J. Physiol. (1984), 352, pp. 517-526 517 With 2 text-figures Printed in Great Britain

NATRIURETIC RESPONSE OF THE RAT TO PLASMA CONCENTRATIONS OF ARGININE VASOPRESSIN WITHIN THE PHYSIOLOGICAL RANGE BY R. J. BALMENT*, M. J. BRIMBLE*, MARY L. FORSLINGt AND C. T. MUSABAYANE* From the *Department of Physiology, University of Zimbabwe, P.O. Box MP 167, Mount Pleasant, Harare, Zimbabwe and the tDepartment of Physiology, Middlesex Hospital Medical School, London W1P 6DB (Received 7 December 1983)

SUMMARY 1. The relationship ofplasma vasopressin concentrations in the physiological range to renal electrolyte excretion was investigated. 2. Unanaesthetized rats, when normally hydrated, were found to have a plasma vasopressin concentration of 113 + 015 ,su./ml. 16 h water deprivation raised this to 1P98+0-21 ,su./ml. 3. Inactin-anaesthetized rats infused with 0 45 % NaCl had a plasma vasopressin concentration of 1 19 + 0418 su./ml. Administration ofsynthetic arginine vasopressin at 6 and 24 ,uu./min raised plasma vasopressin levels to 1P88+0-17 and 4-26 + 0 43 ,uu./ml respectively. 4. In addition to the expected antidiuresis, vasopressin at a rate of 6 ,uu./min also produced a highly significant increase in Na+ excretion from 89 + 06 to 10-5+0-6/imol/min and Cl- excretion from 9-1+0-7 to 105+07j7mol/min. At 24 ,uu./min it produced larger increases in Na+ and Cl- excretion. 5. Inactin-anaesthetized hypophysectomized rats infused with 0 45 % NaCl had a plasma vasopressin concentration of only 017+004 #zu./ml. Administration of vasopressin at 6 and 24 #su./min raised plasma vasopressin levels in these animals to 0-63 + 0-17 and 2-20 + 0-11 su./ml respectively. 6. Hypophysectomized rats failed to exhibit a natriuresis in response to the lower dose of vasopressin, despite exhibiting an undiminished antidiuresis. The failure of the natriuresis may be related to the lower plasma vasopressin concentration achieved. 7. It is concluded that in the rat plasma vasopressin concentrations within the physiological range do influence Na+ and Cl- excretion by the kidney as well as controlling urine flow rate. INTRODUCTION The antidiuretic hormone, arginine vasopressin, is the major factor controlling the rate of urine flow in most mammalian species including the rat and human (Heller, R. J. Balment was a visiting lecturer to the University of Zimbabwe. Permanent address: Department of Zoology, University of Manchester, Manchester M13 9PL. 518 R. J. BALMENT AND OTHERS 1963). In addition to its antidiuretic effects, arginine vasopressin is known to have a natriuretic action (Jacobson & Kellog, 1956; Kurtzman, Rogers, Boonjarern & Arruda, 1975; Buckalew & Dimond, 1976). It is usually considered that a natriuretic response is seen only when supraphysiological levels of the hormone are present (Dicker, 1957; Atherton, Hai & Thomas, 1969; Kurtzman et al. 1975). In other studies, however, increases in electrolyte excretion have been observed in response to low rates ofadministration oflysine vasopressin (Atherton, Green & Thomas, 1971) and arginine vasopressin (Chan & du Vigneaud, 1970; Fejes-Toth & Szenasi, 1981); however, plasma vasopressin concentrations were not measured. Furthermore, Luke (1973) postulated that the natriuresis observed during hydropenia in the rat was due to vasopressin. An important role for vasopressin in the regulation of salt excretion in the sheep has also been suggested (Kinne, MacFarlane & Budtz-Olsen, 1961; Scott & Morton, 1976). In view of these conflicting opinions it seemed important to re-investigate the natriuretic effect of arginine vasopressin and to determine the plasma hormone level necessary to increase renal electrolyte excretion. Accordingly, plasma hormone concentrations and renal function have been assessed during infusions of synthetic arginine vasopressin. In the light of earlier reports (Buckalew & Dimond, 1976) of the involvement of other pituitary factors in the natriuretic response to vasopressin, these studies have been performed in hypophysectomized as well as in intact rats.

METHODS Animak8 Experiments were performed on male Sprague-Dawley rats (320-460 g body weight) bred and housed in the Medical Faculty animal house at the University of Zimbabwe. Animals were maintained on a 12 h light/12 h dark regime and allowed free access to food (Mouse comproids, National Foods, Harare) and water.

Renal8tudime Thirteen rats were anaesthetized with an intraperitoneal injection ofInactin (5-ethyl-5-(1'- methylpropyl)-2-thiobarbiturate; Byk Gulden) at 0-11 g/kg body weight. The right jugular vein was cannulated (Portex PP90). The urinary bladder was also cannulated (Portex PP90) via an incision in the abdominal wall (the dead space in the bladder cannula was approximately 150,ul). Body temperature was maintained at 37° 00 by means of a heated table. A further fifteen animals were treated in exactly the same way except that prior to cannulation their pituitaries were first exposed by the parapharyngeal approach, a dental drill (Shick HS2000/18) being used to drill through the base of the skull. The anterior and posterior lobes of the pituitary were then removed by suction, completeness of removal being checked post mortem. Both groups of animals were placed on a continuous jugular infusion of 0 45 % (0-077M) NaCl at 150 Fsl/min (Sage Syringe Pump model 351). An initial equilibration period of 4 h was allowed during which time urine was collected and its volume, Na+, K+ andCl- content measured. Following the equilibration period three consecutive urine collections were made into pre-weighed plastic vials atmin10 intervals. The infusate was then switched to one of identical ionic composition but containing synthetic arginine vasopression (Sigma grade V) at a concentration of either 40 or 160 Itu./ml, allowing delivery of the hormone to rats at 6 or 24 ,uu./min respectively (1 usu. equals approximately 2-5 pg). After a further three collectionsmin)(30 the animals were returned to the vasopressin-free infusate for the final six collectionsmin).(60 At the end of the experiment a 2 ml blood sample was taken by cardiac puncture, the plasma separated and assayed for Na+, K+ CCl.and VASOPRESSIN-INDUCED NATRIURESIS 519

Analytical methods Urine volume was determined gravimetrically. Urinary Na+ and K+ were determined by flame photometry (Corning model 435 flame photometer) and Cl- by conductivity meter (Corning 925 chloride analyser). Determination of plasma vasopressin concentrations (a) Follouing vasopre8sin administration. Parallel groups of 'intact' (n = 8) and 'hypophysecto- mized' (n = 6 or 7) were prepared and infused in the same way as the renal study groups. However, at the end of the 30 min period of vasopressin administration, at either 6 or 24 ,/u./min, the jugular cannula was clamped (to prevent contamination of the blood sample with infusate), the animals decapitated and trunk blood collected into pooled heparinized containers. All blood collections were completed within 30 s of stopping the infusion. Blood was also collected in a similar manner from control groups of 'intact' (n = 7) and 'hypophysectomized' (n = 5) rats which had received an infusion of 0 45 % NaCl for 5 h, but no vasopressin. Plasma was separated by centrifugation, and a 2 ml aliquot of each plasma sample was freeze-dried (Virtis Freeze-drier) and stored at -20 0C until dispatched by air to the Middlesex Hospital Medical School for determination of arginine vasopressin content by radioimmunoassay. The validity of this procedure for plasma storage and transport was verified by assay of vasopressin in plasma from hypophysectomized rats to which known concentrations (1 and 5 ,au./ml) of vasopressin had been added. (b) In unanaesthetized rats. Two groups of unanaesthetized rats were killed by decapitation and trunk blood collected, plasma separated and a known volume (1 or 2 ml) freeze-dried and assayed for arginine vasopressin. Rats in the first group (n = 12) were permitted free access to water up to the time ofdecapitation, while rats in the second group (n = 10) were deprived ofwater overnight (16 h). Assay of vasopressin Plasma samples were extracted with bentonite (Skowsky, Rosenbloom & Fisher, 1974) and then vasopressin concentrations determined by radioimmunoassay using radiolabelled (1261) vasopressin prepared with a solid phase lactoperoxidase method (Stromberg, Forsling & Akerlund, 1981). The results are presented in terms of the First International Standard for arginine vasopressin (77-501). There was no displacement of binding of labelled hormone in this assay by extracts of plasma from rats homozygous for congenital diabetes insipidus (Brattleboro strain). Statistical methods Values are presented as means + s.. of means. In the renal studies the mean excretory rate in the three control urine collections was used as a base line for comparison with subsequent collections, except where there was a progressive change through the control period as occurred for Na+ and Cl- excretion in hypophysectomized rats. In these latter cases the sloping base line was projected forwards for comparison with subsequent 10 min clearances (Balment, Brimble & Forsling, 1980). The significance of any deviations from the base line was assessed using a paired t test. All other comparisons of grouped data were by unpaired t test.

RESULTS Fluid and electrolyte balance in 8udine-infused rats During the 4 h equilibration period the rats received 36 ml fluid and 2-77 mmol Na+ and C1- from the infusate. The total volume of urine produced and Na+, K+ and C1- excreted through this period is presented in Table 1. The amounts of water, Na+ and Cl- excreted were significantly less (P < 0 01) than the amounts infused both in intact and hypophysectomized animals. The hypophysectomized animals, however, excreted markedly less Na+ and Cl- (P < 0 01) than did intact rats, though the amount ofwater excreted was similar in both groups. The greater degree of salt retention in hypophysectomized animals extended into the experimental period (see Figs. 1 and 520 R. J. BALMENT AND OTHERS 2) and resulted in elevated plasma Na+ by comparison with intact rats (P < 0 01; Table 1). Plasma chloride also appeared higher in hypophysectomized rats though this difference was not statistically significant. Effects of arginine vasopressin on urine flow and electrolyte excretion in intact rats (Fig. 1) Administration of arginine vasopressin at 6 /zu./min (n = 7) significantly reduced urine flow from an average 140 ± 3 #sl/min in the control period to 87 + 11 #I/min (P < 0 01) in the last collection during hormone infusion. The decrease in urine flow

TABLE 1. Fluid and electrolyte balance Intact rats Hypophysectomized (n= 13) rats(n= 15) Fluid and electrolyte excretion in 4 h equilibration period Urine volume (ml) 22+1 22 + 2 Sodium (mmol) 1P04+ 0 35 0-21 + 005 Potassium (mmol) 0-48 + 012 0-24 + 006 Chloride (mmol) 1 11 + 032 0-26+0{06 Terminal plasma electrolyte concentrations (mmol/l) Sodium 148+1 155+2 Potassium 46 +±02 4-6 +0-2 Chloride 116+2 123+3 was accompanied by an increase in both Na+ and Cl- excretion. Na+ excretion increased from 8-9 + 0-6 ,mol/min to a peak of 10-5 + 0-6 ,smol/min (P < 0 01), while Cl- excretion rose from 91 +07 to 10-5+0-7 ,mol/min (P < 001). There was no significant change in the rate of K+ excretion. Arginine vasopressin at 24 ,uu./min (n = 6) had similar but more pronounced effects on urine flow and electrolyte excretion. Urine flow decreased from 138+8 to 42 + 6 ,sl/min (P < 0 01), while Na+ excretion increased from 8-2 + 1-5 to 13-7 + 2-3 mol/min (P < 001) and Cl- excretion rose from 76 + 09 to 13-9 + 21 ,umol/min (P < 0 01). With this dose maximum Na+ and Cl- excretory rates occurred in the 10 min period following cessation of vasopressin administration. At this time K+ had also increased significantly from 36 + 02 to 49 + 04 pmol/min (P < 002). By 20-40 min after stopping the hormone infusion at 6 or 24 pu./min, urine flow had risen to levels significantly exceeding control values, reaching peaks of 186 + 15 Fl/min (P < 0 05) and 223 + 16 Fl/min (P < 0 01) respectively. Cl- excretion was significantly depressed below control values at this time, falling to 7 0 + 04 ,umol/min (P < 0 05) and to 6-0 + 06 ,umol/min (P < 0 05) respectively. Na+ excretion appeared to show similar transitory falls but the changes were not statistically significant. VASOPRESSIN-INDUCED NATRIURESIS 521

Effects of arginine vasopressin on urine flow and electrolyte excretion in hypo- physectomized rats (Fig. 2) The reduction in urine flow with both rates of vasopressin administration in hypophysectomized rats was similar to that observed in intact animals. Infusion of vasopressin at 6 ,su./min (n = 9) reduced urine flow from 154 + 15 to 103 +13 #Il/min A B 1IAVPi61iAVP 24 T

-200 1 S

12 -~~~~~~~~~~~ TT~~~~~~~~ 0 X I ~~I

0

4-~~~I

C

0 60 120 0 60 120 Time (min) Time (min) Fig. 1. A, the effect of arginine vasopressin administration at 6 juu./min (AVP 6) on urine flow and Na+ and K+ excretion rates in seven intact rats. B, the effect of arginine vasopressin administration at 24,uu./min (AVP 24) on urine flow and Na+ and K+ excretion rates in six intact rats. Values are presented as means for each 10 min collection period; vertical bars indicate S.E. of means.

(P < 001) while 24 #zu./min (n = 6) reduced urine production from 179+17 to 36 + 4 #zl/min (P < 0 01). In both groups of hypophysectomized rats there was slow progressive increase in the rate of Na+ or Cl- excretion during the experimental period. There were, however, no changes in Na+ or Cl- excretion that could be attributed to vasopressin administration at 6 ,uu./min. Vasopressin at 24 gtu./min did appear to cause a small increase in Na+ and Cl- excretion superimposed on the progressive upward trend. When allowance was made for the upward trend the mean increase in Na+ excretion was 14 ± 06 #smol/min (005 < P < 0 1) and the mean increase in Cl- excretion was 18±+0-7 #smol/min (P < 0 05). K+ excretion was also increased by 0-8 +03 Itmol/min (P < 0 05). There was a gradual recovery of urine flow over the 40 min after vasopressin 522 R. J. BALMENT AND OTHERS

A B LAVP6j AVP24] 200 _

0 C 8f

0 0 4I Na'

Na' M 01 KI K 0 60 120 0 60 120 Time (min) Time (min) Fig. 2. A, the effect of arginine vasopressin administration at 6 #uu./min (AVP 6) on urine flow and Na+ and K+ excretion rates in nine hypophysectomized rats. B, the effect of arginine vasopressin administration at 24 /su./min (AVP 24) on urine flow and Na+ and K+ excretion rates in six hypophysectomized rats. Values are presented as means for each 10 min collection period; vertical bars indicate S.E. of means. administration had stopped. However, the overshoot in urine flow and depression in electrolyte excretion seen in the intact rats was not observed in the hypophysecto- mized animals. Plasma vasopressin concentrations Arginine vasopressin was detected in the plasma of both intact and hypo- physectomized rats after 5 h of hypotonic saline infusion (Table 2). Hormone levels were, however, markedly lower (P < 0'01) in hypophysectomized than in intact rats. Dose-related increases in plasma hormone concentrations were produced by infusion of arginine vasopressin at 6 and 24 ,uu./min for 30 min. The levels achieved in hypophysectomized rats were as might be predicted from the lower basal hormone levels, lower (P < 0-01) than in intact animals at both rates of administration (Table 2). The plasma arginine vasopressin concentration in normally hydrated, unanaes- thetized rats was 1-13 + 0'15 puu./ml (n = 12). After a 16 h (overnight) period of water deprivation, vasopressin concentration was elevated to 1-98+0-21 #tu./ml (n = 10; P < 0-01). VASOPRESSIN-INDUCED NATRIURESIS 523

TABLE 2. Plasma arginine vasopressin Plasma arginine vasopressin concentration Vasopressin administration Hypophysectomized rate Intact rats rats (,u./min) (,mu./ml) (uu-/ml) 0 lP19+018 0-17+0-04 (7) (5) 6 1P88+0417 0-63+0-17 (8) (6) 24 4-26+043 2*20+±011 (8) (7) Values are means ±s.E. of means; n values are shown in parentheses.

DISCUSSION The importance of pituitary factors, directly or indirectly, in the regulation of renal function was underlined by the contrasting renal management of water and electrolytes in intact and hypophysectomized rats. During the equilibration period both groups of animals were found to excrete less water, Na+ and Cl- in urine than they received in the infusate. No account was taken of extrarenal losses of water and electrolyte, but, as these should be small in comparison with renal excretion, it seems reasonable to conclude that there was retention of water and electrolyte during this period in both groups of animals. Although the degree of water retention was similar in both groups, hypophysectomized rats retained considerably more Na+ and Cl- than did intact rats and consequently exhibited elevated plasma Na+ concentrations. Na+ retention has been reported previously in acutely hypophysectomized rats (Lichardus & Ponec, 1972) and appears related to the loss of posterior pituitary factors (Lichardus & Ponec, 1973). During the 30 min control period preceding vasopressin administration the mean urine flow and Na+ and Cl- excretion rates of intact rats were still somewhat lower than the rates ofinfusion. They were, however, similar to the flow and excretion rates previously reported for Sprague-Dawley rats (Garland, Balment & Brimble, 1983), under comparable experimental conditions. Electrolyte excretion rates in hypophy- sectomized rats were much lower than in intact animals during this period, indicating a continuance of the exaggerated renal retention of Na+ and Cl- in this group. Normally hydrated, unanaesthetized Sprague-Dawley rats were found to have similar plasma vasopressin levels to those previously reported in the same (Dunn, Brennan, Nelson & Robertson, 1973; Keil & Severs, 1977; Husain, Manger, Rock, Weiss & Frantz, 1979) and other strains ofrats (Mohring & M6hring, 1976; Dogterom, Greidanus & De Wied, 1978; Forsling, Brimble & Balment, 1982; Fyhrquist, Tikkanen & Linkola, 1981). Water deprivation overnight caused a small but significant elevation of plasma vasopressin. Plasma vasopressin concentration in intact, Inactin-anaesthetized Sprague-Dawley rats infused with saline was similar to that in unanaesthetized rats and to that in comparably treated Long Evans rats in 524 R. J. BALMENT AND OTHERS an earlier study (Forsling et al. 1982). Although elevated, plasma vasopressin levels following hormone administration at 6,uu./min remained within the range induced by mild (overnight) dehydration. The plasma vasopressin concentration induced at the higher rate of administration was beyond this range but remained lower than the hormone levels seen after 48 h water deprivation (M6hring & Mdhring, 1976; Keil & Severs, 1977; Fyhrquist et al. 1981). Thus the low rate of vasopressin administration produced plasma hormone levels within the range the animal might experience on a day-to-day basis, while the higher rate produced hormone concentrations probably only achieved under more adverse conditions. The low dose of vasopressin not only reduced urine flow as expected, but also produced a highly significant increase in Na+ and Cl- excretion. A similar but much larger natriuresis was associated with the higher rate of hormone administration. The present investigation provides no evidence as to the site within the nephron at which vasopressin exerts this natriuretic action. However, a renal micropuncture study by Fejes-Toth & Szena'si (1981) using a vasopressin administration rate similar to our higher dose, provided evidence that in male Sprague-Dawley rats vasopressin produces a natriuresis by reducing NaCl re-absorption in the loop of Henle, without significantly affecting distal tubular function. There have also recently been a series of investigations of the effects of vasopressin on Na+ or Cl- transport in isolated perfused medullary segments of the loop of Henle from rat, mouse and rabbit (Sasaki & Imai, 1980; Hall & Varney, 1980; Herbert, Culpepper & Andreoli, 1981 a, b). These studies have shown that vasopressin poten- tiates NaCl re-absorption from the lumen of the medullary thick ascending limb. This is opposite to the conclusions of the in vivo study by Fejes-Toth & Szena'si (1981) and also appears to conflict with the natriuretic action described by ourselves and others. The reasons for this apparent conflict between in vivo and isolated tubule studies may be related to a differential action of vasopressin on superficial and juxtamedullary nephrons. Following cessation of vasopressin administration at both dose levels, there was a rebound increase in urine flow to well above control levels. This rebound has been described previously in a study involving unanaesthetized rats (Jacobson & Kellog, 1956). One possible explanation is that endogenous vasopressin release is inhibited by the water retained during antidiuresis, and that this inhibition of endogenous vasopressin allows urine flow to increase above control values once the effect of the exogenous vasopressin has worn off. The depression in electrolyte excretion observed at this time would also be consistent with this concept. Trace amounts of vasopressin were consistently found in the plasma of Inactin- anaesthetized, hypophysectomized rats infused with saline. This probably represents residual release of hormone either from the cut ends of neurohypophysial axons or from extrapituitary vasopressinergic nerve terminals (Sterba, 1979). In view of the low endogenous vasopressin level it was not surprising that the plasma vasopressin concentrations produced by equivalent rates of exogenous hormone administration were lower in hypophysectomized than in intacts rats. Indeed, vasopressin administra- tion at 6 ,uu./min in intact rats produced plasma hormone levels similar to those achieved by infusion at 24 ,#u./min in hypophysectomized animals. The absence of VASOPRESSIN-INDUCED NATRIURESIS 525 a natriuretic effect of the low vasopressin dose in hypophysectomized rats may, therefore, simply reflect the low circulating hormone level induced in these animals. Indeed the natriuretic response to vasopressin at 24 ,uu./min approached that produced with the lower dose in intact animals. Thus there may be no need to invoke an additional pituitary factor as suggested by Buckalew & Dimond (1976). Clearly, however, the possibility that vasopressin exerts its natriuretic action indirectly requires further investigation. Although the natriuretic responses to equivalent doses of vasopressin were less marked in hypophysectomized than in intact animals, the antidiuretic responses were similar in both groups. This lends support to the finding of Lote, McVicar & Smyth (1983) that the natriuretic action of vasopressin is not directly linked to the antidiuretic effect of the hormone. This is reminiscent of the observed renal actions of oxytocin in which there is also a clear separation of the effects on electrolyte excretion and urine flow (Chan, 1976; Balment et at. 1980; Mehta, Arruda, Kurtzman, Smith & Walter, 1980). In conclusion we believe we have demonstrated that plasma concentrations of arginine vasopressin within the normal physiological range do influence renal Na+ and Cl- excretion, and we agree with Luke (1973) that vasopressin is probably involved in the normal day-to-day regulation of electrolyte excretion in the rat. This contrasts with the otherneurohypophysial hormone, oxytocin, for which we concluded there was no evidence for a significant role in the day-to-day regulation ofsalt balance (Balment et al. 1980). We wish to thank Byk Gulden, Konstanz for a gift of Inactin.

REFERENCES ATHERTON, J. C., GREEN, R. & THOMAS, S. (1971). Influence of lysine vasopressin dosage on the time course of changes in renal tissue and urinary composition in the conscious rat. J. Physiol. 213, 291-309. ATHERTON, J. C., HAI, M. A. & THOMAS, S. (1969). Acute effects of lysine vasopressin injection (single and continuous) on urinary composition in the conscious water diuretic rat. Pfligers Arch. 310, 281-319. BALMENT, R. J., BRIMBLE, M. J. & FORSLING, M. L. (1980). Release of oxytocin induced by salt loading and its influence on renal excretion in the male rat. J. Physiol. 308, 439-449. BUCKALEW, V. M. & DIMOND, K. A. (1976). Effect of vasopressin on sodium excretion and plasma antinatriferic activity in the dog. Am. J. Physiol. 231, 28-33. CHAN, W. Y. (1976). An investigation of the natriuretic and oxytocic actions of neurohypophysial hormones and related peptides: delineation of separate mechanisms of action and assessment of molecular requirements. J. Pharmac. exp. Ther. 196, 746-757. CHAN, W. Y. & DU VIGNEAUD, V. (1970). Natriuretic, diuretic and anti-arginine-vasopressin (ADH) effects of two analogs of oxytocin: 4-leucine (-oxytocin) and 2,4 diisoleucine (-oxytocin). J. Pharmac. exp. Ther. 174, 541-549. DICKER, S. E. (1957). Urine concentration in the rat during acute and prolonged dehydration. J. Physiol. 139, 108-122. DOGTEROM, J., VAN WIMERSMA GREIDANUS TJ, B. & DE WIED, D. (1978). Vasopressin in cerebro- spinal fluid and plasma of man, dog and rat. Am. J. Physiol. 234, E463-467. DUNN, F. L., BRENNAN, T. J., NELSON, A. E. & ROBERTSON, G. L. (1973). The role of blood osmolality and volume in regulating vasopressin in the rat. J. clin. Invest. 52, 3212-3219. FEJES-T6TH, G. & SZENASI, G. (1981). The effect of vasopressin on renal tubular 22Na efflux in the rat. J. Physiol. 318, 1-7. 526 R. J. BALMENT AND OTHERS

FORSLING, M. L., BRIMBLE, M. J. & BALMENT, R. J. (1982). The influence of vasopressin on oxytocin-induced changes in urine flow in the male rat. Acta endocr. 100, 216-220. FYHRQUIST, F., TIKKANEN, I. & LINKOLA, J. (1981). Plasma vasopressin concentration and renin in the rat: effect of hydration and hemorrhage. Acta phy8iol. 8cand. 113, 507-510. GARLAND, H. O., BALMENT, R. J. & BRIMBLE, M. J. (1983). Oxytocin and renal function in the rat; an investigation of a possible proximal site of action. Acta endocr. 102, 517-520. HALL, D. A. & VARNEY, D. M. (1980). Effect of vasopressin on electrical potential difference and chloride transport in mouse medullary thick ascending limb of Henle's loop. J. clin. Invest. 66, 792-802. HELLER, H. (1963). Neurohypophyseal hormones. In Comparative Endocrinology, vol. 1.Glandular Hormones, ed. VON EULER, U. S. & HELLER, H., pp. 25-80. New York and London: Academic Press. HERBERT, S. C., CULPEPPER, R. M. & ANDREOLI, T. E. (1981 a). NaCl transport in mouse medullary thick ascending limbs. I. Functional nephron heterogeneity and ADH-stimulated NaCl co- transport. Am. J. Physiol. 241, 412-431. HERBERT, S. C., CULPEPPER, R. M. & ANDREOLI, T. E. (1981 b). NaCl transport in mouse medullary thick ascending limbs. II. ADH enhancement of transcellular NaCl cotransport; origin of transepithelial voltage. Am. J. Physiol. 241, 432-442. HUSAIN, M. K., MANGER, W. M., ROCK, T. W., WEISS, R. J. & FRANTZ, A. G. (1979). Vasopressin release due to manual restraint in the rat: role of body compression and comparison with other stressful stimuli. Endocrinology 104, 641-644. JACOBSON, H. N. & KELLOG, R. H. (1956). Isotonic NaCl diuresis in rats, antidiuresis and chloruresis produced by posterior pituitary extracts. Am. J. Physiol. 184, 376-389. 376-389. KEIL, L. C. & SEVERS, W. B. (1977). Reduction in plasma vasopressin level of dehydrated rats following acute stress. Endocrinology 100, 30-38. KINNE, R., MACFARLANE, W. V. & BUDTZ-OLSEN, 0. E.(1961). Hormones and electrolyte excretion in sheep. Nature, Lond. 192, 1084-1085. KURTZMAN, N. A., ROGERS, P. W., BOONJARERN, S. & ARRUDA, J. A. L. (1975). Effect of infusion of pharmacologic amounts of vasopressin on renal electrolyte excretion. Am. J. Physiol. 228, 890-894. LCHARDUS, B. & PONEC, J. (1972). Effects of hypophysectomy on sodium excretion in rats without blood dilution during blood volume expansion. Experientia 28, 471-472. LCHARDUS, B. & PONEC, J. (1973). On the role of the hypophysis in the renal mechanism of body fluid volume regulation. Endokrinologie 61, 403-412. LOTE, C. J., MCVICAR, A. J. & SMYTHE, D. G. (1983). Antidiuretic action of vasopressin-glycine- lysine-arginine in the rat. J. Physiol. 342, 50-51P. LUKE, R. G. (1973). Natriuresis and chloruresis during hydropenia in the rat. Am. J. Physiol. 224, 13-20. MEHTA, P. K., ARRUDA, J. A. L., KURTZMAN, N. A., SMITH, C. W. & WALTER, R. (1980). Effect of neurohypophysial peptides on electrolyte excretion. Miner. Electr. Metab. 3, 10-20. M6HRING, B. & MSHRING, J. (1976). Plasma ADH in normal Long-Evans rats and in Long-Evans rats heterozygous and homozygous for hypothalamic diabetes insipidus. Life Sci. Oxford 17, 1307-1314. SASAKI, S. IMAI,& M. (1980). Effects of vasopressin on water and NaCl transport across the in vitro perfused medullary thick ascending limb of Henle's loop in mouse, rat and rabbit kidneys. Pflugers Arch. 383, 215-221. SCOTT, D. & MORTON, J. J. (1976). Changes in urinary water and electrolyte excretion in sodium loaded sheep in response to intravenous infusion of arginine vasopressin. Q. Jl exp. Physiol. 61, 57-70. SKOWSKY, W. R., ROSENBLOOM, A. A. & FISHER, D. A. (1974). Radioimmunoassay measurement of arginine vasopressin in serum: development and application. J. dlin. Endocr. Metab. 38, 278-287. STERBA, B. (1979). Exohypothalamic vasopressinergic and oxytocinergic pathways. In Recent Resultu in Peptide Hormone and Androgenic Steroid Research, ed. LASLO, F. A., pp. 102-110. Budapest: Akademiai Kiado. STROMBERG, P., FORSLING, M. L. & AKERLUND, M. (1981). Effects of prostaglandin inhibition on vasopressin levels in women with primary dysmenorrhea. Obstet. Gynaec. 58, 206-208.