Mechanisms of activation of renal (Na+ + K+)-ATPase in the rat. Effects of acute and chronic administration of dexamethasone

The effects of dexamethasone on the Na,K-ATPase activity and pump function of corneal endothelial cells

PURPOSE

The Na(+)- and K(+)-dependent ATPase (Na,K-ATPase) expressed in the basolateral membrane of corneal endothelial cells plays an important role in the pump function of the corneal endothelium. We investigated the possible role of dexamethasone in the regulation of Na,K-ATPase activity and pump function in corneal endothelial cells.

METHODS

Confluent monolayers of mouse corneal endothelial cells were exposed to dexamethasone. ATPase activity of the cells was evaluated by spectrophotometric measurement of phosphate released from ATP with the use of ammonium molybdate, with Na,K-ATPase activity being defined as the portion of total ATPase activity sensitive to ouabain. Pump function of the cells was measured with the use of an Ussing chamber, with the pump function attributable to Na,K-ATPase activity being defined as the portion of the total short-circuit current sensitive to ouabain. Western blot analysis was examined to measure the expression of the Na,K-ATPase alpha(1)-subunit.

RESULTS

Dexamethasone (1 or 10 microM) increased the Na,K-ATPase activity and pump function of the cultured cells. These effects of dexamethasone were blocked by cycloheximide, a protein synthesis inhibitor. Western blot analysis also indicated that dexamethasone increased the expression of the Na,K-ATPase alpha(1)-subunit, whereas it decreased the expression of the phospho-Na,K-ATPase alpha(1)-subunit.

CONCLUSIONS

Our results suggest that dexamethasone stimulates Na,K-ATPase activity in mouse corneal endothelial cells. The effect of dexamethasone activation in these cells is mediated by Na,K-ATPase synthesis and increase in an enzymatic activity by dephosphorylation of Na,K-ATPase alpha(1)-subunits.

Animal and human tissue Na,K-ATPase in normal and insulin-resistant states: regulation, behaviour and interpretative hypothesis on NEFA effects

The sodium(Na)- and potassium(K)-activated adenosine-triphosphatase (Na,K-ATPase) is a membrane enzyme that energizes the Na-pump by hydrolysing adenosine triphosphate and wasting energy as heat, so playing a role in thermogenesis and energy balance. Na,K-ATPase regulation by insulin is controversial; in tissue of hyperglycemic-hyperinsulinemic ob/ob mice, we reported a reduction, whereas in streptozotocin-treated hypoinsulinemic-diabetic Swiss and ob/ob mice we found an increased activity, which is against a genetic defect and suggests a regulation by hyperinsulinemia. In human adipose tissue from obese patients, Na,K-ATPase activity was reduced and negatively correlated with body mass index, oral glucose tolerance test-insulinemic area and blood pressure. We hypothesized that obesity is associated with tissue Na,K-ATPase reduction, apparently linked to hyperinsulinemia, which may repress or inactivate the enzyme, thus opposing thyroid hormones and influencing thermogenesis and obesity development. Insulin action on Na,K-ATPase, in vivo, might be mediated by the high level of non-esterified fatty acids, which are circulating enzyme inhibitors and increase in obesity, diabetes and hypertension. In this paper, we analyse animal and human tissue Na,K-ATPase, its level, and its regulation and behaviour in some hyperinsulinemic and insulin-resistant states; moreover, we discuss the link of the enzyme with non-esterified fatty acids and attempt to interpret and organize in a coherent view the whole body of the exhaustive literature on this complicated topic.

Corticosteroid modulation of Na(+)-K+ pump-mediated relaxation in maturing airway smooth muscle

1. The ontogeny of the relaxant influence of the airway electrogenic Na(+)-K+ pump and its potential modulation by corticosteroids were examined in airway smooth muscle (ASM) segments isolated from newborn and adult rabbits. 2. Control and methylprednisolone-treated (MP) ASM segments were half-maximally contracted with methacholine in K(+)-free buffer and the ASM relaxant responses to Na(+)-K+ pump activation were subsequently evaluated. Relative to adult ASM, control newborn ASM showed significantly enhanced maximal relaxation (Rmax) to KCl (62.5 +/- 5.2% vs. 47.8 +/- 5.2%), but no difference in sensitivity (pC2 = -log concentration producing 50% Rmax: 2.18 +/- 0.12 vs. 2.29 +/- 0.09-log M). 3. Exposure of ASM segments to 500 microM methylprednisolone for 1 h potentiated the airway Na(+)-K+ pump activity. A more pronounced effect was obtained in newborn ASM, where both the Rmax and pC2 values were significantly enhanced. In mature ASM, only the Rmax response to KCl was increased in the presence of MP. 4. Collectively, these data demonstrate that: (i) the functional activity of the airway electrogenic Na(+)-K+ pump decreases with post-natal maturation in the rabbit: (ii) corticosteroid treatment potentiates Na(+)-K+ pump activity in rabbit ASM; and (iii) the latter effect of corticosteroids is enhanced in immature airways. 5. The above findings provide new evidence that the airway relaxant response to activation of the electrogenic Na(+)-K+ pump varies ontogenetically and that corticosteroids potentiate the Na(+)-K+ pump activity in an age-dependent manner.

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.

Adrenocorticoid regulation of Na+,K(+)-ATPase in adult rat kidney: effects on post-translational processing and mRNA abundance

The mechanisms by which adreno-corticoid hormones regulate Na+,K(+)-ATPase in adult kidney were studied in adrenalectomized (Adx) rats. Five days after adrenalectomy, Na+,K(+)-ATPase activity was significantly reduced in the renal cortex homogenate (C = 13.0 +/- 0.8 vs. Adx = 7.1 +/- 0.7 mumol Pi mg-1 protein h-1) and in renal microsomes (C = 30.3 +/- 1.9 vs Adx = 14.6 +/- 1.3 mumol Pi mg-1 protein h-1). Glucocorticoid replacement treatment of adrenalectomized rats with betamethasone (20 micrograms kg-1 body wt twice daily for 5 days) effectively counteracted the observed reduction in Na+,K(+)-ATPase activity. In cortical homogenate the protein level of alpha 1 and beta 1 subunits measured in immunoblots was not significantly different in Adx and control rats, indicating that 5 days after adrenalectomy the alpha 1 and beta 1 subunits were present in renal cortical cells to almost normal extent but could not be assembled into a transmembrane functional unit. In support of this conclusion we found that the protein level of both the alpha 1 and beta 1 subunits was significantly lower (P less than 0.001 for both subunits) in microsomes from Adx than in control rats. The mRNA abundance for alpha 1 and beta 1 subunits were not lower in Adx as compared to control rats 1 and 5 days after surgery. However, if Adx rats were given a single dose of betamethasone (600 micrograms kg-1 body wt), a significant 2-fold increase in both alpha 1 and beta 1 mRNAs was observed (P less than 0.05 for both subunits).(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular mechanisms of action of mineralocorticoid hormones

Mineralocorticoid hormones are a subset of steroid hormones that act primarily in epithelial tissues to regulate ion transport of Na+, K+ and H+. Cellular specificity is conferred by receptors which act in the nucleus to stimulate gene expression. Transcription and subsequent translation result in the production of new proteins which mediate the physiologic effects. The mechanisms involved in receptor specificity and localization, in regulation of gene activation, and in expression of transport effects are reviewed. The cellular actions of mineralocorticoids fit well with the general model of steroid hormone action but considerable questions remain at each step in the process.

Involvement of second messenger systems in stimulation of angiotensin converting enzyme of bovine endothelial cells

We have demonstrated previously that a variety of agents including corticosteroids, thyroid hormone, cationophores, methylxanthines, and analogues of cAMP--all of which have diversified functions in various tissues--elevate cellular angiotensin converting enzyme (ACE) activity of bovine endothelial cells in culture. In addition to these agents, we have now found that direct and receptor-mediated stimulators of adenylate cyclase, i.e., forskolin and cholera toxin, increase cellular ACE activity after 48 h incubation in culture. In an attempt to search out a more unifying concept of these stimulatory effects, we have further investigated the roles of second messengers in the stimulatory actions. Ca2+ ionophore A23187 produced significant increases in both intracellular Ca2+ and ACE of endothelial cells. In contrast to Ca2+ ionophore, agents that transiently mobilize Ca2+ from intracellular reserves such as bradykinin, acetylcholine, and ATP have no effect on the level of cellular ACE. Representative agents that elevate cellular cAMP (e.g., isobutyl methylxanthine [IBMX] and dibutyryl cAMP) elevated cellular ACE, but the slightly increased [Ca2+]i produced by these agents did not reach statistical significance. While IBMX, cholera toxin, and forskolin elevated cellular cAMP, other ACE stimulatory agents (hormones and cationophores) had no effect on cAMP. Ca2+ ionophore and the agents that elevated intracellular cAMP potentiated the effect of dexamethasone, thyroid hormone, and aldosterone in elevating cellular ACE activity. Increases in ACE activity produced by all stimulants were inhibited by the presence of 10-50 nM ouabain in the culture medium. Inhibition of ACE elevation by oubain was reversed by increasing the extracellular [K+], thereby implicating Na+, K(+)-ATPase in the ACE regulatory mechanism. These results support the presence of multiple independent mechanisms for the regulation of cellular ACE. In addition to possible involvement of intracellular Ca(2+)- and cAMP-dependent pathways, ACE is also increased by corticosteroids and thyroid hormone through mechanisms unrelated to Ca2+ and cAMP.

Regulation of renal kallikrein synthesis and activation by glucocorticoids

The effects of endogenous and exogenous glucocorticoids on renal active and prokallikrein levels (ng/mg protein) and in vivo kallikrein synthesis rate were studied in the conscious rat. Within two hours after low dose methylprednisolone (MP, 0.0125 to 0.05 mg/100 g body wt), active kallikrein and prokallikrein fell (29.1 +/- 2.3 and 35.1 +/- 2.7 ng/mg protein, respectively, compared to 38.4 +/- 3.7 and 42.7 +/- 3.4 in vehicle-treated rats, P less than 0.05 or less). These changes were accompanied by a significant fall in prokallikrein synthesis rate relative to total protein synthesis. The reductions in active and prokallikrein levels were transient, dissipating by six hours. With increasing MP doses, there was further dose-dependent reduction in active kallikrein. However, prokallikrein levels increased to normal as the MP dose was increased despite continued suppression of synthesis, suggesting that prokallikrein activation was inhibited. Renal kallikrein levels were also examined in relation to changes in endogenous glucocorticoid levels. In intact rats, three hours after plasma corticosterone peaked (10 p.m.), active and prokallikrein levels were 30.2 +/- 2.9 and 27.0 +/- 1.6 ng/mg protein, respectively, compared to 36.9 +/- 2.3 and 37.2 +/- 2.6 (P less than 0.005) three hours after the corticosterone nadir (11 a.m.). Furthermore, adrenalectomy increased active and prokallikrein (47.3 +/- 4.8 and 87.3 +/- 6.0 ng/mg protein, respectively), compared to levels in intact or shamoperated rats (intact: 32.9 +/- 2.9 and 54.9 +/- 5.3 ng/mg protein, P less than 0.01 or less). Adrenalectomy also eliminated the diurnal changes in kallikrein levels seen in intact rats. These data suggest that renal prokallikrein synthesis and activation are physiologically regulated by glucocorticoids.

Effect of angiotensin II infusion in rats on Na,K-ATPase activity in renal cortical microsomal preparations

Direct dose-dependent effects of angiotensin II on renal tubular sodium reabsorption have been demonstrated. Alterations in tubular sodium reabsorption may occur via modulation of renal Na,K-ATPase activity. Thus, these experiments were undertaken to ascertain whether angiotensin II could influence renal cortical Na,K-ATPase activity. Angiotensin II, 495 ng/microliters/h, or vehicle (controls) was infused for 24 h via miniosmotic pumps 48 h after rats were adrenalectomized and implanted with osmotic pumps containing 12.5 micrograms/microliters corticosterone (Treatment I) or both corticosterone and 0.2 microgram/microliter aldosterone (Treatment II), and in rats receiving 3% NaCl in their food (sodium loaded, Treatment III). Rats receiving Treatments I and III received saline to drink. Renal cortical microsomal membranes were prepared, and the effects of angiotensin II infusion on the K1/2 and Vmax for Na, K, and ATP determined. Angiotensin II infusions were associated with (i) a decrease (P less than 0.001) in the K1/2 for Na activation of Na,K-ATPase from 14 +/- 3 to 6 +/- 1 (n = 4 experiments), 16 +/- 1 to 12 +/- 1 (n = 5), and 12 +/- 3 to 7 +/- 1 (n = 5) mM (means +/- SE) for treatments I, II, and III, respectively; (ii) no changes in the K1/2 for K activation or the Km for ATP; (iii) no changes in the Vmax for Na, K, or ATP; and (iv) no change in Mg-ATPase activity. We conclude that angiotensin II infusion is associated with a decrease in the K1/2 of renal cortical Na,K-ATPase activity for sodium. This action of angiotensin II on the enzyme activity may contribute to the regulation of tubular sodium transport.

Regulation of glucocorticoid receptors and Na-K ATPase activity by hydrocortisone in proximal tubular epithelial cells

The effect of hydrocortisone (HC) in modulating glucocorticoid receptors (GR) and sodium-potassium adenosine triphosphatase (Na-K ATPase) activity was studied in primary cultures of immunoisolated murine proximal tubular epithelial cells (PTEC). Utilizing monoclonal antibody against stage-specific embryonic antigen-1, a homogeneous population of PTEC was obtained in high yield. The cells were cultured to confluence and further treated for 48 h in serum-free growth medium containing no HC (control); 50 nM HC; or 50 nM HC plus 20 nM of the antiglucocorticoid, RU 38486. PTEC treated with 50 nM HC had 56% of GR binding and 160% Na-K ATPase activity as compared to controls (P less than 0.01). GR binding was abolished by incubation in RU 38486 whereas Na-K ATPase fell below control values (P less than 0.05). Brief incubations of HC-treated PTEC with 0.5 mM ouabain resulted in a fall in GR binding without a change in Na-K ATPase activity. These data indicate that in PTEC, HC regulates GR binding and they suggest that stimulation of Na-K ATPase activity is a direct biological response to this receptor-hormone interaction. Thus, primary cultures of immunoaffinity-isolated PTEC offer a good model system for investigating the molecular basis underlying the regulation of GR binding and postreceptor events influenced by glucocorticoids.

Regulation of fetal Na+/K+-ATPase in rat kidney by corticosteroids

The enzymatic differentiation of various tissues is under hormonal control in the perinatal period. Since the regulation of Na+/K+-ATPase has not been explored prenatally, the aim of this study was to determine the corticosteroid sensitivity of sodium pump maturation in the fetal period. Na+/K+-ATPase activity was both measured in kidney homogenates of fetal rats and localized by in-situ histochemistry. Sodium pump activity was first quantifiable on day 18 of fetal development as 1.4 +/- 0.17 mumol Pi/h per mg protein, and was increased 3.4-times by day 22 of gestation. While the Na+/K+-ATPase activity was the most intense in cortical tubules at an earlier fetal age (18th and 19th day), the reaction product in the medullary tubules increased with fetal age, becoming highly intense on the 21st and 22nd day of gestation. From the 18th to 21st day of fetal development homogenate Na+/K+-ATPase activity increased as a function of chronologic age. While mineralocorticoids were without any effect on Na+/K+-ATPase activity, on the last day of the fetal development, the glucocorticoid dexamethasone proved to be successful in stimulating enzyme activity in corticosteroid-suppressed animals. According to our results, glucocorticoid hormones seem to be operating as an endogenous driving force for sodium pump maturation at the end of fetal development.

The human leucocyte sodium pump in Cushing's syndrome

Because corticosteroids have important effects on sodium homeostasis, we studied leucocyte 22Na efflux in patients with Cushing's syndrome. The ouabain-sensitive 22Na+ efflux rate constant was raised in Cushing's syndrome (mean +/- SD 2.84 +/- 0.32 vs 2.35 +/- 0.53/h, P less than 0.001, n = 15). This efflux rate constant correlated with the urinary free cortisol (rs = 0.61, P less than 0.02), but less significantly with the 0900 h plasma cortisol (rs = 0.46, P less than 0.08). There was no correlation with the supine plasma aldosterone. Intracellular sodium content was significantly lower in Cushing's syndrome (21.1 +/- 4.6 vs 27.8 +/- 9.5 mmol/kg dry weight, P less than 0.01), with a raised intracellular potassium to sodium content ratio (16.1 +/- 3.3 vs 12.9 +/- 3.6, P less than 0.01). After treatment of the Cushing's syndrome by trans-sphenoidal adenomectomy or adrenalectomy, these defects in cellular sodium balance were corrected. Thus, the distribution of sodium between intra- and extracellular fluid may be affected by adrenal corticosteroids via an action on the sodium pump.

Glucocorticoids modulate renal glucocorticoid receptors and Na-K ATPase activity

Primary cultures of mouse renal tubular epithelial cells were used to study the effect of hydrocortisone on the regulation of glucocorticoid receptors (GR) and on sodium-potassium adenosine triphosphatase (Na-K ATPase) activity. A GR assay was developed and performed directly on cell monolayers maintained in serum-free medium to which hydrocortisone at 5 nM, 50 nM, and 5 X 10(-4) M was added. Compared with control cells grown in medium without hydrocortisone, GR levels per cell decreased by 50% after 48 hours of growth in medium containing 5 nM hydrocortisone concentrations (50 nM or 5 X 10(-4) M), GR levels decreased to less than or equal to 28% of control values. In all hydrocortisone treatment groups there was an inverse relation between GR concentrations and Na-K ATPase activity. Binding of cell GR by the addition of the antiglucocorticoid RU 38486 in hydrocortisone-supplemented medium eliminated the glucocorticoid-induced stimulation of Na-K ATPase activity. These results demonstrate a time- and dose-dependent effect of glucocorticoids on GR binding activity and a direct relation between this receptor-hormone interaction and Na-K ATPase activity in intact renal tubular epithelial cells.

The human leucocyte sodium pump in adrenocortical insufficiency

In view of the known effects of cortisol and related compounds on sodium balance, we examined leucocyte 22Na+ efflux in patients with adrenocortical insufficiency. The ouabain-sensitive 22Na+ efflux rate constant, which reflects sodium pump activity, was lowered in secondary hypoadrenalism (mean 1.95 +/- SD 0.25 vs 2.33 +/- 0.45 h-1, P less than 0.002, n = 6). Patients with hypoadrenalism also had higher intracellular sodium content (mean 34.9 +/- SD 3.5 vs 27.8 +/- 9.5 mmol/kg dry weight, P less than 0.008) and lower intracellular potassium to sodium ratios (mean 9.7 +/- SD 0.7 vs 12.9 +/- 3.6, P less than 0.001). These defects in cellular sodium balance were corrected following replacement therapy with cortisol, prednisolone or dexamethasone. In-vitro incubation of normal leucocytes with 5-20 nmol/l dexamethasone led to an increase in the Na+/K+-ATPase activity. The effect of cortisol and related compounds on the distribution of sodium between intra- and extra-cellular fluid may be mediated by their action on the sodium pump.

Altered parathyroid hormone- or calcitonin-stimulated adenosine 3', 5'-monophosphate release by isolated perfused bone from glucocorticoid-treated rats

The present studies were designed to examine in vivo effects of glucocorticoid on PTH-or calcitonin (CT)-stimulated adenosine 3',5'-monophosphate (cAMP) release from the isolated perfused bone of rat and to test whether the duration of glucocorticoid administration influenced such effects. We assessed the ability of acute (24 hour) or chronic (2 week) dexamethasone administration to modulate the cAMP response to 5 micrograms human PTH-(1-34) or 1 micrograms eel CT. Acute treatment with dexamethasone (1 mg/100 g body wt) increased the cAMP response to PTH, but decreased the response to CT. This enhanced effect on PTH-stimulated cAMP release was not apparent in the presence of phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX, ImM). In contrast, chronic dexamethasone treatment (0.2 mg daily for 2 weeks) led to a decrease in both PTH- and CT-stimulated cAMP release. Such impaired response of the dexamethasone-treated bones to PTH was also found in rats that underwent parathyroidectomy 24 hours before sacrifice. These data indicate that 1) the duration of glucocorticoid administration may influence the effect of PTH on bone and 2) glucocorticoid may decrease cAMP-mediated CT function, regardless of the duration of treatment.

The effect of ionophores and related agents on the induction of doming in a rat mammary epithelial cell line

Addition of dimethyl sulfoxide (DMSO) and the mammotropic hormones prolactin, hydrocortisone, insulin, and estradiol to confluent cultures of the epithelial cell line Rat Mammary (Rama) 25 increases dramatically the formation of domes in the cell monolayer after 48-72 hr. Associated with the increase in doming is an increase of 24% in the activity of the Na+/K+ ATPase. Both Ca2+ (A23187) and Na+ (monensin, gramicidin J, melittin) ionophores can replace DMSO in inducing domes, whilst the K+ ionophore valinomycin inhibits doming. However, there are no synergistic nor additive effects, respectively, with suboptimal or optimal concentrations of A23187 and melittin together. Ouabain, at concentrations which inhibit the Na/K ATPase in vitro, and amiloride, at concentrations reported to inhibit the passive transport of Na+, both inhibit completely the formation of domes induced by DMSO, A23187, and melittin. EGTA, however, inhibits only the induction of doming by DMSO and A23187; it is without effect with melittin. A23187 and melittin induce the major polypeptide changes that occur in doming cultures with DMSO, and most of these changes are also inhibited with ouabain. It is suggested that one possible interpretation of the findings is that the induction of doming by DMSO in Rama 25 cells occurs by means of sequential increases in Ca2+ and Na+ influxes into the cell, and that the increased intracellular concentration of Na+ so produced stimulates the Na+/K+ ATPase, with a net effect of pumping liquid beneath the cellular monolayer.

Receptor occupancy vs. induction of Na+-K+-ATPase and Na+ transport by aldosterone

In the urinary bladder of the toad Bufo marinus aldosterone (between 0.8 and 100 nM) stimulates Na+ transport [half-maximal induction concentration (K1/2) = 6.5 nM]. At low hormone concentrations (0.8-8 nM), the increase of Na+ transport between 0.75 and 2.5 h is accompanied by a fall in transepithelial resistance (R). Higher hormone concentrations (30-800 nM) induce an additional resistance-independent fraction of Na+ transport within 2.5-8 h. From 6 h on, aldosterone (between 0.2 and 20 nM) stimulates in the same tissue the biosynthesis rate of the alpha- and beta-subunits of Na+-K+-ATPase (K1/2 = 3 and 1.5 nM, respectively). New pump synthesis is thus not a prerequisite for the early mineralocorticoid response but might be linked to the late transport event. The mineralocorticoid response is usually ascribed to interaction with the higher affinity type 1 receptor. In the present study we show, however, that at least 55% of the overall Na+ transport response is linked to nuclear occupation of the lower affinity type 2 receptors [dissociation constant (Kd) = 50 nM, maximum number of binding sites (Nmax) = 315 fmol/mg protein]. Distinct aldosterone effects, such as the fall in R and the increase in Na+-K+-ATPase synthesis, are more closely related to occupation of type 1 receptors (Kd = 0.3 nM, Nmax = 23 fmol/mg protein). At maximal induction of these latter parameters, only about 20% of type 2 receptors are occupied. These results suggest that both types of aldosterone receptors are involved in the mediation of the full mineralocorticoid response: type 1 in the early and late and type 2 particularly in the late tissue response.

Regulation of renal Na-K-ATPase in the rat. Role of the natural mineralo- and glucocorticoid hormones

Both mineralo- and glucocorticoids stimulate renal Na-K-ATPase, but their relative role in the regulation of the enzyme remains controversial. In this study we measured Na-K-ATPase activity in the cortical collecting tubule (CCT) of adrenalectomized rats replaced with either the native mineralocorticoid (aldosterone) or glucocorticoid (corticosterone) in doses calculated to yield previously determined physiologic concentrations of these hormones (5 ng X dl-1 and 5 micrograms X dl-1, respectively). This was achieved by continuous delivery of aldosterone (1 microgram X 100 g-1 X d-1) from an osmotic minipump or of corticosterone (2 pellets of 20 mg each), implanted subcutaneously either at adrenalectomy or 7 d later, when Na-K-ATPase activity reached its nadir. Adrenalectomized rats not receiving hormone replacement and adrenal-intact animals served as controls. The CCT was chosen because it contains the highest concentration of binding sites for both hormones. Na-K-ATPase activity declined 52% in the CCT of untreated adrenalectomized rats after 7 d, and remained unchanged thereafter. Physiologic replacement doses of aldosterone prevented this decline and restored the activity of the enzyme after it had been allowed to decrease maximally following adrenal ablation, whereas similar replacement of corticosterone was without effect. These observations suggest that under physiologic conditions Na-K-ATPase in the CCT, a probable target nephron segment of both hormones, is under mineralocorticoid rather than glucocorticoid control.

Effect of corticosterone and triiodothyronine on (Na+ + K+)-adenosine triphosphatase activity in rat submandibular gland

The present study concerns the effect of glucocorticoids and triiodothyronine (T3) on rat salivary (Na+ + K+)-adenosine triphosphatase (NaK-ATPase). The results indicate that corticosterone increases submandibular gland NaK-ATPase activity with a single dose of hormone and with two daily doses on three successive days. Multiple injections of dexamethasone also increases submandibular and sublingual gland NaK-ATPase activity. The effect of glucocorticoids on salivary NaK-ATPase is selective since Mg-ATPase shows no significant differences under different steroidal states. We subsequently investigate the possibility of interaction between the effects of corticosterone and triiodothyronine on submandibular gland NaK-ATPase activity. Our studies indicate that each of these hormones increases NaK-ATPase activity in the absence of the other.

Short-term effects of aldosterone and dexamethasone on Na-K-ATPase along the rabbit nephron

Both glucocorticoids and mineralcorticoids stimulate the renal Na-K-ATPase. However, the exact site of their respective action is not precisely determined and it is still unknown whether these effects are cumulative or not. We studied the effects of dexamethasone and aldosterone on Na-K-ATPase activity in microdissected nephron segments from adrenalectomized rabbits. In proximal convoluted tubule (PCT) the enzyme activity was altered neither by adrenalectomy nor by any steroid replacement. In the medullary thick ascending limb of the loop of Henle (MAL) and the distal convoluted tubule (DCT), Na-K-ATPase activity decreased by 40% after adrenalectomy, and was restored to control level three hours after administration of dexamethasone (100 micrograms/kg) but not by aldosterone (up to 10 micrograms/kg). In the cortical (CCT) and medullary (MCT) collecting tubule the enzyme activity decreased by 75% after adrenalectomy but in contrast with the MAL and the DCT, these two segments were sensitive to both dexamethasone (100 micrograms/kg) and aldosterone (10 micrograms/kg) and recovered their activities within 3 h after the hormone injection. These effects were not additive. Spironolactone (100 micrograms/kg) abolished the action of each of the two hormones on the CCT and MCT. In contrast, spironolactone did not curtail the effect of dexamethasone on MAL and DCT. These results indicate that whereas glucocorticoid action is localized in MAL, DCT, CCT and MCT, the mineralocorticoid effect is restricted to the CCT and MCT exclusively. They also suggest that, in the CCT and MCT, the two types of hormones share the same receptors.

Mineralo- and glucocorticoid effects on renal excretion of electrolytes

The acute effects of mineralo- and glucocorticoids on urinary electrolyte excretion were studied in the conscious, acutely potassium deprived, adrenalectomized rat. Sodium, potassium, and creatinine were measured in the urine excreted from 2.5 to 5.5 h after injection of one or more of the following steroids: aldosterone (Aldo), 9-alpha fluorocortisol (FC), deoxycorticosterone (DOC), dexamethasone (Dex), and spironolactone (Spiro). The hierarchy (a) for increasing creatinine excretion was Dex greater than FC greater than Aldo greater than DOC greater than Spiro greater than none, a hierarchy consistent with glucocorticoid potency; and (b) for producing anti-natriuresis was Aldo greater than DOC greater than or equal to FC greater than or equal to none = Spiro greater than Dex, a hierarchy consistent with mineralocorticoid potency. In contrast, the kaliuresis produced by mineralo- and glucocorticoids appears different. A "mineralocorticoid" kaliuresis is 1) elicited by anti-natriuretic doses of Aldo and FC, 2) approximately twice control UKV, 3) unrelated to changes in glomerular filtration rate (GFR), and 4) inhibited by Spiro. A "glucocorticoid" kaliuresis is 1) elicited by Dex and high doses of Aldo and FC, 2) about seven to twenty-fold greater than control UKV, 3) possibly dependent, in part, on changes in GFR, and, 4) not inhibited by Spiro. DOC was not kaliuretic at anti-natriuretic doses. The urinary Na/K ratio was an unreliable index of mineralocorticoid action.