Plasma concentrations of arginine vasotocin, prolactin, aldosterone and corticosterone in relation to oviposition and dietary NaCl in the domestic fowl

The osmolality and concentrations of Na, K, Cl and the hormones arginine vasotocin (AVT), prolactin, aldosterone and corticosterone were measured in plasma as functions of time in relation to oviposition, changing NaCl content of the diet, and feeding-inanition. AVT was significantly increased immediately after oviposition (but not during the hour before) with a calculated average value of 38.0 +/- 4.1 pg/ml at oviposition. A moderate increase in concentrations of prolactin and corticosterone were observed immediately after oviposition. Oviposition was not associated with detectable changes in plasma osmolality (and electrolyte concentrations) nor with the concentration of aldosterone. After a sudden change from a high NaCl diet to a low NaCl diet the plasma osmolality and concentrations of NaCl, AVT and prolactin reached new stable levels in 24 hr, whereas the plasma aldosterone concentration required more than 4 days to reach a steady level. After resalination plasma aldosterone was suppressed in less than 8 hr. Both osmolality and concentrations of AVT and prolactin showed transient overshoots during the first 24 hr. NaCl depletion resulted in a transient increase of corticosterone.     

Plasma levels of arginine vasotocin,prolactin, aldosterone and corticosterone during prolonged dehydration in the domestic flowl: effect of dietary NaCl

Three groups of White Plymouth Rock laying hens were adapted to three levels of dietary NaCl: low-NaCl food with tap water (LOW), high-NaCl food (1% NaCl w/w added) with tap water (HT), and high-NaCl food with 0.5% NaCl for drinking (HS). The birds were subjected to water deprivation (dehydration) for 18 days. Blood sampling was done at 2-4 day intervals. Plasma concentrations of arginine vasotocin (AVT), prolactin (PRL), aldosterone (ALDO) and corticosterone (CS) were determined by radioimmunoassay. Plasma osmolality, sodium, chloride, and potassium were also determined. In the normally hydrated hens fully adapted to the diets, there was a stepwise increase from LOW to HS in plasma osmolality (305, 315, 332 mOsm, for LOW, HT and HS, respectively), [Na+] (144, 153, 161 mM) and [Cl-] (109, 119, 127 mM) as well as in [AVT] (6, 14, 18 pg/ml) and [PRL] (16, 24, 34 ng/ml). Regressing [AVT] on osmolality gave a slope of 0.30 pg . ml-1/mOsm and a threshold of 273 mOsm. The slope of [PRL] on osmolality was 0.73 ng . ml-1/mOsm. The correlation coefficient of [AVT] and [PRL] was 0.67. LOW had high [ALDO] (165 pg/ml) which was suppressed to low levels in HT and HS (5-8 pg/ml), while [CS] was the same in all groups (0.9-1.1 ng/ml). Plasma [K+] was decreased in the high-NaCl groups (5.8 mM in LOW, 4.4 and 4.7 mM in HT and HS). Dehydration resulted within 2 days generally in a sharp (5-15%) increase in osmolality, [Na+] and [Cl-], which thereafter increased more slowly during the remaining 16 days in all groups, with the slowest increase in LOW. The levels of osmolality [Na+] and [Cl-] were 5% lower in LOW than in HT and HS, which showed the same levels during the dehydration period. Plasma [AVT] and [PRL] increased 2-4 fold within 2 days of dehydration; [AVT] reached a plateau at 29 pg/ml in all groups, but [PRL] continued to rise in all groups, fastest in LOW, reaching similar levels in all groups after 14-18 days of dehydration, about 85 ng/ml. The correlation coefficient of [AVT] and [PRL] was decreased by half (to 0.32) during dehydration. Plasma [ALDO] increased in all groups with dehydration, 1.7 fold in LOW and 3-6 fold in HT and HS, but the levels reached in HT and HS were only 15-30% of that seen in LOW.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of melatonin and natural and synthetic analogues of arginine vasotocin on plasma prolactin levels in adult male rats

A significant elevation in plasma prolactin was observed 10 min following the intravenous injection of 100 microgram of melatonin into either estrogen-progesterone (EP) primed or into nonsteroid-treated male rats. 60 min postinjection in the EP primed rat, the groups treated with 100 microgram or 10 mg of melatonin had signficantly elevated plasma prolactin levels while no effect was observed with these same doses in the nonsteroid-treated rats. Compared to diluent-treated controls, a significant elevation in plasma prolactin was observed at 10, 20 and 60 min following the intravenous injection of either 1 microgram arginine vasotocin (AVT) or 1 mg melatonin into EP primed male rats. A consistent rise in plasma prolactin was also evident after the injection of 1 microgram of either arginine vasopressin, lysine vasopressin or AVT. Oxytocin had no effect on plasma prolactin values. The intravenous administration of 1 microgram of (deamino-1,6 dicarba, 8-arginine)-vasotocin caused a significant elevation of plasma prolactin 10 and 20 min after injection. However, the injection of another analogue of AVT, (4-leucine, 8-arginine)-vasotocin, had no effect on prolactin release at the time points measured.     

The pneumostome rhythm in slugs: a response to dehydration controlled by hemolymph osmolality and peptide hormones

1. One response of the terrestrial slug, Limax maximus to dehydration is the initiation and modulation of the pneumostome rhythm. When a slug has lost 15-20% of its initial body weight by evaporation, the frequency of pheumostome closures, which is less than 0.5 closures/min in fully hydrated slugs, begins to increase. 2. The frequency increases with further dehydration, but the average duration of each closure remains constant. Thus, the proportion of time during which the pneumostome is closed increases. Simultaneously, the area of the pneumostome opening decreases. 3. This behavior appears to be controlled in part by both the osmolality of the slug's hemolymph and by a peptide closely related to arginine vasotocin (AVT) and arginine vasopressin (AVP). Injecting intact slugs with mannitol, which increases the osmolality of the hemolymph, or with AVT or AVP, can initiate the pneumostome rhythm. 4. Mannitol injections, however, do not provoke the decrease in the area of the pneumostome opening which is induced by natural dehydration or by AVT or AVP injection. This suggests that at least two systems may be involved in the overall control of the pneumostome.     

Physiological and hormonal control of thermal depression in the tiger snake, Notechis scutatus

Plasma sodium concentrations in field-caught Western tiger snakes, Notechis scutatus, from semi-arid Carnac Island (CI) varied seasonally, with snakes exhibiting significant hypernatraemia during summer and normal concentrations following autumn rain. In contrast, field-caught tiger snakes from a perennial fresh-water swamp (Herdsman Lake, HL) exhibited no significant increase in plasma sodium concentrations during summer. Laboratory-induced hypernatraemia caused thermal depression in both populations; there was a weak negative relationship between plasma sodium concentration and temperature selection that was significant for CI snakes. Hypernatraemia significantly elevated circulating concentrations of the neuropeptide arginine vasotocin (AVT) in both CI and HL snakes. CI snakes injected with a physiological dosage of AVT also evidenced thermal depression. Despite the positive correlation between AVT and both plasma sodium concentration and osmolality for laboratory snakes, field samples from CI snakes indicate that circulating levels of AVT may be influenced more by plasma osmolality than sodium levels. The data suggest that, in CI snakes, chronic dehydration in the field leads to hypernatraemia which may lead to elevated levels of AVT if plasma osmolality also increases. This will in turn invoke a depression in thermal behaviour that may improve the water economy and survival of snakes on semi-arid CI. Although HL snakes do not experience seasonal dehydration, physiological changes away from the stable homeostatic state appear to prompt the same behavioural shifts, illustrating the intrinsic nature of the thermal behaviour in different populations of the same species of snake.     

Osmoregulatory effects of hypophysectomy and homologous prolactin replacement in hybrid striped bass

The effects of ovine prolactin (oPRL) and striped bass prolactin (sbPRL; Morone saxatilis) on plasma osmolality, electrolyte balance, and gill Na(+),K(+)-ATPase activity were investigated in hypophysectomized (Hx), freshwater (FW)-acclimated, hybrid striped bass (M. saxatilisxMorone chrysops). They were kept in dilute (isoosmotic) seawater for about 10 days after surgery. Seven days after transfer to FW, Hx fish had lower plasma osmolality and lower levels of Na(+), Cl(-), and Ca(2+) than sham-operated and intact fish. Fish were injected four times with oPRL (1, 5, or 20 microg/g body mass), sbPRL (10 or 100 ng/g), or hormone vehicle (0.9% NaCl) at 48-h intervals (days 0, 2, 4, and 6) in FW and then sampled for blood plasma 24 h after the fourth injection (day 7). In Hx fish, oPRL (5 and 20 microg/g) and sbPRL (10 and 100 ng/g) were effective in maintaining plasma osmolality and levels of Na(+), Cl(-), and Ca(2+) above values seen in saline-injected controls. Hypophysectomy did not affect branchial Na(+),K(+)-ATPase activity, but enzyme activity was significantly reduced in Hx fish receiving oPRL (20 mug/g) or sbPRL (10 or 100 ng/g). These results indicate that PRL acts to maintain plasma osmotic and ionic balance in FW-adapted hybrid striped bass, and that this may involve downregulation of branchial Na(+),K(+)-ATPase activity.     

Shift in body fluid compartments after dehydration in humans

To investigate the influence of [Na+] in sweat on the distribution of body water during dehydration, we studied 10 volunteer subjects who exercised (40% of maximal aerobic power) in the heat [36 degrees C, less than 30% relative humidity (rh)] for 90-110 min to produce a dehydration of 2.3% body wt (delta TW). After dehydration, the subjects rested for 1 h in a thermoneutral environment (28 degrees C, less than 30% rh), after which time the changes in the body fluid compartments were assessed. We measured plasma volume, plasma osmolality, and [Na+], [K+], and [Cl-] in plasma, together with sweat and urine volumes and their ionic concentrations before and after dehydration. The change in the extracellular fluid space (delta ECF) was estimated from chloride distribution and the change in the intracellular fluid space (delta ICF) was calculated by subtracting delta ECF from delta TW. The decrease in the ICF space was correlated with the increase in plasma osmolality (r = -0.74, P less than 0.02). The increase in plasma osmolality was a function of the loss of free water (delta FW), estimated from the equation delta FW = delta TW - (loss of osmotically active substance in sweat and urine)/(control plasma osmolality) (r = -0.79, P less than 0.01). Free water loss, which is analogous to "free water clearance" in renal function, showed a strongly inverse correlation with [Na+] in sweat (r = -0.97, P less than 0.001). Fluid movement out of the ICF space attenuated the decrease in the ECF space.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of AVT antisense oligodeoxynucleotides on AVT release induced by hypertonic stimulation in chicks

In birds, arginine vasotocin (AVT) and mesotocin (MT) are the neurohypophyseal hormones. AVT is known to be an avian antidiuretic hormone and is released from the neurohypophysis by dehydration or hyperosmotic stimulation. The purpose of this study was to determine whether the mechanism of AVT synthesis is related to the mechanism of hormone release from the neurohypophysis. Four-day-old chicks received an AVT antisense oligodeoxynucleotide (ODN) injection into the cerebral ventricle (icv). Following antisense administration, the chicks received hypertonic saline stimulation. Plasma levels of AVT and MT were measured by radioimmunoassays. In control birds, a hypertonic saline injection resulted in the increase of plasma AVT level. The administration of a high dose (50 microg) of antisense ODN inhibited the increase of plasma AVT level induced by the hypertonic saline stimulation. Plasma levels of MT did not change with the administration of hypertonic saline or antisense ODN. These results suggest that the mechanisms that regulate the secretion of AVT from the neurohypophysis may be coupled to the mechanisms that regulate the synthesis of AVT.     

Hormonal regulation of chicken intestinal NHE and SGLT-1 activities

The effects of aldosterone and arginine vasotocin (AVT) on intestinal Na(+)/H(+) exchange (NHE) and Na(+)-sugar cotransport (SGLT-1) activities have been investigated using brush-border membrane vesicles isolated from Hubbard chicken small and large intestines, and they were compared with those induced by either Na(+) depletion or dehydration. Na(+) depletion was induced by feeding the chickens with either a low- or a high-Na(+) diet for either 0.5, 1, 2, 4, or 8 days. Ileal and colonic NHE2 activity increased with the duration of the Na(+) depletion, whereas that of intestinal SGLT-1 decreased, reaching a plateau after 2 days of treatment. Three-hour incubation of the intestine with aldosterone produced the same effects on NHE activity as does Na(+) depletion, without altering SGLT-1 activity. However, 3-h incubation of the intestine with AVT increased intestinal SGLT-1 activity, without affecting intestinal NHE activity. It is concluded that aldosterone regulates apical ileal and colonic NHE2 activity, whereas that of SGLT-1 is regulated by AVT.     

Drinking induced by cellular dehydration in the quail, Coturnix coturnix japonica

1. Drinking was induced in water-replete quail 5-10 min after intravenous injection of hypertonic NaCl (0.69 osmol/l) or sucrose (1.06 osmol/l), but hypertonic urea (2.78 osmol/l) failed to induce drinking. 2. The birds drank approximately the amount required to dilute the injected solutes to isotonicity for each given dose of NaCl or sucrose. 3. The plasma angiotensin II level decreased after injection of 7% NaCl (2.5 osmol/l), but it increased after injection of an equi-osmolar solution of sucrose (65%). 4. Plasma osmolality and Na+ concentration returned quickly to control levels, and then decreased further, after injection of 7% NaCl or 65% sucrose. 5. Blood volume and blood pressure increased immediately after injection of 7% NaCl or 65% sucrose. 6. These results show that drinking is induced after injection of hypertonic solutions exclusively by cellular dehydration, and other regulatory mechanisms for thirst, such as extracellular dehydration and the renin-angiotensin system, are rather inhibitory after injection of hypertonic NaCl.     

Rehydration with fluid of varying tonicities: effects on fluid regulatory hormones and exercise performance in the heat.

This study examined the effects of rehydration (Rehy) with fluids of varying tonicities and routes of administration after exercise-induced hypohydration on exercise performance, fluid regulatory hormone responses, and cardiovascular and thermoregulatory strain during subsequent exercise in the heat. On four occasions, eight men performed an exercise-dehydration protocol of approximately 185 min (33 degrees C) to establish a 4% reduction in body weight. Following dehydration, 2% of the fluid lost was replaced during the first 45 min of a 100-min rest period by one of three random Rehy treatments (0.9% saline intravenous; 0.45% saline intravenous; 0.45% saline oral) or no Rehy (no fluid) treatment. Subjects then stood for 20 min at 36 degrees C and then walked at 50% maximal oxygen consumption for 90 min. Subsequent to dehydration, plasma Na(+), osmolality, aldosterone, and arginine vasopressin concentrations were elevated (P < 0.05) in each trial, accompanied by a -4% hemoconcentration. Following Rehy, there were no differences (P > 0.05) in fluid volume restored, post-rehydration (Post-Rehy) body weight, or urine volume. Percent change in plasma volume was 5% above pre-Rehy values, and plasma Na(+), osmolality, and fluid regulatory hormones were lower compared with no fluid. During exercise, skin and core temperatures, heart rate, and exercise time were not different (P > 0.05) among the Rehy treatments. Plasma osmolality, Na(+), percent change in plasma volume, and fluid regulatory hormones responded similarly among all Rehy treatments. Neither a fluid of greater tonicity nor the route of administration resulted in a more rapid or greater fluid retention, nor did it enhance heat tolerance or diminish physiological strain during subsequent exercise in the heat.     

Effects of changes in plasma hepatic-portal and systemic osmolality on plasma concentrations of arginine vasopressin in conscious newborn calves

Systemic plasma concentrations of arginine vasopressin (AVP) were studied in three groups of 10-15 day-old conscious newborn calves. Animals in the first group (control group) and in the second group (systemic-hypertonic-injected group) received respectively isotonic and hypertonic (8 mmol NaCl/kg body weight) saline injection into the right jugular vein. Animals in the third group were fitted with chronic mesenteric and hepatic-portal catheters and received a 1 h-hypertonic saline infusion (2 mmol NaCl/kg body weight) into the main mesenteric vein. In animals in the second group there were parallel increases in systemic plasma concentration of Na+ (from 148.0 +/- 2.6 to 177 +/- 8 mmol/l; P less than 0.01), osmolality (from 289 +/- 2 to 319 +/- 4 mOsmol/kg H2O; P less than 0.01) and systemic plasma concentrations of AVP (from 4.2 +/- 0.4 to 11.1 +/- 0.6 pmol/l; P less than 0.01) 10 min after the injection. There were no significant changes in control animals. Hypertonic saline infusion into the main mesenteric vein in the third group induced an increase in concentration of Na+ (from 147.3 +/- 2.0 to 165.0 +/- 5.0 mmol/l; P less than 0.01) and osmolality (from 288 +/- 5 to 315 +/- 10 mOsmol/kg H2O; P less than 0.01) in hepatic-portal vein plasma but did not alter systemic plasma osmolality or concentrations of Na+ and AVP. This study demonstrates that the relationship between plasma concentrations of AVP and systemic osmolality is operative in the newborn calf but does not support the hypothesis that hepatic portal osmo-receptors sensitive to hyperosmolality influence AVP release.     

Prolactin-releasing activity of arginine vasotocin in vitro.

A significant elevation in both luteinizing hormone (LH) and prolactin release was observed in the culture medium of hemipituitaries from castrated estrogen-progesterone (EP) primed female rats incubated for 5 h with arginine vasotocin (AVT) and luteinizing hormone-releasing hormone (LRH) compared to corresponding halves incubated with LRH alone. However, AVT alone did not significantly alter the discharge of LH or prolactin from hemipituitaries of EP-treated rats in vitro. Arginine vasopressin, a natural analogue of AVT, inhibited prolactin release using this same model system. Normal male rat hemipituitaries incubated with AVT released significantly more LH and prolactin into the culture medium than did their corresponding halves. A log-dose response curve indicated that any dose from 100 ng to 10 mug AVT significantly promoted prolactin release. However, the terminal tripeptide of AVT, Pro-Arg-Gly(NH2), failed to modify the discharge of LH or prolactin into the culture medium.     

Plasma arginine vasotocin and angiotensin II in the water deprived Kelp gull (Larus dominicanus), Cape gannet (Sula capensis) and Jackass penguin (Spheniscus demersus)

1. The basal levels of the osmoregulatory hormones, arginine vasotocin (AVT) and angiotensin II (AII) were measured (by radioimmunoassay) in the plasma of conscious Kelp gulls, Cape gannets and Jackass penguins. 2. The responses of the hormones to similar degrees of hypertonicity and hypovolemia caused by water deprivation have also been determined. 3. Dehydration elevated plasma AVT and plasma AII in all three species. 4. The AVT concentration was increased by 2-4 fold and although in each case the correlation between plasma osmolality and plasma AVT was highly significant (2P less than 0.01), the sensitivity of release was greater in the gull (1.13 pg/ml per mOsm/kg) than in the gannet (0.36 pg/ml per mOsm/kg) or penguin (0.44 pg/ml per mOsm/kg). 5. Dehydration increased plasma AII 3-fold in the three bird types.     

Renal function and plasma arginine vasotocin during an acute salt load in feral chickens

Summary Renal clearance experiments were conducted on feral chickens descended from birds collected from a coral island off the coast of Queensland, Australia. Following a control period when 0.13 M NaCl was used as a vehicle for the renal function markers, a salt load was imposed by infusion of 1 M NaCl. The hypertonic NaCl infusion resulted in increases in glomerular filtration rate (GFR), effective renal blood flow (ERBF), and urine flow rate (V), whereas filtration fraction decreased. Haematocrit was reduced and plasma osmolality, sodium, chloride and potassium concentrations increased. Plasma arginine vasotocin (P_AVT) levels increased from 31.4±2.3 pg·ml^-1 during the control infusion to 56.0±1.7 pg·ml^-1 following a salt load of 12 mmol Nacl·kg^-1 The sensitivity of release of AVT was 0.69±0.11 pg·ml^-1 per mosmol·kg^-1. The concentrations of Na and Cl in urine increased, whereas urine osmolality and potassium concentration decreased. The expansion of the extracellular fluid volume induced by the salt loading would tend to suppress the release of AVT, whereas the osmotic stimulus would provide a stimulus for the release of AVT. In this study, GFR, ERBF and ERPF increased at the same time as P_AVT increased.Abbreviations AVP arginine vasopressin - AVT arginine vasotocin - ERBF effective renal blood flow - ERPF effective renal plasma flow - GFR glomerular filtration rate - P_avt plasma arginine vasotocin concentration - PAH paraaminohippuric acid - SEM standard error of mean - SNGER single nephron glomerular filtration rate - U/P urine to plasma ratio - V urine flow rate     

Prolactin and its involvement in fluid regulation in the bovine

Plasma prolactin (PRL) concentrations, osmolality, water consumption, feed intake, urine excretion, and fecal water output were determined in twelve steers of 3 breeds exposed to 5 feed and water regimes. Breed differences were found in water intake and plasma PRL concentrations when feed and water were ad lib., however, during any of the other 4 treatments, responses were similar between breeds. During dehydration and feed restriction, water intake, urine, fecal water, and plasma prolactin decreased; however, during hydration and refeeding such changes were not as clearly related. No consistent relationships between plasma prolactin and osmolality were found. Data suggests that PRL's role in fluid regulation in the bovine is most likely associated with alterations in renal hemodynamics rather than by changes in plasma osmolality.     

Antidiuretic action of prolactin in the rat with diabetes insipidus.

Rats with hereditary hypothalamic diabetes insipidus, devoid of endogenous ADH, exhibited a prompt antidiuresis when injected subcutaneously or intraarterially with ovine prolactin. The antidiuresis was accompanied by a decrease in free water clearance and an increase in urine osmolality without a change in osmolal clearance or creatinine excretion. Measurement of PAH and insulin clearances indicated that prolactin had no effect on renal plasma flow or glomerular filtration rate. Prolactin injection caused a transient decrease in urinary sodium excretion, but proximal tubular sodium reabsorption, estimated by lissamine green transit time, was unaffected. The antidiuretic effect of prolactin could not be attributed to ADH contamination of the ovine prolactin preparation. Kidney cyclic AMP content was increased significantly 5 min after injection of prolactin. Thus, prolactin has an antidiuretic effect similar to that which occurs as a result of ADH action on the kidney and does not require either the release or the presence of ADH in order to cause the antidiuresis. Further, the impaired water excretion cannot be attributed to an increase in proximal tubular sodium reabsorption or to alteration of renal hemodynamics. It is suggested that prolactin has a direct ADH-like action on the kidney resulting in antidiuresis.     

Effects of a dopaminergic stimulant,amfonelic acid,on anterior pituitary hormone release in conscious rats

The response of plasma LH, Prolactin, GH and TSH levels to systematic administration of a specific central dopaminergic stimulant, amfonelic acid (AFA), by intravenous pulse injection in ovariectomized (OVX) and OVX estrogen-progesterone primed conscious rats has been evaluated. Intravenous injection of 0.2 mg/kg of AFA had no influence on plasma LH concentration until 60 min after injection when it was significantly elevated. Increasing the dose to 1 mg/kg reduced LH titers at 15 and 30 min with a return to preinjection levels by 60 min. AFA produced a dose-dependent decrease in plasma prolactin levels; the decrease occurred as early as 5 min after injection. AFA, both at 0.2 and 1 mg/kg doses, was effective in producing a sharp, dose-related rise in plasma GH levels. By contrast, TSH levels were significantly suppressed by both doses of AFA. Injection of the 1 mg/kg dose of AFA did not modify plasma LH levels in OVX-steroid-primed animals, white producing a comparable effect on plasma prolactin, GH and TSH levels to that observed in OVX animals. The present results indicate that endogenously released DA can have profound effects on pituitary hormone release, inhibiting PRL and TSH discharge, stimulating GH release and either inhibiting or stimulating LH release.     

Water balance and arginine vasotocin in the cocooning frog Cyclorana platycephala (hylidae)

It is well established that forming a cocoon, for frog species capable of doing so, markedly reduces evaporative water loss; however, the capacity of cocooned frogs to maintain hydration during extended estivation is not well understood. The combined effects of long-term estivation and water loss were examined in the cocoon-forming species Cyclorana platycephala by assessing the hydration state of the frogs throughout a 15-mo estivation period. Frogs lost mass throughout the 15-mo period to a maximum of 36%+/-6.5% of their initial standard mass. Plasma osmolality reached maximal levels by the ninth month of estivation at 487 mOsm kg(-1) and then remained stable to the fifteenth month of estivation. Urine osmolality continued to increase to the fifteenth month of estivation, at which point plasma and urine concentrations were isosmotic. The use of bladder water to counter losses from circulation was indicated by the relatively slow rate of increase in plasma osmolality with mass loss and the progressive increase in urine osmolality. For estivating frogs, evidence was found for a possible threshold relationship between plasma osmolality and plasma arginine vasotocin (AVT) concentration. After estivation, plasma AVT concentrations decreased markedly after 15-mo estivators were placed in water for 2 h, suggesting that high levels of AVT may not be integral to rapid rehydration in this species.