The effect of adrenocortical hormones on water permeability of the collecting duct of the rat

Dexamethasone enhances vasopressin-induced aquaporin-2 gene expression in the mpkCCD cells

The mouse cortical collecting duct cell (mpkCCD) has been an instrumental cell model for studying vasopressin-mediated aquaporin-2 regulation. This cell line was first developed by Vandewalle's group from a transgenic mouse carrying the transforming SV40 antigens driven by the pyruvate kinase promoter. To immortalize the cells, four hormone supplements (dexamethasone, epidermal growth factor, insulin, and triiodothyronine) were used to enhance SV40 antigen expression; however, these hormones appear to have various effects on aquaporin-2 gene expression in the cells. Here, we evaluated the effects of each hormone supplement and found that dexamethasone enhanced vasopressin-induced aquaporin-2 gene expression at both mRNA and protein levels in a dose- and time-dependent manner, without affecting mRNA or protein stability. The effects of dexamethasone were attributed largely to enhanced aquaporin-2 mRNA transcription in association with an enhanced aquaporin-2 promoter activity. Dexamethasone did not affect vasopressin-regulated aquaporin-2 phosphorylation and trafficking. In summary, we optimized the conditions to enhance vasopressin-induced endogenous aquaporin-2 gene expression in the mpkCCD cells. By increasing the amount of aquaporin-2 protein in the cells, our method will facilitate the study of aquaporin-2 cell physiology regulated by vasopressin.

Molecular analysis of impaired urinary diluting capacity in glucocorticoid deficiency

Urinary diluting ability and protein abundance of renal aquaporins (AQPs) and ion transporters in glucocorticoid-deficient (GD) rats were examined at baseline and in response to oral water loading. Rats underwent bilateral adrenalectomy followed by aldosterone (GD) or aldosterone + dexamethasone (CTL) replacement. Before oral water loading, urinary output was significantly decreased and urinary osmolality (U(osm)) was increased in GD compared with CTL rats. Protein abundance of inner medullary AQP2 (148 +/- 18%), phosphorylated AQP2 (pAQP2, 156 +/- 13%), and AQP3 (145 +/- 8%) was significantly upregulated in GD compared with CTL rats (all P < 0.05). GD rats also demonstrated a marked reduction in urinary Na(+) excretion compared with pair-fed CTL rats. Na(+)-K(+)-2Cl(-) cotransporter, Na(+)/H(+) exchanger type 3, and cortical beta- and gamma-subunits of the epithelial Na(+) channel were significantly upregulated in GD rats. At 1 h after an acute water load (40 ml/kg by oral gavage), GD rats demonstrated a decrease in percent water excretion (5 +/- 1 vs. 33 +/- 9%, P < 0.01) and urinary output (33 +/- 12 vs. 250 +/- 65 microl x kg(-1) x min(-1), P < 0.05) and an increase in U(osm) (1,894 +/- 292 vs. 316 +/- 92 mosmol/kgH(2)O, P < 0.001) compared with CTL rats. Plasma AVP was increased (1.6 +/- 0.2 vs. 0.9 +/- 0.2 pg/ml, P < 0.05), as was protein expression of inner medullary AQP2 (149 +/- 5%) and pAQP2 (177 +/- 9%, P < 0.01), in GD compared with CTL rats; apical expression of AQP2 was maintained in GD rats. The vasopressin V(2) receptor antagonist OPC-31260 increased percent water excretion and urinary output and reduced U(osm) compared with vehicle-treated GD rats. OPC-31260 also reversed the increased abundance and apical trafficking of inner medullary AQP2 and pAQP2 protein in GD rats. In conclusion, enhanced protein abundance of Na(+) transporters and Na(+) channels with Na(+) retention occurred with GD. OPC-31260 reversed upregulation and apical trafficking of AQP2 and pAQP2 in association with improved urinary diluting capacity and increased water excretion after oral water loading.

The effect of adrenalectomy on water permeability in rat papillary collecting duct

1. Chronic adrenal insufficiency impairs maximal urine concentration, probably in part due to reduced medullary tonicity but also possibly by inhibition of distal nephron water transport. This latter defect has been demonstrated in rabbit but not in rat. 2. Since the time between adrenalectomy and experiment was different in rabbit and rat studies, diffusional water permeability was evaluated in the papillary collecting duct in the absence and presence of submaximal (20 microU/mL) and supramaximal (200 microU/mL) arginine vasopressin (AVP) in adrenalectomized rats at 7, 14 and 21 days. 3. Experimentation 7 days after adrenalectomy failed to demonstrate significantly altered basal or AVP-induced water permeability which increased by 23 and 78% with submaximal and supramaximal concentrations, respectively. Submaximal AVP concentrations also induced a comparable change in water permeability in adrenalectomized rats at 14 days; however, 21 days after adrenalectomy, diffusional water permeability was not increased by 20 microU/mL AVP (3.31 +/- 0.22 to 3.31 +/- 0.24 microns/s). Nevertheless, the effect of a supramaximal AVP concentration (200 microU/mL) was not altered by adrenalectomy (4.54 +/- 0.39 to 8.08 +/- 0.96; P < 0.01). Incubation of collecting ducts in aldosterone for 2 h did not reverse the inhibitory effect of chronic adrenalectomy on AVP-stimulated water transport. 4. These studies suggest that mineralocorticoid withdrawal does impair the hydro-osmotic action of AVP in the rat papillary collecting duct but that this effect takes between 14 and 21 days to occur.

Endocrine control of electrolyte balance during development

The endocrine control of electrolyte balance during development is reviewed. It is suggested that the high urinary sodium excretion observed in premature infants may be secondary to the immaturity of the adrenal gland to adequately increase the secretion of aldosterone (Sulyok et al, 1979b), and to the inability of the distal tubule to respond appropriately to a rise in circulating aldosterone levels (Sulyok et al, 1979a). On the other hand, the elevated plasma aldosterone levels observed in term newborn infants may play an important role in the blunted response of the newborn kidney to saline loading (Sulyok et al, 1979a; Spitzer, 1982). The ability of ANP to induce a natriuresis and to contribute to fluid and electrolyte homeostasis during development has been investigated. It has been found that the immature kidney is less responsive to ANP than later in life (Chevalier et al, 1988; Robillard et al, 1988). On the other hand, it has been suggested that a rise in plasma ANP during the first five days of life may contribute to the physiological weight loss associated with the extracellular volume contraction occurring shortly after birth (Tulassay et al, 1987). The role of glucocorticoids, prostaglandins and the kallikrein-kinin system in regulating electrolyte balance during development is also reviewed.

Addison's disease, psychosis, and the syndrome of inappropriate secretion of antidiuretic hormone

A case of tuberculous Addison's disease presenting with psychosis, profound hyponatraemia, and detectable plasma antidiuretic hormone is reported. Clinical and biochemical improvement after corticosteroid replacement was followed by relapse with further psychosis and inappropriate antidiuretic hormone secretion: both were promptly reversed by demethylchlortetracycline. The association of psychological symptoms with Addison's disease, the role of anti-diuretic hormone secretion in Addison's disease, and the inter-relationship between Addison's disease, psychosis and anti-diuretic hormone secretion are discussed.

Concentrating defect in the adrenalectomized rat. Abnormal vasopressin-sensitive cyclic adenosine monophosphate metabolism in the papillary collecting duct

Among other defects in water metabolism, adrenal insufficiency is associated with an inability to concentrate urine maximally in both man and experimental animals. Recent studies in the rabbit cortical collecting tubule have suggested indirectly that this defect may result from impaired cyclic AMP (cAMP) formation in response to antidiuretic hormone stimulation. In the present study, we examined key elements of arginine vasopressin (AVP)-dependent cAMP metabolism in the papillary collecting duct (PCD), microdissected from 8-d adrenalectomized (ADX) and sham-operated control rats. AVP-sensitive adenylate cyclase (ADC) activity in PCD did not differ between control and ADX rats. cAMP-phosphodiesterase activity (cAMP-PDIE), measured at 10(-6) M cAMP substrate concentration, was significantly higher (delta + 31.6%) in PCD of ADX rats compared with controls. Incubation of intact PCD from ADX rats with AVP resulted in an accumulation of cAMP (delta - 48.5%) significantly lower than observed in control PCD. Chronic administration of dexamethasone reduced cAMP-PDIE activity in PCD of ADX rats to levels close to or below those observed in control rat PCD, and also resulted in a restoration of AVP-stimulated cAMP accumulation to levels approaching control values. Results indicate that the impaired maximal urinary concentrating ability associated with adrenal insufficiency may be due, at least in part, to a reduced accumulation of cAMP in response to AVP in the PCD. This decreased cAMP accumulation results from increased cAMP-PDIE activity in the PCD of ADX rats and can be corrected by administration of glucocorticoid.

Role of vasopressin in the impaired water excretion of glucocorticoid deficiency

The mechanism whereby glucocorticoid deficiency impairs renal water excretion was studied in the conscious mineralocorticoid-replaced, adrenalectomized rat. Control animals received physiologic replacement with prednisolone, and experimental animals were deprived of glucocorticoid hormone for either 1 or 14 days. The control animals excreted 95 +/- 1.9% of an acute water load (30 ml/kg) in 3 hours, a value significantly higher than the volume excreted by animals deprived fo glucocorticoid hormone for 1 day (70.0 +/- 3.6%, P less than 0.01) and 14 days (40.0 +/- 3.9%, P less than 0.01). Following the acute water load, plasma vasopressin levels, as measured by radioimmunoassay, was 1.08 pg/ml in the control rats, a value significantly lower than values obtained after the water load in rats deprived of glucocorticoid hormone for 1 day (2.5 +/- 0.2 pg/ml, P less than 0.01) and 14 days (2.4 +/- 0.3 pg/ml, P less than 0.01). To further examine the effect of plasma vasopressin in the impaired water excretion of glucocorticoid deficiency, we performed studied in Brattleboro rats with central diabetes insipidus. In these animals with absence of vasopressin, a defect in water excretion was observed after 14 days, but no 1 day, of glucocorticoid deficiency. In Sprague-Dawley rats, the impaired water excretion after 14 days of glucocorticoid deficiency was associated with a significantly lower cardiac index (209 +/- 14 vs. 291 +/- 11 ml/min/kg, P less than 0.01) and renal blood flow (3.8 +/- 0.3 vs. 5.7 +/- 0.2 ml/min/g, P less than 0.01) than that observed after 1 day of glucocorticoid deficiency. In diabetes insipidus rats, after 14 days of glucocorticoid deficiency, the percentage of an acute water load excreted (121 +/- 7% vs. 158.7 +/- 7.0%, P less than 0.01) was lower than that observed after 1 day of glucocorticoid deficiency. In summary, the present results indicate that glucocorticoid deficiency impairs renal water excretion by both vasopressin-dependent and vasopressin-independent mechanisms. The vasopressin-dependent renal mechanism is associated with a marked decrease in both systemic and renal hemodynamics.