Diabetes Insipidus Rats Research Articles

A structural study of the rat proximal and distal nephron: effect of peptide and thyroid hormones.

The effects of the absence of various hormones (antidiuretic hormone, thyroid hormone, parathyroid hormone, and calcitonin) on proximal and distal structures were studied in diabetes insipidus (DI) Brattleboro rats. The cross-sectional area of the first segment of proximal convoluted tubules (S1) was significantly reduced in thyroparathyroidectomized (TPTX) DI rats compared with Long-Evans rats (the strain of origin of DI rats) and untreated DI rats. Administration of triiodothyronine (T3, 10 micrograms/day for 7 days) to TPTX-DI rats restored the proximal tubule structure. In the distal convoluted tubule (DCT) the cross-sectional area of the epithelium and the number of nuclei per cross-sectional area were significantly greater in untreated ADH-deficient DI rats than in the control Long-Evans rats. Daily administration of 1-desamino-8-D-arginine vasopressin (dDAVP, 500 ng/day for 3 wk) significantly reduced the size and the number of DCT cells in DI rats. Cortical micropuncture data indicated that the Na+ concentration in the fluid delivered to the DCT and the absolute amount of Na+ reabsorbed along the DCT were higher in DI than in dDAVP-treated DI rats. It is concluded that functional changes in the PCT, subsequent to chronic TPTX, are accompanied by marked alteration of the cell anatomy of this nephron segment, and that the processes that modify the Na load delivered to the DCT and the Na transport in the DCT are accompanied by structural modifications of this segment.

Effects of the α<sub>2</sub>-Adrenoceptor Antagonists Yohimbine and Idazoxan on Kidney Function in Intact and Diabetes insipidus Rats

The purpose of the present study was to examine the effect of the alpha 2-adrenoceptor antagonists yohimbine and idazoxan on kidney function in intact Sprague-Dawley rats and in Brattleboro rats with familial hypothalamic diabetes insipidus. Both drugs, injected intravenously at 0.5 mg/kg, led to a marked decrease of urine flow, sodium, potassium and chloride excretion in both strains of rats. Glomerular filtration rate and effective renal plasma flow did not change significantly in either strain. The study demonstrates that the alpha 2-adrenoceptor antagonists induce an antidiuretic effect without changes in renal hemodynamics. Antidiuretic hormone does not play a role in yohimbine- or idazoxan-induced antidiuresis. The data suggest a role for alpha 2-adrenoceptors in the renal tubular handling of water and salt.

Mechanism of diuretic action of U-62,066E, a κ opioid receptor agonist

The mechanism of the diuretic action of U-62,066E, a highly selective κ opioid agonist, was examined in unanesthetized rats and in isolated perfused inner medullary collecting ducts (IMCD). In Long-Evans rats, U-62,066E caused a dose-dependent increase in urine flow and a decrease in urine osmolality without affecting urinary excretion of Na +. The diuretic effect of U-62,066E was blocked by MR-2266, a κ opioid receptor antagonist. U-62,066E showed no diuretic effect in homozygous hereditary diabetes insipidus rats (Brattleboro strain). In water-deprived rats, U-62,066E markedly inhibited plasma arginine vasopressin (AVP) levels through a κ receptor-mediated mechanism. In rat IMCD perfused in vitro, 10 −5 M U-62,066E did not inhibit either the baseline or the AVP-stimulated osmotic water permeability. We conclude that the inhibition of the release of AVP is the major if not the entire mechanism of the diuretic action of U-62,066E in rats. Although we ruled out the effect of this drug on the water permeability of IMCD, possible direct effects on other nephron structures remain to be established.

In situ hybridization demonstrates vasopressin gene transcription in hypothalamic neurons of crossbred hypertensive × diabetes insipidus rats

In situ hybridization, Northern blotting, and solution hybridization assay were used to examine vasopressin-gene messenger ribonucleic acid (mRNA) transcripts in hypothalamic tissue from live strains of rats: Long Evans, Wistar Kyoto, and diabetes insipidus (Brattleboro) rats, spontaneously hypertensive stroke-prone rats, and cross-bred diabetes insipidus × spontaneously hypertensive strokeprone rats. A 290 base-pair, single-stranded RNA probe, with 221 bases complementary to exon (of the vasopressin gene was synthesized by the SP6 transcription vector system. This probe labeled appropriate neurohypophysial hypothalamic neurons, as well as suprachiasmatic nucleus cells, in tissue samples from all five strains of rats. Confirming other recent hybridization results with diabetes insipidus rat brain tissue, diabetic animals were found to transcribe their mutated vasopressin gene. In addition, this investigation found that the cross-bred diabetic hypertensive rat also synthesizes a vasopressin-gene messenger ribonucleic acid transcript. Quantitative analyses of solution and in situ hybridization results suggested the cross-bred diabetic hypertensive rat exhibits a level of vasopressin-gene messenger ribonucleic acid similar to diabetes insipidus rats. This observation is consistent with previous data on the cross-bred diabetic hypertensive rat which suggests they inherit the mutant vasopressin gene from Brattleboro rats.

Regulation of hypothalamic magnocellular neuropeptides and their mRNAs in the Brattleboro rat: coordinate responses to further osmotic challenge

A paradigm was developed for the chronic osmotic stimulation of homozygous diabetes insipidus rats of the Brattleboro strain, a strain that fails to synthesize vasopressin. This study examines the adaptation of 2 sets of coexisting peptide hormone magnocellular neurons in the hypothalamoneurohypophyseal system (HNS) of Long Evans (LE), Brattleboro heterozygote (HZ), and Brattleboro homozygote (DI) rats: (1) the arginine8-vasopressin (AVP)/dynorphin (DYN) neurons, and (2) the oxytocin (OT)/cholecystokinin (CCK8) neurons of the paraventricular and supraoptic nuclei, which project to the posterior pituitary. The regimen of chronic intermittent salt-loading (CISL) involved the replacement of 2% saline for normal drinking water for 18 hr/d. This protocol effectively increased plasma levels of AVP and OT in LE and HZ rats, oxytocin in DI rats, and maintained the posterior pituitary in a state depleted of AVP, OT, CCK, and peptides derived from pro-dynorphin: DYN A 1-17, DYN A 1-8, and DYN B 1-13. The ratio of pituitary DYN A 1-17 to DYN A 1-8 content in DI rats or in LE, HZ, and DI rats following 6 d of CISL suggests a preferential release of DYN A 1-17 during periods of chronic secretory activity. In response to chronic secretory activity, mRNAs for AVP, OT, DYN, and CCK increased 1.5-2-fold in all 3 AVP rat strains, with mRNAs for coexisting peptide hormones displaying parallel increases. Mutant AVP mRNA in the DI rat was expressed at very low levels and DYN mRNA in very high levels, with each of these mRNAs continuing to be regulated by CISL in a normal manner. These results suggest a regulatory relationship between AVP and OT neurons, in which vasopressin neurons are feedback-regulated by AVP, most likely via plasma osmolarity, and that oxytocin neurons are modulated by peptides derived from pro-dynorphin.

Effects of Cysteamine on Blood Pressure: Possible Mediation through Vasopressin Release

Cysteamine (beta-mercaptoethylamine, CSH) has been reported to have various effects on the neuroendocrine system. Reports indicate CSH decreases pituitary oxytocin (OT) without affecting pituitary vasopressin (VP). However, preliminary studies from our laboratory strongly indicate that CSH has an effect on VP release. Experiments were conducted with dibenzyline-treated, urethane-anesthetized, male Sprague-Dawley (SD) rats. Rats were injected with 4 mU of standard VP and 4 mg/100 g of CSH. Administration of VP resulted in an increase in mean arterial pressure (MAP) of 23.5 +/- 3.2 mm Hg. Administration of CSH resulted in a consistent, immediate decrease in MAP of 13.0 +/- 2.0 mm Hg prior to an increase of 21.0 +/- 2.6 mm Hg. The effects due to VP and CSH were strikingly different; the CSH-induced MAP rise took longer to peak and to return to baseline. Both the VP- and CSH-induced MAP rise were markedly inhibited by a prior administration of a specific VP antagonist d(CH2)5[Tyr(Me)]AVP. In addition, the typical increase in MAP observed in SD rats following CSH administration was substantially reduced when the same dose was administered in homozygous diabetes insipidus (HODI) rats. The data presented here strongly suggest that CSH-induced MAP elevation is due to the release of VP from the pituitary gland.

Involvement of prostaglandin E2, cAMP, and vasopressin in lithium-induced polyuria

The involvement of prostaglandin E2 (PGE2), adenosine 3',5'-cyclic monophosphate (cAMP), and vasopressin in lithium-induced polyuria was investigated in rats. Administration of LiCl (4 mmol/kg body wt) for 7 days induced a marked polyuria with a significant excretion of urinary PGE2. Administration of indomethacin (IND, 5 mg/kg body wt) for 4 days to lithium-induced diabetes insipidus (LiDI) rats diminished urine volume by 80% and urinary PGE2 by 85%. The in vitro data of the intact rat kidney showed that lithium stimulated arginine vasopressin (AVP)-induced PGE2 production and suggested that PGE2 suppressed cAMP synthesis in rat renal medulla. The AVP-induced PGE2 synthesis was greater and the AVP-stimulated cAMP production lower in the LiDI rat kidney in vitro. Interference of the vasopressin-associated cAMP system and the increased PGE2 synthesis in the kidney may be involved in the development of LiDI. The reduced cAMP production in the LiDI rat kidney might be partly due to the increased PGE2 synthesis. In LiDI rats plasma vasopressin increased, whereas AVP concentration in the hypothalamus and the neurohypophysis significantly decreased. It is postulated that lithium stimulates vasopressin release from the central nervous system and that elevated plasma vasopressin potentiates PGE2 production in the kidney synergistically with lithium.

Taurine in the osmoregulation of the Brattleboro rat

The function of taurine in mammalian osmoregulation was studied in the Brattleboro rat with hereditary hypothalamic diabetes insipidus (DI). DI rats are chronically dehydrated because of their inability to synthesize vasopressin. One day of water deprivation did not affect the water balance in rats with normal vasopressin synthesis, whereas DI rats were markedly dehydrated and lost considerably body weight. Taurine content and 3H-taurine accumulation by platelets were significantly higher in DI rats, with a further increase after one day of water deprivation. In DI rats, water deprivation also evoked a clear taurine increase in skeletal muscle and in the brain. These findings indicate that taurine has an osmoregulatory function in mammals.

Vasopressin gene expression in hypertensive, normotensive, and diabetes insipidus rats.

This investigation examined the presence and abundance of vasopressin-gene messenger ribonucleic acid (mRNA) transcripts in hypothalamic tissue from five strains of rats: Long Evans, Wistar-Kyoto, and diabetes insipidus (Brattleboro) rats, stroke-prone spontaneously hypertensive rats, and cross-bred diabetes insipidus x stroke-prone spontaneously hypertensive rats. A single-stranded RNA probe complementary to exon C of the vasopressin gene was utilized for in situ hybridization and identified hypothalamic 'vasopressinergic' neurons in tissue from all five strains of rats. The results obtained by solution and in situ hybridization suggested the cross-bred diabetic-hypertensive rat exhibits a level of vasopressin-gene messenger ribonucleic acid similar to diabetes insipidus rats. This observation is consistent with previous physiological data which suggests cross-bred diabetic-hypertensive rats inherit the mutated vasopressin gene of the Brattleboro rat.

Passive and active avoidance learning in homozygous and heterozygous diabetes insipidus rats (brattleboro strain)

In conclusion, the rat primary motor cortex appears to be organized into irregularly shaped patches of cortex devoted to particular movements. The location of major subdivisions such as the forelimb or hindlimb areas is somatotopic and is consistent from animal to animal, but the internal organization of the pattern of movements represented within major subdivisions varies significantly between animals. The motor cortex includes both agranular primary motor cortex (AgL) and, in addition, a significant amount of the bordering granular somatic sensory cortex (Gr(SI)), as well as the rostral portion of the taste sensory insular or claustrocortex (Cl). The rat frontal cortex also contains a second, rostral motor representation of the forelimb, trunk and hindlimb, which is somatotopically organized and may be the rat's supplementary motor area. Both of these motor representations give rise to direct corticospinal projections21,42,51,57, some of which may make monosynaptic connections with cervical enlargement motorneurons16. Medial to the primary motor cortex, in cytoarchitectonic field AgM, is what appears to be part of the rat's frontal eye fields, a region which also includes the vibrissae motor representation. The somatic motor cortical output organization pattern in the rat is remarkably similar to that seen in the primate, whose primary, supplementary and frontal eye field cortical motor regions have been extensively studied.

Atrial natriuretic peptide in dehydrated Long–Evans rats and Brattleboro rats

Atrial natriuretic peptide (ANP) was measured by radioimmunoassay in atrial and plasma extracts from normal Long-Evans (LE) rats and Brattleboro-strain diabetes insipidus (DI) rats. LE rats, dehydrated for 72 hours, had an increased plasma osmolality and plasma vasopressin. They also demonstrated a higher atrial immunoreactive ANP (IR-ANP) content than hydrated animals (72 hr dehydration: 178.2 +/- 30.4 micrograms/g wet weight atria, mean +/- SE, control: 60.4 +/- 8.2; P less than 0.001). Plasma IR-ANP in dehydrated LE rats tended to be lower than hydrated LE but this was not statistically significant [72 hr dehydration: 61.9 +/- 5.9 pg/ml, control: 82.4 +/- 8.2]. IR-ANP concentration in atrial extracts from DI rats, without detectable plasma vasopressin levels but with increased plasma osmolality, was not different from that in hydrated LE rats (DI: 100.6 +/- 13.2 micrograms/g). There was also no significant difference between plasma IR-ANP in DI and hydrated LE rats (DI: 100.2 +/- 11.9 pg/ml). The atrial IR-ANP concentration in DI rats was decreased by infusion with either arginine-vasopressin (AVP) or 1-deamino-8-arginine vasopressin (DDAVP), and plasma IR-ANP was increased significantly by both infusions (AVP: 171.3 +/- 18.1 pg/ml, DDAVP: 179.5 +/- 24.6). Thus, changes in atrial and plasma IR-ANP concentration appeared to be associated with changes in water balance but not with plasma AVP levels, indicating that the changes in volume may be a more important factor controlling ANP release in vivo than vasopressin itself.

Functional adaptation of thick ascending limb and internephron heterogeneity to urine concentration

The kidney is able to adapt to a variety of chronic changes in excretory needs, for example, to high or low sodium or potassium intake [1] and to acidosis and alkalosis [2]. One of the main functions the mammalian kidney assumes is the concentration of solutes in the urine. The diversity of mammals affords examples of renal adaptation to water conservation. Comparison among animals living in fresh water, in temperate regions, or inhabiting desert areas reveals several differences in kidney morphology and function [3]. Recent experiments have shown, not only among different species but within a given species as well, remarkable functional adaptation of the kidney to chronic variations in urine concentration. Urine concentration can be altered for sustained periods of time by modifying either antidiuretic hormone (ADH) or fluid intake. The Brattleboro rat offers a unique model in which the effects of the total absence of ADH can be studied [4]. By studying homozygous Brattleboro rats with diabetes insipidus (DI) and comparing their kidney function and morphology with that of heterozygous rats of the same strain, normal Long Evans rats (the strain from which the Brattleboro are derived), and DI rats chronically receiving antidiuretic hormone, we were able to demonstrate several features of the renal adaptation to water conservation. Several of these features were also observed in normal rats in response either to an increase or to a decrease in water intake for several weeks. These changes are described in the first part of this paper. The second part will address the mechanisms by which these changes are induced, and how they improve the urine concentrating process.

Conditioned taste aversion in vasopressin-deficient rats (Brattleboro strain)

Brattleboro rats are homozygous for diabetes insipidus (DI), lacking the ability to synthesize vasopressin. Previous studies reported learning deficits in DI rats on passive avoidance tasks using footshock. Other studies, however, could not replicate these results. In two experiments, we studied the learning of DI and control Long Evans (LE) rats in a different avoidance paradigm: conditioned taste aversion (CTA). In the first experiment a mild CTA to saccharin was established gradually using low levels of an illness-inducing agent (lithium chloride). In the second experiment a strong CTA was established in one acquisition trial and the extinction of the conditioned aversion was followed for 12 trials. The two experiments found no differences between the DI and LE rats in either the magnitude or the rate of acquisition and extinction of the CTA. These results suggest that vasopressin is not involved in the acquisition and retention of CTA, and support previous studies indicating that vasopressin may not be involved in avoidance learning.

Salt and water intake in Brattleboro rats with hypothalamic diabetes insipidus

Brattleboro rats lacking vasopressin have an elevated plasma osmolality and a stimulated renin-angiotensin system relative to Long-Evans rats (LE). The current studies were performed to elucidate the factors controlling water and salt intake in the Brattleboro rat with diabetes insipidus (DI). DI and LE rats were (1) given the choice of water and saline solutions ranging from 0.1–1.0% to assess palatability, (2) dialyzed with isotonic glucose to test for sodium appetite after sodium depletion, and (3) infused intracranially with an angiotensin II analogue (saralasin) to assess the role of angiotensin II in spontaneous salt and water intake. DI rats exhibited spontaneous salt intake which was not significantly different from LE rats and increased their intake of 3.0% NaCl following sodium depletion, although less reliably than LE rats. A significant proportion of those DI rats not developing a sodium appetite showed attenuation of their diabetes following dialysis. No evidence for involvement of angiotensin II in spontaneous salt and water intake was found.

Sympathetic nervous system and age-dependent salt hypertension in Brattleboro rats.

The effects of early sympathectomy on the development of salt hypertension were studied in prepubertal and adult rats with hereditary diabetes insipidus (DI). Early guanethidine administration caused a pronounced and long-term destruction of sympathetic nervous system (SNS) in Brattleboro rats in which blood pressure (BP) was significantly decreased until the age of 22 weeks. This SNS impairment did not abolish the age-dependent BP response of salt-loaded rats that was still greater in young than in adult sympathectomized DI rats. BP of young uninephrectomized DI rats was higher in the late than in the early phase of salt hypertension development. The early sympathectomy lowered BP and increased mortality in all groups of saline drinking DI rats except young uninephrectomized animals in which hypertensive response was attenuated but not prevented. It could be suggested that 1) increased BP response of young rats to high salt intake occurs even in animals with attenuated principal pressor systems, 2) the effects of early sympathectomy on the development of salt hypertension depend on the actual hemodynamic pattern, and 3) moderate BP increase might be a part of homeostatic mechanisms defending the organism threatened by chronic salt overload.

Atrial natriuretic factor is biologically active also in the absence of vasopressin: studies on Brattleboro-strain diabetes insipidus rats.

Since atrial natriuretic factor (ANF) has been shown to inhibit vasopressin secretion, the role of this effect in the acute biological actions of ANF was investigated using Brattleboro-strain diabetes insipidus (DI) rats. Under thiobarbital anesthesia, synthetic rat ANF of a 25 amino acid sequence was administered intravenously as a bolus (8 micrograms/kg) into the jugular vein. The urine volume, urinary sodium and potassium concentration, blood pressure, and heart rate were determined. It was found that ANF administered exogenously can exhibit its diuretic, natriuretic and vasorelaxant activities even in the absence of vasopressin. This indicates that the inhibition of vasopressin secretion is not an indispensible mechanism for acute biological effects of ANF.

Dexamethasone does not inhibit antidiuretic hormone-stimulated prostaglandin production in vivo.

Glucocorticoids are potent inhibitors of prostaglandin (PG) release in vitro, but they fail to influence basal PG production in vivo. To test one possible explanation for this apparent discrepancy in the present study we investigated the effect of dexamethasone (DEX) on basal and antidiuretic hormone-stimulated renal PG synthesis in vivo. In long term experiments, Brattleboro diabetes insipidus rats were treated with DEX (250 micrograms kg-1 day-1) for 7 days, and the effects of deamino-8-D-arginine vasopressin were investigated on the fifth day of DEX treatment. Administration of deamino-8-D-arginine vasopressin was accompanied by a marked increase in PGE2 and PGF2 alpha excretion in both control and DEX-treated animals, but the response was significantly higher during DEX treatment. Furthermore, DEX, by itself, caused a significant increase in the excretion of both PGs. In short term experiments, diabetes insipidus rats received a 24-h iv infusion of DEX (1 mg kg-1 day-1), then renal PG synthesis was stimulated acutely by the infusion of arginine vasopressin (AVP) for 40 min. DEX treatment failed to inhibit basal PG excretion, and the AVP-induced enhancement of PGE2 and PGF2 alpha production was also identical in control and DEX-treated animals. DEX was ineffective in altering the hydroosmotic effect of AVP, confirming that intrarenal PG synthesis was not inhibited. We conclude that glucocorticoids in vivo do not inhibit anti-diuretic hormone-stimulated PG production, and thus, a differential effect of glucocorticoids on basal vs. stimulated PG synthesis cannot account for the discrepancy between in vivo and in vitro studies.

NADH-dependent prostaglandin E 2-9-ketoreductase activity and prostaglandin synthesis in the Brattleboro rat kidney: effects of the antidiuretic hormone

The activity of prostaglandin (PG)E 2-9-ketoreductase (9KR), an enzyme catalyzing the conversion of PGE 2 to PGF 2α, was significantly increased in glomerular and cortical homogenates of diabetes insipidus (DI) rats, as compared to normal Long Evans (LE) rats, and did not change with ADH treatment. Medullary 9KR was similar in the three groups and papillary 9KR was increased, but not significantly, in both groups of DI rats. K m values for PGE 2 and NADH were compared in the various compartments of the kidney. Levels of 9KR were not correlated with the PGE/PGF ratio in urine or supernatants. The synthesis of PGE 2 and PGF 2α by isolated glomeruli was increased in DI rats. This was not reversed by ADH treatment, PGE 2 synthesis increasing even further, especially in the presence of arachidonic acid. In contrast, medullary slices produced significantly less PGs in DI than in LE rats and returned to normal with ADH treatment. Papillary slices produced similar quantities of prostaglandins in all groups. The results do not support the concept that the alterations in PG synthesis observed in DI rat are related only to changes in 9KR activity, but do not exclude the possibility that the enzyme participates in the regulation of PG biosynthesis.

Hyperprolactinaemia alleviates behavioral alterations of rats with hereditary hypothalamic diabetes insipidus (Brattleboro strain)

Male rats homozygous for hereditary hypothalamic diabetes insipidus (HO) and their heterozygous (HE) and normal (NO) variants (Brattleboro rats) were made hyperprolactinaemic by homografting two adenopituitaries under the kidney capsule. The high water intake and urine output of homozygous diabetic sham-operated rats (sham-HO) were similar to those of homografted HO animals. Also, hyperprolactinaemia failed to change the water intake and urine output of HE and NO rats, as compared to those of heterozygous (sham-HE) and normal (sham-NO) sham-operated animals. Compared to sham-HE and sham-NO animals, sham-HO rats showed a slow acquisition of active avoidance responses, a facilitated extinction of a pole jumping avoidance behavior and a reduced retention of a passive avoidance response. However, an improved performance of acquisition and retention behaviors up to the level of sham-HEs and sham-NOs was observed in homografted HO rats. Hyperprolactinaemia resulted in a reduced responsiveness to electrical footshock in HO, HE and NO animals, and in facilitated acquisition of active avoidance responses in HE and NO rats, but it failed to affect avoidance extinction and retention in the latter variants. These results suggest that the behavioral alterations shown by homozygous diabetes insipidus rats are alleviated by hyperprolactinaemia although high levels of plasma prolactin do not interfere with the mechanisms regulating water intake and urine output. In addition, hyperprolactinaemia affects the behavior of heterozygous and normal variants of Brattleboro strain but in a selective way.

Relation between potassium excretion and proximal tubular fluid output in conscious unoperated rats adapted to different dietary potassium contents.

The effect of dietary potassium content on the proximal tubular fluid output (Vprox), and the relation between Vprox and potassium excretion was examined in conscious unoperated diabetes insipidus rats adapted to five dietary potassium contents in the range 25-400 mmol/kg. The lithium clearance (CLi) and the urine flow (V) were used as measures of Vprox, CLi being equal to and V being about one-third of Vprox. Increase of the dietary potassium content from 25 to 400 mmol/kg led only to a slight, barely statistically significant increase of Vprox. However, repeated measurements in the same rats on different days showed considerable intraindividual variation of Vprox and the potassium excretion. At each dietary potassium level except the lowest one the potassium excretion was linearly correlated to CLi (r = 0.60-0.92) and to V (r = 0.56-0.85). Errors in bladder emptying could not explain these correlations. The slopes of the regression lines increased with increasing dietary potassium content, and the regression lines all intercepted the CLi axis at approximately the same CLi value, i.e., on the average about 70 microliter/min./100 g body weight. The results indicate that an increase of the dietary potassium content leads to a slight increase of Vprox. Spontaneous intraindividual variation of Vprox is considerable and is associated with linearly related variation of the potassium excretion. The change of the potassium excretion for a given change of Vprox increases with the dietary potassium content.