The handling of immunoreactive vasopressin by the isolated perfused rat kidney

Effect of chronic central endothelin-1 on hemodynamics and plasma vasopressin in conscious rats

OBJECTIVES

These studies were designed to test whether chronic central administration of endothelin-1 induces changes in systemic hemodynamics and plasma vasopressin similar to those observed with acute microinjections of endothelin-1.

METHODS

Sprague Dawley rats underwent sham denervation or sinoaortic denervation. Three days later, baseline mean arterial blood pressure, heart rate, and vasopressin were assessed in conscious rats. Then, a cannula was stereotaxically inserted into the lateral ventricle and attached to an osmotic minipump that delivered one of the following: (i) artificial cerebrospinal fluid; (ii) endothelin-1, 10 pmol/hour; (iii) BQ-123, 400 pmol/hour; or (iv) endothelin-1+BQ-123. Mean arterial blood pressure and heart rate were monitored daily and blood was obtained for plasma vasopressin on days 3 and 9. On day 10, the rats were euthanized, the hypothalami were removed, and vasopressin messenger ribonucleic acid content was assessed.

RESULTS

The pressor effect of intracerebroventricular endothelin-1 was similar in intact and sinoaortic denervation rats and was prevented by endothelin receptor A antagonism with BQ-123. Administration of BQ-123 alone resulted in a depressor and bradycardia in sinoaortically denervated rats. Chronic endothelin-1 administration did not change plasma vasopressin but resulted in a significant decrease in hypothalamic vasopressin messenger ribonucleic acid levels, which was reversed by endothelin receptor A inhibition.

DISCUSSION

Although the pressor effect of chronic central endothelin-1 is similar to that reported with acute endothelin-1, plasma vasopressin levels do not increase, at least in part, due to downregulation of hypothalamic vasopressin gene expression. Sinoaortic denervation increases endogenous central endothelin receptor A tone. Furthermore, these observations confirm that the pressor effect of central endothelin-1 is not mediated by plasma vasopressin.

Biological half-life and organ distribution of [3H]8-arginine vasopressin following administration of vasopressin receptor antagonist OPC-31260

The effects of the antidiuretic (V(2)) non-peptide receptor antagonist OPC-31260 on the plasma vasopressin level and the biological half-life and organ distribution of radiochemically pure, biologically active [(3)H]8-arginine vasopressin [spec. act.: 15.9 mCi/mmol (588 GBq/mmol)] were studied in Wistar rats. The plasma vasopressin level increased significantly throughout the whole experimental period (24 h). There was no change in the fast phase of the curves of total radioactivity disappearance from the plasma after the administration of [(3)H]arginine vasopressin (control: 1.51+/-0.17 min, OPC-31260-treated: 1.42+/-0.12 min, n=10). The fast phase of the disappearance curves of intact [(3)H]arginine vasopressin did not change either following the administration of OPC-31260 in a dose of 30 mg/kg p.o. (control: 1.06+/-0.19 min, OPC-31260-treated: 1.00+/-0.15 min, n=6). The slow phase of the biological half-life, which is characteristic for the examined compound, proved to be significantly longer (total radioactivity control: 9.29+/-0.61 min, OPC-31260-treated: 12.33+/-0.42 min, P<0.05, n=10; [(3)H]arginine vasopressin radioactivity: control: 5.96+/-0.58 min, OPC-31260-treated: 8.90+/-0.37 min, P<0.05, n=6). In the control rats, the radioactivity was accumulated to the greatest extent in the neurohypophysis, adenohypophysis and kidney. Following OPC-31260 administration, significantly more radioactive compounds accumulated in the kidney (control: 0.30+/-0.052 total radioactivity %/100 mg organ weight, OPC-31260-treated: 0.50+/-0.133 total radioactivity %/100 mg organ weight, P<0.05, n=10) and neurohypophysis (control: 0.37+/-0.053 total radioactivity %/100 mg organ weight, OPC-31260-treated: 0.52+/-0.076 total radioactivity %/100 mg organ weight, P<0.05, n=10). Our results permit the conclusion that the antidiuretic antagonist OPC-31260 not only blocks the V(2) receptors, but also increases the biological half-life of vasopressin. The longer biological half-life of vasopressin following OPC-31260 administration may play a role in the elevation of the plasma vasopressin level.

Luminal arginine vasopressin stimulates Na(+)-H+ exchange and H(+)-ATPase in cortical distal tubule via V1 receptor

Bicarbonate reabsorption was evaluated by stationary microperfusion of in vivo early distal (ED) and late distal (LD) segments of rat kidney. Intratubular pH was recorded by double-barreled H ion-exchange resin/reference (1 M KCl) microelectrodes for the determination of HCO3- reabsorption. In the presence of luminal arginine vasopressin (AVP, 10(-9) M), a significant increase in HCO3- reabsorption was observed both in ED (from 0.931 +/- 0.061 to 2.12 +/- 0.171 nmol.cm-2.s-1] and LD segments [from 0.542 +/- 0.086 to 1.67 +/- 0.111 nmol.cm-2.s-1]. The addition of the V1-receptor antagonist [(d (CH2)5, Tyr (Et)2) arginine vasopressin] (10(-5) M) to luminal perfusion blocked luminal AVP mediated stimulation in ED and LD segments. 5-(N, N-hexamethylene) amiloride (10(-4) M) added to luminal perfusion inhibited luminal AVP-mediated stimulation in ED (by 63.7%) and LD (by 34.1%) segments. The addition of Bafilomycin A1 (2 x 10(-7) M) to the luminal perfusion did not affect luminal AVP-mediated stimulation in ED segments, but reduced it (by 31.7%) in LD segments. Our results indicate that luminal AVP acts to stimulate the Na(+)-H+ exchange in ED and LD segments via activation of V1 receptors, as well as the vacuolar H(+)-ATPase in LD segments.

Arginine vasopressin disposition in the conscious rabbit

To characterize the urinary kinetics of AVP, and the influence of regional blood flow on the metabolic degradation of AVP, multiple doses of AVP were administered to conscious rabbits. AVP systemic clearance (ClT) was not influenced by changes in dose, in spite of a decrease in AVP urinary clearance following the highest dose. Hepatic blood flow was inversely associated with AVP concentrations, and despite a decrease in hepatic plasma flow of 37% (p < 0.05), following the high dose of AVP, ClT remained unchanged. These results indicate that AVP plasma kinetics are first order and plasma flow independent, and urinary kinetics are zero order.

Furosemide dynamics in conscious rabbits: modulation by arginine vasopressin

The aims of this study were to assess the influence of arginine-vasopressin (AVP) on the pharmacodynamics and kinetics of furosemide. To this purpose, the response and the kinetics of furosemide (5 mg/kg i.v.) were studied in two groups of rabbits, one control and one receiving an infusion of AVP (2.5 ng/kg/min). The infusion of AVP generated mean plasma levels of 35 pg/ml, and in these rabbits osmolal clearance was increased, free water clearance was reduced, and renal plasma flow was reduced by 25% (p < 0.05). High AVP plasma levels increased the natriuresis (p < 0.01) and the urinary excretion of prostaglandin E2 (UPgE2V; p < 0.01). The increase in UPgE2V was associated with AVP plasma concentrations (r = 0.8248; p < 0.001). AVP reduced the increment in natriuresis and diuresis elicited by furosemide from 163 +/- 20 to 87 +/- 20 mumol/min (p < 0.05) and from 1.22 +/- 0.11 to 0.83 +/- 0.13 ml/min (p < 0.05). The infusion of AVP enhanced furosemide metabolic clearance but diminished its renal clearance, resulting in a decrease in the rate of furosemide urinary secretion. It was concluded that high plasma levels of AVP reduce furosemide natriuresis, presumably because of a decrease in furosemide urinary secretion.

Metabolism of neurohypophysial hormones

Over the past decade several new routes of neurohypophysial hormone metabolism have been identified. These include nonhepatic splanchnic clearance and renal clearance in addition to filtration that appears to be receptor mediated. The intraluminal degradation of VP in the proximal tubule, and distal tubular secretion, at least in one species, has been identified. The brain has been identified as a site for VP and OT metabolism, and the amniotic sac may be a major site for VP clearance in the guinea pig fetus. There have been generalized findings regarding VP and OT metabolism. First, VP metabolism in the whole body and in the amniotic sac appears to increase with increasing concentrations of hormone; this does not appear to be the case with OT. Also, evidence has been presented that suggests a potential for the formation of biologically active metabolites. There have been several associations of pathophysiological states with altered VP or OT metabolism, sometimes with plasma levels remaining unchanged. Lastly, caution is emphasized when measuring these hormones by RIA, and differences in specificities of antisera toward hormone metabolites must be considered.

Hereditary vasopressin resistance in man and mouse

The DI +/+ Severe hereditary nephrogenic diabetes insipidus mouse is resistant to the antidiuretic action of vasopressin (VP) because of failure to accumulate cAMP and subsequent inability to form intramembranous particles on the apical (luminal) surface of kidney cells that normally respond to VP. The defect is primarily, if not exclusively, due to excessive activity of specific cAMP-phosphodiesterases. The abnormality can be overcome in vitro and in vivo by the phosphodiesterase inhibitor, rolipram. Most cases of hereditary NDI in man have sex-linked recessive inheritance, which appears to be due to an abnormality of the V2 receptor. The chromosomal locus of the defect is Xq28. Sporadic cases of congenital NDI have been described in females who appear to have a defect beyond the V2 receptor and the guanine nucleotide-binding stimulatory protein. There is no information on the biochemical defect in very rare cases with other types of inheritance patterns. No abnormalities of V1a and V1b receptor function have been found in patients with NDI. Mice and patients with NDI have evidence of increased AVP synthesis. AVP release in relation to plasma osmolality is increased in patients during infusion of hypertonic saline. This is the opposite of what has been described in patients with primary polydipsia (dipsogenic diabetes insipidus) who are chronically overhydrated. Together, these studies indicate that chronic dehydration and overhydration can cause up- and downregulation of the osmotic release of AVP.

Abnormal hemodynamics and elevated angiotensin II plasma levels in polydipsic patients on regular hemodialysis treatment

To investigate the cause and the mechanisms responsible of the compulsive thirst and excessive fluid intake observed in many patients on chronic dialysis treatment, we measured plasma antidiuretic hormone (ADH), angiotensin II (Ang II) and some hemodynamic parameters in seven polydipsic and in six normodipsic patients before hemodialysis, at the end of it and several times during the interdialytic interval. Before dialysis we found that ADH was elevated in both groups (6.9 +/- 1.9 vs. 6.9 +/- 1.3 pg/ml, respectively in polydipsics and controls), whereas Ang II was abnormally high only in polydipsics (51 +/- 12 vs. 11 +/- 3 pg/ml, P < 0.01); these patients also had significantly higher heart rate and cardiac indices and lower total peripheral resistances than control patients. Overall these hemodynamic indices were related with Ang II but not with ADH. Ang II rose markedly in polydipsics after dialysis, reaching a peak at the fourth hour after its termination (136 +/- 12 pg/ml) and remained consistently elevated throughout the interdialytic period, whereas in controls Ang II was practically unchanged with respect to baseline. In contrast, ADH had minor and similar modifications in both groups, in whom also the hemodynamic changes were superimposable. Significant correlations were found between the absolute and percent changes of Ang II and those of plasma volume during the interdialytic interval (P < 0.001 for both), and between the individual values of Ang II measured during the whole study and the interdialytic weight gain (P < 0.05). These results demonstrate that polydipsic patients have abnormally high levels of Ang II before and after the hemodialysis-induced volume depletion.(ABSTRACT TRUNCATED AT 250 WORDS)

The intrarenal site of calcitonin gene-related peptide degradation in the isolated perfused rat kidney

1. Calcitonin gene-related peptide (CGRP) is a potent vaso-active 37 amino acid peptide, typically elevated in plasma from patients with medullary thyroid cancer (MTC), but undetectable in the plasma of normal subjects. 2. The kidney is a major site for the clearance of exogenously infused CGRP but the intrarenal site of this clearance is unknown. Extra-organ clearance is also significant for CGRP, and whereas the site and mechanism of this degradation remain uncertain, the vasculature has been postulated as the most likely site. 3. The isolated perfused rat kidney (IPRK) was studied to (i) localize the intrarenal site of CGRP clearance and (ii) determine the contribution of the renal vasculature to the clearance of CGRP. The half-life of CGRP in the filtering IPRK was 63.9 +/- 4.5 min, whereas blocking of filtration by elevation of the perfusate osmolarity abolished the degradation. This suggests that (i) renal CGRP degradation occurs after glomerular filtration with intratubular metabolism and (ii) that there is no active CGRP degradation in the (glomerular) capillary endothelium. 4. These results do not support the theory that renal vascular endothelium plays a major active role in CGRP degradation.

Metabolic clearance rate of arginine vasopressin in patients with cirrhosis

Metabolic clearance rate and half-time of arginine vasopressin were measured in 43 cirrhotic patients and 10 control subjects. Synthetic arginine vasopressin was infused intravenously at a rate of 500 pg/min/kg of body weight for 75 min. The metabolic clearance rate was significantly reduced, and the half-time of arginine vasopressin after stopping the infusion was significantly increased in patients with cirrhosis, particularly in those with ascites and in those with moderate or severe liver dysfunction. Changes in metabolic clearance rate and half-time of arginine vasopressin correlated with the score of the liver dysfunction, prothrombin activity and levels of serum albumin and bilirubin but not with parameters of kidney function (serum creatinine levels and clearance of creatinine). We conclude that reduced metabolic clearance rate and prolonged half-time of vasopressin in plasma are frequent findings in cirrhotic patients with poor liver function. This impaired catabolism of antidiuretic hormone may contribute to maintaining elevated plasma levels of this hormone in these patients and may be an additional factor leading to fluid retention and to dilutional hyponatremia.

Luminal vasopressin modulates transport in the rabbit cortical collecting duct

We explored the action of luminal AVP in rabbit CCD perfused in vitro at 37 degrees C. Nanomolar concentrations of luminal AVP induced a sustained hyperpolarization of transepithelial voltage (Vt) in contrast to a transient hyperpolarization caused by basolateral AVP. 10 microM basolateral ouabain abolished the latter but not the former change in Vt. Despite a sustained hyperpolarization (from -20.7 +/- 2.9 to -34.1 +/- 4.7 mV; P less than 0.01), 10 nM luminal AVP only slightly altered net Na+ and K+ fluxes (7.6% stimulation and no significant change, respectively). Instead, luminal AVP appeared to modulate an acetazolamide-sensitive electrogenic ion transport because 200 microM basolateral acetazolamide suppressed the luminal AVP-induced hyperpolarization (percentage of Vt from -50.4 +/- 10.8 to -5.1 +/- 1.4; P less than 0.005). In terms of water transport, 10 nM luminal AVP did not change hydraulic conductivity (Lp, x 10(-7) cm/atm per s) (from 3.9 +/- 0.8 to 5.0 +/- 1.2), but suppressed the increase in Lp induced by 20 pM basolateral AVP (134.9 +/- 19.2 vs. 204.3 +/- 21.1 in control; P less than 0.05). These findings demonstrate distinct luminal action of AVP, suggesting amphilateral regulation of epithelial transport by AVP in the CCD.

Vasopressin in aged rats: longitudinal studies of vasopressin excretion in Sprague-Dawley and Fischer 344 strains

In order to provide physiological baseline values for future experimental procedures, indices of vasopressin secretion were assessed in male Sprague-Dawley (SD) and Fischer 344 (F344) rats at 3 and 20 months of age. Daily water intake, urine volume, urine osmolality, and urinary vasopressin excretion were monitored in SD rats for 30 days, and in F344 rats for 60 days. In the SD strain, daily water and urine volumes, expressed as ml/24 hr/100 g b.wt., were consistently lower in aged animals, as was a calculation of water balance (water intake-urine output volumes/24 hr). Although mean VP concentration in urine appeared higher in aged rats (33.9 +/- 20.4 pg/ml) than in young (16.3 +/- 7.7 pg/ml), total daily VP excretion was comparable for both ages when expressed as a function of body weight [80.6 +/- 37.3 pg for 3 months old (m.o.) and 81.9 +/- 47.2 pg/24 hr/100 g b.wt. for 3 and 20 m.o. respectively]. Young and old F344 males showed comparable daily drinking and urine volumes, and water balance, during two months of monitoring, but VP excretion was lower (p less than 0.025) in aged rats (83.8 +/- 19.0 pg/24 hr/100 g b.wt.) than in 3 m.o. rats (213.0 +/- 48.1 pg/24 hr/100 g b.wt.). Urine VP concentration was comparable (69.6 +/- 20.6 for 3 m.o.; 59.8 +/- 25.6 pg/ml for 20 m.o.). Mean urine osmolality was not significantly different among groups. Urine osmolality and daily urine volumes showed a significant correlation with daily VP excretion among young, but not aged, rats of both strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of vasopressin and determination of its corticomedullary levels in rat kidney tissue

To investigate the kidney tissue level of arginine vasopressin (AVP), tissue AVP was extracted, identified and assayed. Rat kidney tissue was separated into the papilla (P), medulla (M), and cortex (C). After homogenization, AVP was extracted with acetone and determined by radioimmunoassay. Fractionation of the tissue extract by Sephadex G25 column chromatography showed that the peak of immunoreactive AVP was identical to that of synthetic AVP. There was good correlation between bioassayable and immunoassayable AVP for the tissue extracts. Tissue AVP concentrations (pg/g wet weight) after 36 hr of dehydration were: P, 164.3 +/- 13.0; M, 93.8 +/- 8.4; and C, 28.9 +/- 3.9 with a plasma AVP (pg/ml) of 11.7 +/- 1.0 (mean +/- SE, N = 18). Thus, the papilla/plasma AVP concentration ratio was high. When 125I-AVP was administered, the papilla/plasma ratio was very low. Under water diuresis, the tissue AVP level was very low. The kidney tissue level of dDAVP, which was administered exogenously, was much higher than that of AVP when compared at similar plasma levels. Thus, the present study demonstrated the existence of a corticomedullary concentration gradient of AVP with a medullary level much higher than the plasma level. The accumulation of AVP seemed to result from receptor-mediated processes, since 125I-AVP, which is biologically inactive and does not bind to hormone receptors, showed only a slight accumulation. The higher tissue level of dDAVP suggested that the tissue level was determined by the balance between accumulation and inactivation, since dDAVP is known to be scarcely inactivated in the kidney.

The renal metabolism of insulin

The kidney plays a pivotal role in the clearance and degradation of circulating insulin and is also an important site of insulin action. The kidney clears insulin via two distinct routes. The first route entails glomerular filtration and subsequent luminal reabsorption of insulin by proximal tubular cells by means of endocytosis. The second involves diffusion of insulin from peritubular capillaries and subsequent binding of insulin to the contraluminal membranes of tubular cells, especially those lining the distal half of the nephron. Insulin delivered to the latter sites stimulates several important processes, including reabsorption of sodium, phosphate, and glucose. In contrast, insulin delivered to proximal tubular cells is degraded to oligopeptides and amino-acids by one of two poorly delineated enzymatic pathways. One pathway probably involves the sequential action of insulin protease and either GIT or non-specific proteases; the other probably involves the sequential action of GIT and lysosomal proteases. The products of insulin degradation are reabsorbed into the peritubular capillaries, apparently via simple diffusion. Impairment of the renal clearance of insulin prolongs the half-life of circulating insulin by a number of mechanisms and often results in a decrease in the insulin requirement of diabetic patients. Much needs to be learned about these metabolic events at the subcellular level and how they are affected by disease states. Owing to the heterogeneity of cell types within the kidney and to their anatomical and functional polarity, investigation of these areas will be challenging indeed.

Metabolic clearance rate of immunoreactive vasopressin in man

Metabolic clearance of synthetic arginine vasopressin (AVP) has been measured in sixteen healthy subjects and ten uraemic patients on maintenance haemodialysis. Plasma AVP was measured using a specific radioimmunoassay at different intervals after a single injection of 2 micrograms AVP. The theoretical curve which fitted best with the disappearance curve was the sum of two exponentials in twenty-two subjects and of three exponentials in the other four. Metabolic clearance rate and the volume of fast initial distribution were 287.1 ml min-1 (m2)-1 and 219.3 ml/kg b.w., respectively, in normal subjects. Metabolic clearance rate was considerably lower in the uraemic group. This emphasizes the role of kidneys in the degradation of AVP and may account, at least in part, for the higher basal plasma value of this hormone observed in uraemic patients.

Insulin binding and degradation by luminal and basolateral tubular membranes from rabbit kidney

Insulin influences certain metabolic and transport renal functions and is avidly degraded by the kidney, but the relative contribution of the luminal and basolateral tubular membranes to these events remains controversial. We studied (125)I-insulin degradation [TCA and immunoprecipitation (IP) methods] and the specific binding of the hormone by purified luminal (L) and basolateral (BL) tubular membranes. These were prepared from rabbit kidney cortical homogenates by differential and gradient centrifugation and ionic precipitation steps in sequence, which resulted in enrichment vs. homogenate of marker enzymes' activities (sodium-potassium-activated adenosine triphosphatase for BL and maltase for L) of 8- and 12-fold, respectively. Both fractions degraded insulin avidly and bound the hormone specifically without saturation even at pharmacologic concentrations (10 muM). At physiologic insulin concentrations (0.157 nM) BL membranes degraded substantial amounts of insulin (44.2+/-2.6 and 40.7+/-2.2 pg/mg protein per min by the TCA and IP methods, respectively), even though at lesser rates (P < 0.001) than the luminal fraction (67.2+/-2.3 and 75+/-6.2 pg/mg protein per min, respectively); the rate of insulin catabolism by BL membranes was significantly higher (P < 0.001) than that which could be attributed to their contamination by luminal components [12.2+/-1.9 pg/mg per min (TCA method), or 13.7+/-1.9 pg/mg per min (IP method)]. Competition experiments suggested that insulin-degrading activity in both fractions includes both specific and nonspecific components. In contrast to degradation, insulin binding by both membranes was highly specific for native insulin and was severalfold higher in BL than L membranes [17.5+/-1.3 vs. 4.5+/-0.4 fmol/mg protein (P < 0.001) at physiologic insulin concentrations]. Despite the marked difference in the binding capacity for insulin by the two membranes, the patterns of labeled insulin displacement by increasing amounts of unlabeled hormone were superimposable (50% displacement required approximately 3 nM), suggesting that their receptors' affinity for insulin was similar. These observations provide direct evidence that interaction of insulin with the kidney involves binding and degradation of the hormone at the peritubular cell membrane.

Renal filtration, absorption and catabolism of human alpha interferon

The renal handling of human interferon-alpha has been evaluated by using an isolated and perfused rabbit kidney. IFN-alpha disappears from plasma with a t1/2 of 81 min and the fractional turnover rate is 0.84%/min. About 47 molecules of IFN-alpha are filtrated with 100 molecules of creatinine but most of the IFN is absorbed by tubular cells. This is the first report showing that human IFN-alpha is filtrated by the kidney, largely absorbed, most probably catabolized within tubular epithelium and excreted in negligible amounts with the urine.