Polyuria of thyrotoxicosis: downregulation of aquaporin water channels and increased solute excretion.

Thyrotoxicosis is a common disorder causing cardiovascular and renal irregularities. In this study, thyrotoxicosis was produced in rats by 14 days of daily thyroxine injection. This was associated with an increase in cardiac index, mean arterial pressure, and renal blood flow compared with euthyroid controls. Food and water intake along with urine output were significantly increased in the thyrotoxic rats compared with control animals associated with a significant increase in solute excretion. Polyuria and increased solute excretion still occurred even when food and water intake was equivalent. These renal responses were associated with significant decreases in AQP1 and AQP2 water channel expression in both the ad lib and paired intake studies in the cortex and inner medulla. The downregulation of AQP2 protein occurred in spite of equivalent plasma arginine vasopressin (AVP) in the ad lib and increased AVP in the paired feeding studies. Solute-free water reabsorption was greater in both the ad lib and paired thyrotoxic than euthyroid rats and was associated with increased Na-K-2Cl cotransporter expression. We propose that the AVP-independent downregulation of AQP2, the observed increase in renal arterial pressure, and decrease in filtration fraction contribute to polyuria the increased solute excretion in spite of enhanced ion transporters in thyrotoxicosis.

Role of glucocorticoid hormones in arginine vasopressin gene regulation.

The mechanism underlying increased AVP synthesis and release in glucocorticoid deficiency is not known. Therefore, the present study was undertaken to investigate whether the mechanism was at the level of AVP gene transcription. The AVP gene promoter contains a consensus GRE, a CRE, and four AP2 sites. To assess the functional importance of these sites, 5' deletions of the AVP promoter were created and transient transfections were performed. Promoter activity in hypothalamic cells transfected with deletions lacking the GRE or both the GRE and CRE exhibited higher activity when compared to longer constructs containing both sites. In neuroblastoma cells, only the deletion lacking the GRE exhibited increased AVP promoter activity over the longer construct. These results are consistent with the idea that glucocorticoids suppress AVP gene expression by acting on a GRE in the AVP promoter region. Further, dexamethasone inhibited AVP promoter activity by >50% in hypothalamic cells transfected with the GRE-containing construct. In conclusion, the data presented here support a central mechanism to explain, at least in part, the nonosmotic increase in AVP with glucocorticoid deficiency.

Arginine vasopressin secretion with mutants of wild-type and Brattleboro rats AVP gene.

Defects in peptide processing are associated with several disorders, including central diabetes insipidus (CDI). In the Brattleboro (BB) rat with CDI, the mRNA and protein of arginine vasopressin (AVP) are present in the hypothalamus, but no circulating AVP is detectable, thus suggesting a processing defect. The present study examined AVP secretion in cultured COS cells transfected with various constructs from wild-type and mutated Brattleboro AVP gene precursors. The precursor contains three exons encoding for vasopressin (VP), neurophysin (NP), and glycopeptide (GP). The Brattleboro rat has a deletion of a single base, guanine (G), in the NP coding region that leads to a frameshift, resulting in the loss of normal stop codon. The wild-type pcVP (22.0 +/- 5.2 pg/10[-2] U beta-galactosidase [beta-gal]), but not the mutated BB AVP gene pcBB (1.2 +/- 0.4 pg/10[-2] U beta-gal), was associated with AVP secretion from the COS cells as measured by RIA. The wild-type AVP gene without the GP coding region was associated with AVP release greater (47.4 +/- 13.5 pg/10[-2] U beta-gal, n = 5, P < 0.05, versus pcVP) than the pcVP with intact VP, NP, and GP coding regions. However, the wild-type AVP gene with VP coding region alone was not processed and secreted. Normalizing the pcBB total length with the insertion of a stop codon at the site of the normal stop codon was not associated with AVP secretion (3.0 +/- 1.4 pg/10[-2] U beta-gal). However, insertion of a stop codon so that the pcBB length equaled the length of VP and NP coding regions of the wild type was associated with AVP secretion (13.5 +/- 4.0 pg/10[-2] U beta-gal). When a stop codon was inserted into the wild-type NP coding region at the same site as the G deletion in the pcBB, the AVP secretion was significantly lower (15.1 +/- 5.0 pg/10[-2] U beta-gal) than pcVP with VP + NP but no GP coding regions (47.4 +/- 13.5 pg/10[-2] U beta-gal, n = 5, P < 0.05). In summary, (1) both VP and intact NP, but not GP, coding regions are necessary for AVP processing and secretion; (2) decreasing the length of the NP coding region diminishes but does not abolish AVP processing and secretion; and (3) shortening of the pcBB length with a stop codon at a site comparable to wild-type VP + NP allows AVP secretion, albeit less than with wild-type gene precursor. Thus, the CDI in BB rats is caused by the G deletion in NP coding region. This defect leads to abnormalities that contribute to the abnormal AVP processing. Specifically, the frameshift and absence of a stop codon cause a mutated extended C terminus, which, along with the mutated NP, contribute to the abnormal steps of AVP processing, transport, and secretion in the BB rat. These defects no doubt impair the folding and configuration necessary for normal processing of the AVP gene precursor.

Effect of kappa opioid agonist RU 51599 on osmotic and non-osmotic stimulated arginine vasopressin release and gene regulation in small cell lung carcinoma cells.

Arginine vasopressin (AVP) is synthesized in the hypothalamus, stored in the posterior pituitary, and osmotic and non-osmotic stimuli release AVP into the circulation for antidiuretic and vascular actions on target tissue. The kappa-opioid agonist, RU 51599, exhibits a potent diuretic activity in both experimental animals and humans. This diuretic activity is characterized by a water diuresis without an associated increase in electrolyte excretion. Studies with cultured rat hypothalamo-neurohypophysial system explant showed that AVP mRNA level changed in parallel to the RU 51599-induced changes in AVP secretory rate. There are, however, no hypothalamic neuronal cell lines to study AVP gene regulation system, and it is not known whether RU 51599, regulates AVP secretion and biosynthesis under osmotic and non-osmotic stimulatory conditions of AVP release. The effect of RU 51599 on AVP release, AVP mRNA, and AVP gene promoter activity in osmotic and non-osmotic conditions was therefore studied using cultured small cell lung carcinoma (SCLC) cell lines. RU 51599 significantly inhibited AVP release by osmotic stimulation (330 mOsm) and non-osmotic stimulators, angiotensin II (AII) and endothelin 3 (ET3). However, RU 51599 did not show any effect on the AVP mRNA and AVP gene promoter activity stimulated by high osmolality and ET3. These results indicate, therefore, that RU 51599 suppresses AVP secretion by inhibition at the step of AVP release during osmotic and non-osmotic stimulation but does not affect the AVP gene transcription level in the SCLC cells.

Upregulation of endothelial and neuronal constitutive nitric oxide synthase in pregnant rats.

Pregnancy is characterized by hemodynamic and body fluid alterations. Increased nitric oxide (NO) production has been suggested to play a role in the hemodynamic alterations of pregnancy and has also been reported to increase arginine vasopressin (AVP) release. We therefore hypothesized that gestation could increase both NO synthase (NOS) constitutive isoforms, neuronal NOS and endothelial NOS, and thereby contribute to the hyposmolality and peripheral arterial vasodilation of pregnancy, respectively. The present study was therefore undertaken to examine the constitutive NOS isoforms in aortas, mesenteric arteries, and hypothalami of pregnant rats on day 20 of gestation compared with age-matched nonpregnant rats. Plasma AVP was determined by radioimmunoassay and hypothalamic mRNA AVP by solution hybridization assay. Hypothalamic neuronal NOS was assessed by Northern blot and Western blot; endothelial NOS was assessed by Western blot in arteries and hypothalamus. The results demonstrated that 1) plasma AVP and hypothalamic AVP mRNA are increased in pregnant rats (n = 8), 2) neuronal NOS protein and mRNA are increased in hypothalamus of pregnant rats (n = 4), and 3) endothelial NOS expression, as assessed by Western blot analysis, is increased in both conductance (aorta) as well as resistance (mesenteric) arteries of pregnant rats (n = 4). We conclude that both of the constitutive NOS isoforms are increased in pregnant rats, suggesting that the peripheral arterial vasodilation and hyposmolality of pregnancy could be mediated by these isoforms.

Osmotic and non-osmotic regulation of arginine vasopressin (AVP) release, mRNA, and promoter activity in small cell lung carcinoma (SCLC) cells.

Arginine vasopression (AVP) is synthesized in the magnocellular neurons of the hypothalamus and stored in the posterior pituitary. It has been shown that hypothalamic AVP mRNA is increased during experimental stimulation of osmotic and non-osmotic stimulation of AVP release. The mechanisms underlying the stimulation of AVP biosynthesis in these conditions are not known. The present study was, therefore, performed to measure AVP release, AVP mRNA level, and AVP gene promoter activity during osmotic and non-osmotic stimulation of AVP secretion in the small cell lung carcinoma (SCLC) cells. AVP release was measured by radioimmunoassay, steady state levels of AVP mRNA by solution hybridization, and AVP gene promoter activity exhibited by a 1.5 kb 5'-flanking AVP gene fragment fused to a luciferase reporter after SCLC cells were subjected to osmotic or non-osmotic conditions. High media osmolality (330 mOsm) significantly increased AVP release (control (C) 1.42 +/- 0.27 vs. High Osm 3.67 +/- 0.39 pg/2 x 10(6) cells, N = 9, P < 0.002); AVP mRNA (C 173.6 +/- 16.8 vs. High Osm 280.1 +/- 19.4 pg/2 x 10(6) cells, N = 7, P < 0.001); and AVP gene promoter activity (C 1353 +/- 99 vs. High Osm 2026 +/- 134 L.U./10(-4) U beta-gal, N = 8, P < 0.001). Non-osmotic stimulators. 0.1 microM endothelin 3 (ET3), 1 microM angiotensin II (AII), and 10 microM acetylcholine (Ach) significantly increased AVP release; ET3 (C 1.78 +/- 0.20 vs. ET3 6.85 +/- 1.86 pg/2 x 10(6) cells, N = 8, P < 0.02); AII (C 1.29 +/- 0.38 vs. AII 27.80 +/- 7.09 pg/2 x 10(6) cells, N = 5, P < 0.05) and Ach (C 1.14 +/- 0.33 vs. Ach 2.68 +/- 0.58 pg/2 x x10(6) cells, N = 6, P < 0.05). However, only ET3 significantly increased AVP mRNA (C 166.6 +/- 19.6 vs. ET3 254.4 +/- 25.6 pg/p x 10(6) cells, N = 5, P < 0.05) and AVP promoter activity (C 1515 +/- 163 vs. ET3 2389 +/- 342 L.U./10(-4) U beta-gal, N = 6, P < 0.05). To localize the region of the AVP promoter that mediates the osmotic stimulation and the effect of ET3, 5' deletions of the AVP promoter fragments terminating at -532, -211, and -102, was assessed. Only the promoter activity of the 1.5 kb construct, but not the deletion constructs, was significantly increased by ET3 or high osmolality. These results suggest that modulation of AVP gene transcription is, at least in part, responsible for increased AVP synthesis and release in response to osmotic and non-osmotic stimulation, and that the region of 5' flanking sequence between -1500 and -532 contains the elements responsible for the effects.

Arginine vasopressin gene expression in rats with puromycin-induced nephrotic syndrome.

Nephrotic syndrome is characterized by water and sodium retention, which leads to edema formation. The nonosmotic stimulation of arginine vasopressin (AVP) release from the pituitary gland has been implicated to be one of the important factors of abnormal water retention in patients with nephrotic syndrome. It is not known, however, whether nephrotic syndrome is associated with stimulation of hypothalamic vasopressin gene expression. Puromycin aminonucleoside is known to cause altered glomerular permeability, which results in experimental nephrotic syndrome in rats. In the present study, therefore, AVP gene expression has been studied in the hypothalamus of rats with puromycin aminonucleoside-induced nephrotic syndrome (PNS). Nephrotic syndrome was induced by a single intravenous injection of puromycin aminonucleoside (50 mg/kg body weight). Nephrotic syndrome was confirmed by urinary protein excretion (control 20.8 +/- 3.5 mg/24 hr v PNS 273.9 +/- 41.4 mg/24 hr; P < 0.0001, n = 6) and serum albumin concentrations (control 4.52 +/- 0.07 g/dL v PNS 2.96 +/- 0.22 g/dL; P < 0.001, n = 6). In PNS rats, plasma AVP was significantly higher than in control rats (control 0.77 +/- 0.10 pg/mL v PNS 2.13 +/- 0.42 pg/mL; P < 0.005, n = 12), even though there were no differences in plasma osmolality (control 292.0 +/- 2.0 mOsm/kg H2O v PNS 290.3 +/- 2.5 mOsm/kg H2O; P = NS, n = 12) or serum sodium concentration (control 142.7 +/- 0.7 v PNS 142.1 +/- 1.1; PNS, n = 12).(ABSTRACT TRUNCATED AT 250 WORDS)

Arginine vasopressin receptor internalization and recycling in rat renal collecting tubules.

Arginine vasopressin (AVP) binds to two distinct receptors to initiate vasopressor (V1 receptor) and hydroosmotic actions (V2 receptor). Internalization and recycling of the V1 receptor in cultured vascular smooth muscle cells and hepatocytes have recently been demonstrated. However, the receptor cycle of the AVP V2 receptor in the renal collecting tubules has not yet been well defined. Therefore, the present study was undertaken to investigate the AVP V2 receptor cycle, including AVP binding to the surface receptor, internalization and potential recycling in isolated outer medullary collecting tubules. The maximal AVP surface binding was reached in 10 min, and 25 micrograms/ml trypsin completely inhibited the surface binding. A Scatchard plot of 125I-AVP surface binding indicated a single population of V2 receptors with a Kd of 1.92 x 10(-9) M and a Bmax of 1.77 x 10(-11) M or 590 fmoles/mg protein. 81.7% (72-85%) of specific bound receptor was internalized (specific surface binding: 742.8 +/- 111.1 vs internalized binding: 607.3 +/- 27.8 fmoles bound/mg protein). More than 90% of surface bound receptor was recycled to the cell surface after internalization (control surface binding: 584.0 +/- 64.0 vs recycled surface binding: 546.6 +/- 32.0 fmoles bound/mg protein). Cycloheximide (40 micrograms/ml) did not inhibit the receptor recycling (control recycled surface binding: 546.6 +/- 32.0 vs cycloheximide recycled surface binding: 505.0 +/- 54.8 fmoles bound/mg protein), thus suggesting that the receptors were not resynthesized after dissociation from the receptor-ligand complex. These studies therefore demonstrate that the AVP V2 receptor is internalized and recycled in the rat renal collecting tubule.

Vasopressin gene expression in rats with experimental cirrhosis.

Impaired ability to excrete a water load occurs in a substantial number of patients with advanced cirrhosis and in animals with experimental cirrhosis. The nonosmotic stimulation of arginine vasopressin release from the pituitary has been implicated as an important factor in the abnormal water excretion in patients and animals with cirrhosis. In this study, arginine vasopressin hypothalamic gene expression was studied in cirrhotic rats. Cirrhosis was induced by a combination of phenobarbital treatment in drinking water and weekly intragastric administration of carbon tetrachloride for 13 to 15 wk. Severe cirrhosis was confirmed by morphological analysis and the presence of ascites. Plasma arginine vasopressin was also significantly higher in rats with cirrhosis (control = 1.77 +/- 0.16 and cirrhotic rats = 4.14 +/- 0.62 pg/ml, n = 9, p < 0.002). Hypothalamic arginine vasopressin messenger RNA was also significantly higher in cirrhotic rats (control = 762.1 +/- 132.3 and cirrhotic rats = 1,834.2 +/- 271.9 pg/hypothalamus, n = 9, p < 0.005). Pituitary arginine vasopressin content was significantly lowered in cirrhotic rats (control = 3.69 +/- 0.98 and cirrhotic rats = 1.57 +/- 0.09 micrograms/pituitary, n = 9, p < 0.05). No difference was seen in hypothalamic arginine vasopressin content between the two groups (control = 4.64 +/- 0.34 and cirrhotic rats = 4.23 +/- 0.33 ng/hypothalamus, n = 9, NS). Oxytocin messenger RNA in the hypothalamus was also not significantly different between the two groups (control = 8.61 +/- 0.68 and cirrhotic rats = 9.33 +/- 0.65 unit of density, n = 9, NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Enzymatic and binding effects of atrial natriuretic factor in glomeruli and nephrons.

Atrial natriuretic factor (ANF) has been suggested to exert a tubular effect on the mammalian nephron, perhaps in part by interacting with other hormones. In the present study, the effect of ANF was examined on glomeruli (Gm) and different renal tubule segments including medullary (MAL) and cortical thick ascending limb (CAL) and cortical (CCT), outer medullary (OMCT) and inner medullary collecting tubules (IMCT). This effect of ANF was assessed by alteration in adenylate cyclase and cGMP in the various nephron segments in the presence and absence of arginine vasopressin (AVP), parathyroid hormone (PTH) and calcitonin (SCT). An effect of ANF (10(-8) M) was not demonstrated on adenylate cyclase (fmol cAMP formed/30 min/micrograms protein) in Gm, CAL, MAL, CCT, OMCT or IMCT. Nor did ANF (10(-8) M) interfere with the effect of PTH (5 IU/ml) on the Gm (PTH 35.1 +/- 3.7 vs. PTH + ANF 32.5 +/- 1.8, NS), CAL (PTH 50.5 +/- 10.9 vs. PTH + ANF 46.2 +/- 1.4, NS) or AVP (10(-8) M) on the CCT (AVP 40.8 +/- 6.6 vs. AVP + ANF 33.0 +/- 3.1, NS), OMCT (AVP 56.0 +/- 11.8 vs. AVP + ANF 42.1 +/- 6.7, NS), IMCT (AVP 66.5 +/- 4.6 vs. AVP + ANF 53.5 +/- 7.0, NS) or MAL (AVP 15.5 +/- 1.6 vs. AVP + ANF 14.0 +/- 2.6, NS). ANF also did not affect SCT (1.5 x 10(-8) M)-induced adenylate cyclase on CCT (SCT 69.8 +/- 11.3 vs. SCT + ANF 79.9 +/- 7.2, NS). ANF (10(-8) M), however, significantly increased cGMP in the Gm (6.4 +/- 1.7 to 121.3 +/- 32.4 fmol/micrograms protein, P less than 0.001) and IMCT (0.63 +/- 0.16 to 1.46 +/- 0.29 fmol/micrograms protein, P less than 0.05). However, no effect of ANF on cGMP was observed in the CAL, CCT, OMCT, and MAL even at 10(-7) M ANF. PTH (5 IU/ml) did not alter either basal or ANF-stimulated cGMP in the Gm. Also, specific ANF binding was studied in the microdissected IMCT. Kd was 6.08 x 10(-9) M and Bmax was 8.07 x 10(-11) M.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular action of arginine vasopressin in the isolated renal tubules of hypothyroid rats.

Hypothyroidism has been demonstrated to be associated with an impaired concentrating capacity and specific morphological changes in the thick ascending limbs. This study was performed to evaluate the cellular action of arginine vasopressin (AVP) in the isolated renal tubules from control (C) and hypothyroid (HT) rats. Hypothyroidism was induced by feeding aminotriazole for 4 wk. Urinary volume was higher in HT rats (C 13.5 +/- 0.9, HT 17.7 +/- 0.9 ml/24 h, P less than 0.005) and urinary osmolality was lower in HT rats (C 1,707 +/- 49, HT 1,229 +/- 35 mosmol/kgH2O, P less than 0.001). Plasma AVP levels were significantly higher in HT rats (C 1.93 +/- 0.59, HT 4.12 +2- 0.62 pg/ml, P less than 0.05), thus documenting AVP resistance. The adenylate cyclase response to AVP (10(-6) M) was significantly lower (P less than 0.02) in the medullary thick ascending limb of Henle's loop (mTALH) in HT (14.3 +/- 2.4 to 41.7 +/- 5.8 fm X 30 min-1 X mm-1, P less than 0.001) than in mTALH in C rats (14.4 +/- 2.8 to 110.1 +/- 24.9 fm X 30 min-1 X mm-1, P less than 0.001). In contrast, the adenylate cyclase response to AVP was not significantly different in collecting tubules of cortex, outer medulla, and inner medulla from C and HT rats, although a slight decrease in response to AVP was observed in cortical and outer medullary collecting tubules.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of arginine vasopressin in medullary thick ascending limb on maximal urinary concentration.

Two groups of Sprague-Dawley rats, Harlan (H) and Charles River (CR), were discovered in that the medullary thick ascending limb (MAL) had a profoundly different adenylate cyclase response to arginine vasopressin (AVP). Using these two groups of rats, we studied the correlation between AVP action on the MAL and maximal urinary concentration. AVP (10(-6) M) significantly stimulated adenylate cyclase in MAL of H rats (7.4 +/- 0.9 to 43.8 +/- 4.6 fmol cAMP formed X 30 min-1 X mm-1, P less than 0.001) but not in CR rats (10.3 +/- 1.4 to 12.7 +/- 2.0 fmol cAMP formed X 30 min-1 X mm-1, NS). In contrast, AVP significantly stimulated adenylate cyclase of cortical, outer and inner medullary collecting tubules from both H and CR rats. Glucagon (10(-6) M) significantly stimulated adenylate cyclase of MAL from both H and CR rats. After 48 h of fluid deprivation, urinary osmolality was significantly higher (P less than 0.001) in the H (4,504 +/- 399 mosmol/kg H2O, n = 14) than CR (2,840 +/- 176 mosmol/kg H2O, n = rats. This observation was not attributable to differences in creatinine clearance (CR, 1.30 +/- 0.24; H, 1.24 +/- 0.03 ml/min, NS, n = 4) or plasma AVP (CR, 12.75 +/- 1.44; H, 12.38 +/- 1.17 pg/ml, NS, n = 6) levels. These results therefore suggest that the action of AVP on the MAL, in addition to the effect on collecting tubules, is involved in maximal urinary concentration in rats.

Effects of vasopressin antagonist on vasopressin binding, adenylate cyclase activation, and water flux.

We studied the effect of an arginine vasopressin (AVP) analogue, (1-[beta-mercapto-beta, beta-cyclopentamethylenepropionic acid],2-O-ethyltyrosine, 4-valine)AVP(d[CH2]5Tyr[Et]VAVP), on the stimulation of adenylate cyclase by various hormones in the isolated nephron segments and 3H-AVP binding to renal papillary membranes from the rat. The net water flux across the renal cortical collecting tubules of the rabbit was also examined. We found that d(CH2)5Tyr(Et)VAVP significantly inhibited adenylate cyclase activation by AVP in cortical, medullary, and papillary collecting tubules and in the medullary thick ascending limb. In contrast, the AVP analogue did not alter the stimulation of adenylate cyclase by parathyroid hormone in the cortical thick ascending limb, by glucagon in the medullary thick ascending limb, and by calcitonin in cortical collecting tubules. In addition, d(CH2)5Tyr(Et)VAVP blocked [3H]AVP binding to renal papillary membranes. The enhanced net water transport induced by AVP in isolated, perfused rabbit cortical collecting tubules also was completely blocked by this AVP analogue. These results indicate that d(CH2)5Tyr(Et)VAVP specifically antagonizes the cellular action of AVP on the medullary thick ascending limb and on the cortical, medullary, and papillary collecting tubules. Evidence is also presented for competitive antagonism as the cellular mechanism of action.

The cyclic AMP system in the inner medullary collecting duct of the potassium-depleted rat.

The present study was undertaken to investigate the cyclic AMP system in the isolated inner medullary collecting tubule (IMCT) of hypokalemic (HK) rats. In situ incubation of IMCT with 10(-7) M arginine vasopressin (AVP) at 300 mOsm/kg H2O in control normokalemic rats increased cyclic AMP content (fmoles/mm) from 5.68 +/- 1.41 to 30.3 +/- 5.31 (P less than 0.001). In HK rats the increase in cyclic AMP was blunted from 7.18 +/- 2.0 to 14.78 +/- 3.14 fmoles/mm (P less than 0.05 compared to controls). No such blunting was observed in the outer medullary collecting duct of hypokalemic rats, but was seen in the IMCT when studied at 800 (P less than 0.05), 1200 (P less than 0.01), and 2000 mOsm/kg H2O (P less than 0.05). The increase in cyclic AMP was also blunted in IMCT of HK rats not allowed to become polyuric or polydipsic by pair-watering studied at 300, 800, and 1200 mOsm/kg H2O. To define the process responsible for the failure to normally increase cyclic AMP in HK, adenylate cyclase activity (AC) was determined at 800 mOsm/kg H2O. While basal AC was not different, the response to all concentrations of AVP between 10(-10) and 10(-6) M was markedly depressed in tubules from HK rats. In contrast AC response to 10(-2) M NaF was not different in IMCT of normokalemic and HK rats. While the abnormal cyclic AMP content with AVP could be explained by abnormal generation, a contribution of increased metabolism was also sought.(ABSTRACT TRUNCATED AT 250 WORDS)