RENAL TUBULAR EFFECTS OF HYDROCORTISONE AND ALDOSTERONE IN NORMAL HYDROPENIC MAN: COMMENT ON SITES OF ACTION

Aldosterone Upregulates Transient Receptor Potential Melastatin 7 (TRPM7)

Transient receptor potential melastatin 7 (TRPM7) is a ubiquitously expressed Mg(2+)-permeable ion channel fused to a C-terminal α-kinase domain. Recently, aldosterone was shown to increase intracellular Mg(2+) levels and alter inflammatory signaling in TRPM7-expressing HEK293 cells. This study was undertaken to assess whether these effects were related to an aldosterone-mediated increase of TRPM7 current and/or plasma membrane localization. Using HEK293 cells stably expressing WT-TRPM7, we found that 18-h application of aldosterone significantly increased TRPM7 current and TRPM7 plasma membrane protein expression by 48% and 34%, respectively. The aldosterone-mediated increase of TRPM7 current was inhibited by eplerenone, a mineralocorticoid receptor (MR) blocker, and GSK-650394, an inhibitor of the serum- and glucocorticoid-regulated kinase 1 (SGK1). SGK1 blockade also prevented the aldosterone-induced increase of TRPM7 plasma membrane protein. It was further determined that K1648R-TRPM7, the phosphotransferase-inactive TRPM7 mutant, was unresponsive to aldosterone. Therefore, chronic aldosterone treatment increases the plasma membrane expression of TRPM7, which is associated with an increase of TRPM7 current. This process occurs via an MR-dependent, genomic signaling cascade involving SGK1 and a functioning TRPM7 α-kinase domain. We suggest that this mechanism may be of general relevance when interpreting the effects of aldosterone because the MR receptor is found in multiple tissues, and TRPM7 and SGK1 are ubiquitously expressed.

Regulation of rat renal (Na(+) + K+)-adenosine triphosphatase mRNA levels by corticosterone

We investigated the mechanisms for glucocorticoid regulation of rat renal NaK-ATPase activity. Our findings suggest that the magnitudes of corticosterone-induced increases in alpha 1 mRNA and beta 1 mRNA levels are similar in the kidney of the adult adrenalectomized rats. The results also suggest that corticosterone restores NaK-ATPase activity in adrenalectomized rats prior to any enhanced sodium delivery.

Prednisolone can increase glomerular permeability to proteins in nephrotic syndrome

In patients with a nephrotic syndrome administration of prednisolone causes an increase of proteinuria. To elucidate the mechanism of this effect we have studied the acute proteinuric effect of prednisolone, 125 to 150 mg intravenously, in nine patients (7 M, 2F) with a nephrotic syndrome. Mean age (+/- SD) of the patients was 53 +/- 6 years, mean endogenous creatinine clearance 104 +/- 30 ml/min, and mean proteinuria 7.7 +/- 3.0 g/24 hr. After administration of prednisolone, urinary total protein excretion rose in all patients from a mean (+/- SEM) of 4.89 +/- 0.59 mg/min before to 9.09 +/- 0.99 mg/min at five hours after administration (P less than 0.01). Glomerular filtration rate (inulin clearance), effective renal plasma flow (PAH clearance), and filtration fraction did not change significantly. The increases of urinary excretion of albumin (median %: +92%), IgG (median %: +88%), and transferrin (median %: +76%) were comparable and correlated significantly. Urinary excretion of beta 2-microglobulin did not change significantly however. We conclude that intravenous administration of prednisolone to patients with a nephrotic syndrome causes an increase in urinary protein excretion rate which cannot be explained by changes in renal hemodynamics or tubular protein reabsorption, and which therefore must be the result of a change in glomerular permselectivity characteristics.

Mechanism of increased renin release in the adrenalectomized rat. Adrenal insufficiency and renin

We have previously suggested that inhibition of renin release by sodium chloride is related to absorptive chloride transport in the loop of Henle. Infusion of sodium chloride fails to inhibit renin release in the adrenalectomized (Adx) rat, and dexamethasone restores renin responsiveness to sodium chloride. The purpose of the present study was to evaluate the relationship between loop function (urinary diluting and concentration capacity) and plasma renin concentration (PRC) in the Adx rat. After hypotonic sodium chloride infusion, free water clearance (CH2O) of Adx rats (0.56 ml/hr/100 g +/- 0.17 SE) was decreased (p less than 0.01) compared to controls (2.86 ml/hr/100 g +/- 0.29 SE); PRC of Adx rats (61.9 units/ml +/- 11.2 SE) was increased (p less than 0.01) above controls (6.0 units/ml +/- 1.7 SE). These differences persisted after administration of d(CH2)5Tyr(Et)VAVP, a potent ADH antagonist. In separate groups of animals, after water deprivation, urine concentration of Adx rat (1,401 mOsm/kg +/- 45 SE) was less (p less than 0.01) than that of controls (2,117 mOsm/kg +/- 169 SE). Dexamethasone normalized both CH2O and urinary concentrating ability and also decreased PRC in Adx rats. Thus, in the glucocorticoid deficient rat, increased renin release is associated with impaired loop function. The loop defect may account for high PRC that is not suppressed by sodium chloride.

Nocturnal diuresis in panhypopituitarism

Although impaired water diuresis in adrenocortical insufficiency is well-known, little attention has been paid to the diuretic pattern at night. In two cases of panhypopituitarism, the nocturnal diuretic pattern was found to be quite different from that in the morning, and marked diuresis did occur after water loading at night, without any significant change of serum cortisol which remained at a low level throughout the day. At the antidiuretic stage in the morning, urine osmolality continued to rise gradually, in spite of the water loading, to the level of 666 mOsm/liter. At the water restriction test, the urine was concentrated only to the same level of 600-700mOsm/liter. At the ascending stage of urine osmolality, exogenously injected pitressin showed little antidiuretic effect, although the kidney was able concentrate urine to the higher level later. In the morning, 3 liters of 5% glucose infusion failed to produce a marked diuresis in spite of extreme plasma dilution and expansion. Furosemide immediately induced diuresis even in the morning and the kidney recovered its ability to respond to pitressin. Glucocorticoid also improved the diuretic pattern in the morning, but a latent period of about 2 hr was always observed before the appearance of the effect. What happened during this latent period was unclear, but it was interesting to note that a mechanism similar to that which induces diuretic response seemed to occur without glucocorticoid at night.

Acute effect of prednisolone on renal handling of sodium

The effect of prednisolone on renal handling of sodium (Na) was studied in rats under three experimental conditions: 1) hydropenia, 2) water diuresis, and 3) distal tubular blockade (DTB). Prednisolone, 0.25 mg/100 g per hr, was infused directly into left renal artery and urine was collected separately from each kidney. Predominantly unilateral increases in urine flow (V) and Na excretion were noticed in all experiments during prednisolone infusion. In the hydropenic rats the maximal increments on the infused side were, for V (mean ± SD), from 9.3 ± 1.5 to 21.4 ± 0.8 μl/min (P < 0.001); for C_Na/C_In, from 0.28 ± 0.11 to 2.97 ± 0.71 % (P < 0.005); and for TH2Oc/CIn, from 2.93 ± 2.26 to 5.32 ± 1.92% (P < 0.05). In the rats with water diuresis, the maximal increases were, for V/C_In, from 5.87 ± 1.97 to 10.1 ± 6.0% (P < 0.005); for C_H2O/C_In, from 4.09 ± 0.68 to 6.00 ± 0.44% (P < 0.0005); and for C_Na/C_In, from 0.22 ± 0.07 to 0.70 ± 0.38% (P < 0.01). In DTB-rats the maximal increases were for V from 48.6 ± 9.0 to 72.7 ± 14.1 μl/min (P < 0.0005) and for C_Na/C_In from 9.42 ± 2.97 to 20.23 ± 7.34% (P < 0.005). In the contralateral kidney these changes were less pronounced. These observations suggest that prednisolone depresses directly Na reabsorption. The association of natriuresis with augmented TH2Oc/CIn and C_H2O/C_In during hydropenia and water diuresis, respectively, and the increases in V and C_Na/C_In during DTB, all are consistent with inhibition of Na reabsorption in the proximal tubule.

Effect of methylprednisolone on renal function and the zonal distribution of blood flow in the rat

The effect of acute and chronic administration of methylprednisolone on renal function was studied in rats with clearance and micropuncture techniques. Although acute administration did not influence renal function, chronic administration for 4 days caused a marked increase in glomerular filtration rate and PAH clearance. The rise in the reabsorption of sodium under these conditions was associated with a disproportionate increase in filtration rate and renal blood flow in deep portions of the cortex. There was no evidence of altered fractional sodium reabsorption in the proximal segment of surface nephrons by micropuncture analysis or in the ascending limb by clearance studies during a water or a solute diuresis.

Effect of spironolactone on stool electrolyte losses during human cholera

This study demonstrated that endogenous aldosterone, as determined by its transient competitive block by spironolactone, caused significant sodium and chloride retention during naturally acquired cholera. This beneficial effect of the hormone is accompanied, however, by a deleterious depletion of potassium. In addition, it was found that stool rate significantly altered the sodium and potassium concentrations in cholera stool despite minimal or absent aldosterone activity.

Acute renal response to large doses of intravenous prednisolone in kidney homograft recipients and in normal subjects

The immediate renal response to large intravenous doses of prednisolone was studied in 18 kidney homograft recipients and in 6 normal subjects. Clearance rates of inulin (C_IN), creatinine (C_CR), p-aminohippurate (C_PAH), and electrolytes were measured over 3 one-hour periods following intravenous infusion of prednisolone (1 Gm.) and compared with corresponding clearance rates after a placebo infusion. C_IN, C_CR, and C_PAH rates and (CCRCIN) ratios exhibited a substantial decrease during all collection periods following the infusion of prednisolone, both in the normal subjects and in the patients. Fractional excretion of potassium (CKCIN) increased in a progressive fashion reaching peak values after 3 hours. Biphasic variations were observed in the fractional excretion of sodium (CNACIN); an increase during the first hour was followed by a decrease during the third hour. The changes in the fractional excretions of ultrafiltrable calcium (CCaCIN), ultrafiltrable magnesium (CMgCIN), and phosphorus (CPCIN) were minimal. Normal subjects exhibited significant decreases in CCaCIN and CMgCIN following the infusion of prednisolone; there was no significant change in the patients. CPCIN increased significantly both in the normal subjects and in the patients. These results indicate that acute suppression of kidney function is a general renal response to large doses of glucocorticoids. The marked decrease in the creatinine clearance ratio (CCRCIN) observed after the administration of prednisolone is consistent with a depressed tubular secretion of creatinine and emphasizes the inadequacy of C_CR as an indication of glomerular filtration rate (GFR) under conditions in which large doses of glucocorticoids are employed.

The renal tubular handling of aldosterone and its acid-labile conjugate

Stop-flow studies using infusions of aldosterone-(3)H or its (3)H acid-labile conjugate were done on five rhesus monkeys. The aldosterone-(3)H urine-to-plasma (U/P) ratio decreased in the same distal urine samples as sodium. The (3)H acid-labile conjugate U/P-to-inulin U/P ratio increased in the more proximal samples either with conjugate formed endogenously during aldosterone-(3)H infusions or with labeled conjugate infused alone. Aldosterone reabsorption occurred at a distal site in the renal tubule, and secretion of its acid-labile conjugate occurred at a proximal site.