Breeding condition, temperature, and the regulation of salt and water by pituitary hormones in the red-spotted newt, Notophthalmus viridescens

Aquatic-to-terrestrial habitat shift reduces energy expenditure in newts

Many organisms seasonally modify their standard metabolic rates (SMR). However, the diversity of cues triggering the acclimatization response remains little understood. We examined the influence of experimentally induced aquatic-to-terrestrial habitat shift on the thermal sensitivity of SMR in newts. Standard metabolic rates increased with temperature (13-23°C), although consistently lower in terrestrial than aquatic individuals. Motor activity during respirometry trials decreased with temperature at similar rates in both groups. We conclude that in newts, a habitat shift might represent an important modulator of the seasonal acclimatization response in SMR. Lowered SMR suggests the potential to reduce newt maintenance costs and depletion of caloric reserves during the activity-limited period on land.

Integument structure and function in juvenile Xenopus laevis with disrupted thyroid balance

The skin is the largest organ in the body and is a barrier between the internal and external environment. The present study evaluates how PTU, a goitrogen, that is used to treat hyperthyroidism affects the structure and electrical properties of the frog (Xenopus laevis) skin. The results are considered in the context of the two-membrane model established in the seminal work of Ussing and collegues in the 1940s and 1950s. In vitro experiments with skin from Xenopus adults revealed that PTU can act directly on skin and causes a significant increase (p<0.05, One-way ANOVA) in short circuit current (Isc) via an amiloride-insensitive mechanism. Juvenile Xenopus exposed to waterborne PTU (5 mg/L) had a significantly bigger and more active thyroid gland (p<0.01, Student's t-test) than control Xenopus. The bioelectric properties of skin taken from Xenopus juveniles treated with PTU in vivo had a lower Isc, (3.05±0.4, n=13) and Rt (288.2±39.5) than skin from control Xenopus (Isc, 4.19±1.14, n=14; Rt, 343.3±43.3). A histological assessment of skin from PTU treated Xenopus juveniles revealed the epidermis was significantly thicker (p<0.01, Student's t-test) and had a greater number of modified exocrine glands (p<0.01, Student's t-test) in the dermis compared to control skin. Modifications in skin structure are presumably the basis for its changed bioelectric properties and the study highlights a site of action for environmental chemicals which has been largely neglected.

Hormonal effect on the osmotic, electrolyte and nitrogen balance in terrestrial Amphibia

Two main hormones regulate water balance in amphibian. First, mesotocin (MT) acting as a diuretic agent, and second arginine vasotocin (AVT) being an anti-diuretic hormone. In addition, prolactin (PRL), aldosterone, corticosterone, angiotensin II and atriunatriuretic hormones, play a role too in regulating water and ion balance. The hormones affect the epidermis and bladder permeability to water and ions as well as the kidney through the control of the glomerular filtration rate (GFR). The main questions concern the presence and action of these hormones during the amphibian's life history. Are they present in both larval and adult stages? Are these hormones being synthesized in both aquatic and terrestrial adult phases? Under what circumstances are they being stored or released? Would the target organs (epidermis, bladder, kidney) respond in a similar way during all periods? The problem is the fact that under most circumstances an amphibian while in an aquatic environment responds physiologically differently than when on land. Only partial information concerning hormone presence, release and control of water balance is available at the moment, and even that is fragmentary and based on only a very small number of amphibian species.

Prolactin alters the expression of integumental glycoconjugates in the red-spotted newt, Notophthalmus viridescens

Prolactin (PRL)-mediated changes in the texture and secretory activity of the skin in adult red-spotted newts may involve alterations in the distribution and/or expression of structural and secretory epidermal glycoconjugates. To explore this possibility, skin samples were obtained from groups of conditioned animals that had received injections of either ovine prolactin or amphibian saline over a 14-day period. Glycoconjugates within the epidermis and cutaneous glands were examined by means of lectin histochemistry using a panel of eight HRP-labelled lectins. PRL increased levels of sialic acid and n-acetylglucosamine in the stratum corneum. In contrast, glycoconjugates containing fucose, galactose, n-acetylgalactosamine, and galactose-(1,3)-n-acetylgalactosamine were decreased by PRL within both glands and epidermis. These results suggest that the integumental effects associated with prolactin in the red-spotted newt are mediated, at least in part, through the alteration of epidermal and glandular glycoconjugates.

Amphibian prolactins: activity in the eft skin transepithelial potential bioassay

The effects of purified prolactins isolated from frogs (fPRL; Rana catesbeiana) and newts (nPRL; Cynops pyrrhogaster) were compared with those of ovine prolactin (oPRL) and thyrotropin releasing hormone (TRH) in the eft (Notophthalmus viridescens) skin transepithelial potential (TEP) bioassay. At total doses as low as 0.4 micrograms/animal, both fPRL and nPRL were as effective as oPRL in reducing eft skin TEP. By contrast, TRH at total doses as high as 400 micrograms/animal was ineffective in altering eft skin TEP. These data provide the first direct evidence that amphibian PRLs can exert significant physiological control over salamandrid integumental sodium transport.

Electrophysiology of newt skin: effects of prolactin

Integumental transepithelial potential (TEP) was measured by both in vivo and in vitro (Ussing chamber) techniques in the same adult terrestrial-phase California newts (Taricha torosa). In both types of preparation, TEP showed a logarithmic relation to external sodium (as Na2SO4) concentrations between 0.1 and 10 meq/liter, with in vivo values exceeding in vitro values at all points. KM was ca. 1 and 5 meq/liter for the in vivo and in vitro preparations, respectively. When terrestrial-phase newts were treated with prolactin (PRL; 50 mIU/g/day), in vivo TEP declined significantly within 3 days. In vivo TEP versus [Na+]ext curves of recently collected aquatic-phase newts were found to be not significantly different from those of PRL-treated terrestrial-phase animals. Current/voltage (I/V) plots likewise showed that skin from aquatic-phase and PRL-treated terrestrial-phase newts was electrophysiologically alike. Estimates and calculations based on the I/V relationship suggest that PRL acts primarily to increase the epithelial electrical resistance in the transcellular pathway, with no effect on the electromotive force of the sodium pump.

Effect of prolactin on transcutaneous Na transport in the Japanese newt, Cynops pyrrhogaster

The effect of ovine prolactin (PRL) on the electric parameters related to active Na transport across the skin was investigated in the Japanese newt, Cynops pyrrhogaster. Spring newts living in a pond or stream for breeding and autumn newts migrating from water to land for hibernation were collected and used for the experiments. Autumn newt skin was found to have a higher transepithelial potential difference (TEP) and higher resistances to the active Na current (RNa) and to the shunt pathway (RS) than spring newt skin. Injection of PRL, 20 micrograms/g body wt, into autumn newts every other day for 2 weeks induced an increase in RNa, resulting in decreases in TEP and the short-circuit current (SCC), but barely changed the skin resistance (RM), the electromotive force of the active Na current (ENa), or RS. On the other hand, such injections of PRL into spring newts induced decreases in TEP and SCC, but had no effect on RM, ENa, RNa, or RS. Autumn newts appear to be more sensitive to PRL than spring newts. Insofar as autumn newts are concerned, it is concluded that the effect of PRL is probably due to blockade of the Na channel rather than inhibition of Na pump activity.

Interrenal function in larval Ambystoma tigrinum. II. Control of aldosterone secretion and electrolyte balance by ACTH

Renal clearance techniques were used to assess the role of ACTH on renal electrolyte transport in larval Ambystoma tigrinum. Radioimmunoassay was employed to evaluate changes in circulating aldosterone in these animals. Larvae were hypophysectomized and maintained for 1 week on either ACTH replacement therapy (50 ng/g) or sham injections prior to clearance measurements. Hypophysectomy significantly lowered plasma [Na+] (from 96 to 90 mM), plasma [K+] (from 6 to 4 mM), plasma aldosterone titer (from 157 to 36 pg/ml), fractional Na+ reabsorption (from 97 to 94%), and fractional K+ reabsorption (from 68 to 50%). ACTH replacement restored plasma [Na+] to 96 mM, aldosterone titer to 157 pg/ml, fractional Na+ reabsorption to 96%, and fractional K+ reabsorption to 75%. When steroid synthesis was blocked in a separate set of larvae; ACTH was unable to reverse the sodium depletion which results from adaptation to distilled water. This suggests that ACTH is not acting directly on Na+ transport but acts through a steroid like aldosterone. When larvae were injected intravenously with antialdosterone antibodies their fractional Na+ reabsorption decreased from 95 to 87%. We conclude, therefore, the ACTH works via interrenal steroids, such as aldosterone, to control renal electrolyte transport in this species.

Changes in body water and plasma constituents during bullfrog development: effects of temperature and hormones

The osmoregulatory responses to warmer temperatures and hormone treatment in cold-adapted (5 degrees C) Rana catesbeiana tadpoles and newly metamorphosed frogs were examined. Tadpoles transferred to 11 degrees C and 18 degrees C and left for 5 days lost 7% and 10% of their body weight. Plasma [Na+] was elevated 28% and 21%, respectively. Control (5 degrees C) animals maintained their body weight and plasma [Na+] constant. Daily treatment with either ovine prolactin (oPRL) or ovine growth hormone (oGH) prevented the weight loss and the increase in extracellular [Na+] that occurred when tadpoles were transferred to 18 degrees C. Neither propylthiouracil (PTU) nor arginine vasotocin (AVT) were effective in countering temperature-induced weight loss in tadpoles. Newly metamorphosed frogs transferred to 18 degrees C also lost weight; this was not prevented by daily treatment with saline, oPRL, oGH or PTU. However, in frogs treated daily with AVT, initial BW was regained by day 6. When warm-adapted (18 degrees C) tadpoles were treated daily for 18 days with saline, bPRL, bGH, thyroxine (T4), ergocornine, cortisol, or cortisol + T4, bPRL was most effective in retarding weight loss and maintaining body water content, whereas T4 + cortisol caused the greatest loss of weight and body water. By day 20, the correlations between weight loss and both body water content and hematocrit were highly significant. These data suggest that reported increases in plasma solute concentrations in larval amphibians may actually reflect decreases in extracellular fluid volume, rather than increased amounts of solutes, per se.

Thyroid function in adult rough-skinned newts (Taricha granulosa): effects of hypophysectomy and hormone replacement

This study examines the regulation of thyroid activity in adult rough-skinned newts, Taricha granulosa. Aquatic, breeding-condition newts were hypophysectomized and were injected for 23 days with the mammalian adenohypophysial hormones: ACTH, PRL, and GH. During a fourth week of treatment, these newts also received TSH. Plasma T4 concentrations were lower in hypophysectomized newts than in sham-hypophysectomized newts 23 days after surgery. Both ACTH and PRL increased T4 titers above these in saline-injected control newts; GH alone had no effect. TSH effectively increased the plasma T4 concentrations in newts from all five groups. GH, and possibly PRL, depressed the response to TSH. Plasma T4 concentration was positively correlated with body fluid loss, suggesting that hormone concentration/dilution effects may occur generally in animals such as these that undergo substantial seasonal changes in degree of hydration.

Sensitivity and specificity of salamandrid integumental transepithelial potential to prolactin

The effects of ovine prolactin (oPRL), bovine growth hormone (bGH) and human placental lactogen (hPL) on in vivo integumental transepithelial potential (TEP) were examined in two salamandrid urodeles, adult terrestrial-phase Taricha granulosa and the juvenile red-eft stage of Notophthalmus viridescens. TEP in efts treated with 1.0 microgram oPRL/2 days fell from 73.3 +/- 6.1 to 15.2 +/- 5.5 mV by Day 7 (P less than 0.001), whereas the TEP of efts treated with either 1.0 or 10 micrograms bGH/2 days remained at control levels for as long as 24 days. TEP in efts treated with a single dose of 10 micrograms oPRL dropped from 65.74 +/- 4.1 to 23.0 +/- 3.4 mV (P less than 0.01) in 3 days. Efts treated with various doses of oPRL showed a linear log total dose response over the range of 0.05 to 10.0 micrograms oPRL/animal, with a minimum detectable total dose of 0.4 micrograms/g (0.01 IU/g). In the same experiments, tail height increased by Day 7 in efts treated every other day with 10.0 micrograms oPRL, but not 1.0 microgram oPRL or either 1.0 or 10.0 micrograms bGH/2 days. In Oregon newts injected every other day with 10 micrograms oPRL, TEP decreased by 33% in 8 days (P less than 0.05), whereas in animals treated with 10 micrograms bGH/2 days, TEP did not change from control values even after 23 days. TEP in Oregon newts receiving a single dose of 100 micrograms oPRL dropped to 68% of initial values within 2 days (P less than 0.05), but subsequently recovered to control values 3 weeks after the last injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Osmoregulatory changes during the aquatic-to-terrestrial transition in the rough-skinned newt, Taricha granulosa: the roles of temperature and ACTH

Aquatic breeding-condition newts kept at 5, 11, or 18 degrees showed temperature-dependent changes in body weight (BW), tail height, plasma [Na+], hematocrit, integumental osmotic permeability, urine production, and transepithelial potential. Net urine production could account for the change in weight observed in animals kept at 11 and 18 degrees, suggesting that initial weight change results from a reduction in extracellular volume. Weight loss was correlated with increased hematocrit and plasma [Na+]. In another experiment, newts were sham-hypophysectomized (sham-HX) or hypophysectomized (HX) and injected with saline, ovine prolactin (PRL), ovine growth hormone (GH), or porcine adrenocorticotropic hormone (ACTH) and maintained at 5 degrees for 13 days. All animals lost 3-7% of their initial BW. Plasma [Na+] was reduced in animals injected with hormone compared with both sham-operated and saline-injected controls. Following an increase in water temperature to 11 degrees for 13 days, weight loss in sham-operated and ACTH-injected newts increased to 16% of BW compared with a total of 4-8% in newts injected with PRL, GH, or saline. In this experiment also, loss of body weight was correlated with increased plasma [Na+]. Although measurements of plasma Na+ concentrations indicated that ACTH caused "sodium retention," estimates of changes in total extracellular Na+ revealed that both sham-operated and ACTH-injected newts experienced a net sodium loss. In Taricha granulosa, warm temperatures and ACTH (presumably acting via interrenal hormones) appear necessary for the fluid and electrolyte loss which accompanies the transition from the aquatic to the terrestrial condition.