Na+ and HCO3- concentrations was observed. It was followed by a sustained increase in intracellular K+ and Cl- concentrations. Ouabain (0.1 mmol/l) as well as amiloride (1 mmol/l) reduced K+ accumulation significantly, whereas the H+/K(+)-ATPase inhibitor SCH 28080 had no effect. Hypertonic stress hyperpolarized the cell membrane potential by 19 +/- 2 mV, owing to the decrease of the ratio of Cl- conductance to K+ conductance of the cell membrane. We conclude: (a) acute hypertonic stress activates Na+/H+ exchange in MDCK cells; (b) transient alteration of intracellular Na+ and pH stimulates Na+/K(+)-ATPase and Cl-/HCO3- exchange, exchange, both leading to the sustained intracellular accumulation of KCl; (c) a high intracellular KCl concentration is maintained by the partial reversion of the Cl-/K+ conductance ratio of the plasma membrane.

Rapid hypertonic cell volume regulation in the perfused inner medullary collecting duct

Differential interference contrast microscopic images were used to assess the cell volume regulatory increase (VRI) response of rat IMCD segments isolated from the mid-inner medullary region of pathogen-free Sprague-Dawley rats and perfused in vitro at 37 degrees C. In the absence of ADH. IMCD cells behaved in an osmometric fashion over the range of extracellular osmolalities 290 to 386 mOsm/kg H2O and had an osmotic space equal to 54.2% of total geometric volume. After initial shrinkage in hypertonic perfusing and bathing solutions (340 mOsm/kg H2O using sucrose), cell volume increased rapidly to the isotonic value only in tubules preincubated in ADH (100 microU/ml). The rates of VIR were: (-ADH) 0.0142 +/- 0.0046 nl.min-1.cm-1 or 0.30 +/- 0.10%/min and (+ADH) 0.7225 +/- 0.1278 nl.min-1.cm-1 or 15.42 +/- 2.31%/min (N = 4; P less than 0.01). An overshoot in cell volume was observed on return to isotonic media only in the ADH exposed tubules showing a hypertonic VRI response, indicating that IMCD cells accumulated solute during hypertonic VRI. In the absence of ADH, one mM dibutyryl cyclic AMP mimicked the effect of hormone on hypertonic VRI. This ADH-dependent VRI process required Na+ and (CO2 + HCO3-) in external media and was reduced or abolished by 0.1 mM amiloride, 0.1 mM 4,4'-diisothiocyanatostilbene-2,2-'-disulfonic acid (DIDS) in peritubular solutions. These data suggest that ADH-dependent, rapid hypertonic cell volume regulation in rat inner medullary collecting duct depends on NA+ uptake, which may be mediated by parallel Na+-H+ and an HCO3(-)-dependent. DIDS-sensitive pathway (such as, Cl+-HCO3- exchanger) in basolateral cell membrane. In addition, a luminal amiloride-sensitive pathway (most likely the cation-selective channel) may contribute to cell volume regulation in the rat IMCD.

Sodium-coupled ion cotransport and the volume regulatory increase response

In conclusion, maintenance of volume homeostasis is a fundamental requirement of all cells. For many cell types, this process requires expression of ion cotransport mechanisms as well as accumulation of osmotically-active organic compounds. Recent observations have indicated that the cellular mechanisms responsible for modulating hypertonic volume regulation are complex and appear to involve hormonal, biochemical and physico-chemical stimuli. Knowledge of the specific ion-transport mechanisms involved in the initial phase of VRI, the factors that control their expression, and the interrelationships between inorganic and organic solute accumulation will be required before an in depth understanding of hypertonic cell volume regulation in medullary nephron segments can be achieved.

Hypertonic cell volume regulation in mouse thick limbs. I. ADH dependency and nephron heterogeneity

Differential interference contrast microscopy was used in combination with standard electrophysiological techniques in the in vitro perfused mouse medullary (mTALH) and cortical (cTALH) thick ascending limbs of Henle to evaluate the cell volume responses of these nephron segments to sudden increases in peritubular osmolality and to assess the role of antidiuretic hormone (ADH) and net NaCl absorption on hypertonic volume regulation. In the absence of CO2/HCO3- in external media, the cells of the mTALH behaved in a simple osmometric fashion, with an osmotic space equivalent to 70-80% of the total cell volume. However, in CO2/HCO3- -containing media, the cells of the mTALH, but not the cTALH, were able to increase their cell volume to the original volume after shrinkage in peritubular media made hypertonic with either NaCl or mannitol. This volume-regulatory increase response (VRI) in the mTALH was mediated by an increase in intracellular osmoles, and required peritubular ADH, at concentrations that stimulate maximally the rate of net NaCl absorption. This ADH effect on VRI could be mimicked by addition of dibutyryladenosine 3',5'-cyclic monophosphate to the bath in the absence of hormone. However, 10(-4) M luminal furosemide, a concentration that abolishes ADH-dependent NaCl absorption in the mTALH, had no effect on the VRI response. These results indicate that the cells of the mTALH, but not the cTALH, are capable of hypertonic volume regulation, that ADH (via adenosine 3',5'-cyclic monophosphate) is required for expression of the VRI response in the mTALH, and that the effects of ADH on net NaCl absorption and the VRI response in the mTALH are completely dissociable. Thus these results are consistent with a role for ADH in hypertonic VRI in the mammalian mTALH, which may operate to maintain constant cell volume in this nephron segment during antidiuresis.

Characteristics of ionic binding by rat renal tissue in vitro

A study has been made of Na+ and Cl- binding in metabolically inhibited slices of rat renal cortex and outer medulla incubated in modified Krebs phosphate-bicarbonate Ringer solution. At pH 7.35 in control media (cortex, 147 mmol Na+/l, 105 mmol Cl-/l; outer medulla, 187 mmol Na+/l, 145 mmol Cl-/l) cortical slices bound (mean) 171 nmol Na+ and 56.7 nmol Cl-/mg solute-free dry weight; outer medullary slices bound 188 nmol Na+/mg and negligible amounts of Cl-. In both regions, Na+ was exchangeable on a 1:1 basis for K+ or Li+ in media containing equal concentrations of each cation: Na+ was completely displaced by La3+. In cortical slices in media containing equimolar Cl- and other monovalent anions, binding occurred according to the sequence acetate less than or equal to salicylate less than or equal to Cl- less than SCN-; Cl- was completely displaced by PO4(3-). When medium pH was lowered, Na+ binding was markedly reduced in both regions, whereas Cl- binding increased (and became significant in outer medulla). In NaCl solutions, Na+ binding capacity was saturated at control Na+ concentrations. When [Na+] was progressively reduced (iso-osmolality being maintained by addition of urea), bound Na+ in both regions was nearly linearly related to log medium [Na+]. Raising medium osmolality with urea caused decreased Na+ binding and increased Cl- binding in both regions. Na+ binding in both regions was significantly reduced by pre-treatment with chondroitinase ABC. Binding of both ions was independent of temperature within the range 2-37 degrees C. The possibility is raised that renal ionic binding might influence vectorial ion transport by affecting free ion activity in the region of the transporting cells.

Some effects of monovalent anion replacement on the volume and composition of cells in incubated slices of rat renal cortex

Steady-state cellular water content and cation content and concentration have been studied in slices of rat renal cortex incubated in media in which Cl- was replaced by various monovalent anions at pH 7.35. Anions derived from low molecular weight aliphatic acids caused net uptake of cell water and K+. In the presence of larger anions cell water content fell in a manner related to anionic equivalent weight, but water content in media containing anions from weak aliphatic or alicyclic acids was slightly but consistently higher than that in media containing anions from strong acids. Cell K+ content did not decrease in these media. Aromatic anions caused enhanced cell water loss. When external pH adjusted to 6.8 or 7.8 it was found that in media containing anions from weak acids, but not from strong acids, increasing pH was associated with significantly decreased cell water content.

Techniques and applications of extracellular space determination in mammalian tissues

This review summarizes the ways in which the extracellular space (ECS) may be estimated in mammalian tissues, and briefly describes some of the uses to which the EC confinement of certain molecules (markers or tracers) may be put in the elucidation of physiological functions. The introductory section is followed by a description of the more commonly used marker molecules and their functional characteristics, and of factors likely to lead to the spurious over- or under-estimation of the ECS. Certain alternative methods are also described, in particular those based on morphological and electrical criteria which seek to demonstrate small, functionally important, changes in the size of specialized regions of the ECS (e.g. lateral cellular interspaces) without necessarily being required to provide a quantitatively precise estimate of their size. Section III describes the results of measurements of ECS in various mammalian tissues (muscle, gastro-intestinal tract, nervous tissue, crystalline lens, placenta, lung and kidney) and some applications of EC markers to investigation of cellular function (e.g. uptake of metabolic substrates and epithelial transport) and, in outline, characterization of capillary permeability. The available literature in this field is very extensive, and in the interests of brevity the reader is, where appropriate, referred to previous reviews covering specialized aspects of ECS determination and related topics.

Cellular swelling in slices of rat renal outer medulla incubated in the presence of bovine serum albumin

1. Cells in slices of rat renal outer medulla incubated for up to 100 min in modified phosphate-bicarbonate Ringer undergo reversible, concentration-dependent swelling (increase in water content) in the presence of bovine serum albumin (b.s.a). 2. Swelling also takes place in the presence of dextran sulphate, but neutral dextran or polyvinylpyrrolidone are without effect. 3. Swelling due to b.s.a. is pH-dependent, decreasing (relative to cell volume in the absence of b.s.a.) as pH is reduced. 4. Swelling due to b.s.a. is associated with the net cellular gain of Na+, the magnitude of which increases at pH is raised. 5. There is an accompanying pH-independent net loss of cellular K+; at pH greater than about 6.20 this loss is smaller than the uptake of Na+, and cells thus show net gain of monovalent cation. 6. The relationships between pH and percentage increase in cell water content and between pH and percentage increase in cell osmotic content due to net gain of cation in the presence of 8% b.s.a. are coincident within the range pH 6.15--7.60. 7. Studies on the volume of distribution of [125I]b.s.a. within slices suggests that more may bind to cells at low pH than at high pH.