PHYLOGENETIC DISTRIBUTION OF THE EFFECTS OF NEUROHYPOPHYSIAL HORMONES ON WATER AND SODIUM METABOLISM

Distinct hydro-osmotic receptors for the neurohypophysial peptides vasotocin and hydrins in the frog Rana esculenta

The biological properties of vasotocin, hydrin 1 (vasotocinyl-Gly-Lys-Arg) and hydrin 2 (vasotocinyl-Gly), in particular the hydro-osmotic activities on the frog skin, the frog urinary bladder and the frog kidney, have been compared. Hydrins are as active or more active than vasotocin on the first two organs but they are virtually devoid of antidiuretic activity in the rat and the frog, in contrast to vasotocin. It appears that where the oxytocin ring (residues 1-6), present in the three peptides, is necessary for the action on the three organs, the C-terminal amidated group of vasotocin is necessary for the renal receptor but not for the skin and bladder receptors. It is known that amphibians have two types of vasotocin receptors, V1 and V2, homologous to the vascular/hepatic V1 and the renal V2 vasopressin receptors of mammals, respectively. We suggest that adaptation has led to specialization of (at least) two subtypes of hydro-osmotic V2 receptors, the renal subtype on which vasotocin is mainly active for the reabsorption of tubular water, and the skin/bladder subtype on which hydrin 2 is specifically involved in ensuring the rehydration of the animal. Cooperative evolution might have created in anuran Amphibia, on the one hand, two hydro-osmotic peptides, vasotocin and hydrin 2, derived from a single precursor through differential processing; and on the other hand, two corresponding receptors in kidney and skin for internal and external water recovery.

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.

Vasotocin and hydrin 2 (vasotocinyl-Gly) in the African toad Bufo regularis: study under various environmental conditions

1. The neurohypophysial osmoregulatory hormones of the African toad Bufo regularis, a species adapted to estivate under dry and hot conditions, have been investigated. Vasotocin and hydrin 2 (vasotocinyl-Gly) have been identified by their retention times in high-pressure reverse-phase liquid chromatography and coelution with synthetic peptides, their pharmacological properties (vasotocin) and microsequencing. 2. Vasotocin-associated neurophysin (MSEL-neurophysin type) has been characterized by its N-terminal amino acid sequence. 3. In toads subjected to dehydration by evaporation (20% weight loss) or to osmotic stress by immersion in 2% NaCl for 3 hr (6% weight loss), the molar ratio hydrin 2/vasotocin (about 2:1) remained similar to the one observed in control animals. 4. In toads exposed to saline solution, there was a large decrease (roughly 30%) in the amounts of both hormones in the neuro-hypophysis. Environmental conditions for distinct secretions of vasotocin and hydrin 2 remain to be found.

Role of pars nervosa of the hypophysis in amphibian water economy: a re-assessment

1. Responses in renal function and in water permeability of skin and bladder to wet and dry environments are accomplished within the range of normal hydration of the amphibian organism. 2. Urine production is discontinued at moderate dehydration. 3. Strong dehydration is needed to raise plasma arginine vasotocin (AVT). 4. Surgical interference with hypophysial function may repress water balance responses because of pars distalis dysfunction, with no clear effect of elimination of pars nervosa function. 5. Antidiuretic hormones, along with adrenergic agonists, may be potent stimulators of the water permeability of membranes of variable permeability, such as skin of terrestrial anurans. 6. AVT does not play a key role in amphibian water economy, but may exert a modulatory role in the control of renal function, secondary to nervous control.

Effect of breeding state, moulting, dehydration, exposure to saturated atmosphere, and arginine vasotocin on cutaneous water permeability in the toad Bufo bufo

Cutaneous water permeability was assessed in hydrated male toads under a variety of conditions, including dehydration and rehydration, and the effects of exogenous arginine vasotocin (AVT) were determined. Cutaneous water permeability (the rate of water uptake by toads in water) was high in toads collected in the breeding pond and declined steeply during the first week, coincident with reduced activity of the cutaneous mucus glands. The slopes of the dose-response curves relating AVT to cutaneous water influx were about the same at the transition from the breeding to the nonbreeding state, but the level of influx was higher in the breeding state. The dose-response relationship in long-term terrestrial-acclimated toads was similar to that in water-acclimated toads. The threshold dose for effect on the cutaneous water permeability was about 1 ng AVT. Dehydration had a substantially greater effect on the cutaneous water permeability than AVT. The ratio between dehydration and AVT responses tended to increase with increasing water transport capacity of the skin. Moulting and acclimation to a saturated atmosphere in fully hydrated toads more than doubled the water permeability and abolished the response to AVT. It is suggested that AVT and other factors increase the cutaneous water permeability by similar mechanisms, such as insertion of water channels in the apical membrane of the epidermal cells. The effect of AVT on the toad skin is interpreted as reflecting the general high potency of neurohypophysial hormones in stimulating the water permeability of membranes of variable permeability.

Hydrins, hydroosmotic neurohypophysial peptides: osmoregulatory adaptation in amphibians through vasotocin precursor processing

From neurointermediate pituitary glands of Xenopus laevis and Rana esculenta, previously unreported peptides termed hydrins, active on water permeability of frog urinary bladder and frog skin (Brunn or "water-balance" effect), have been isolated and sequenced. These peptides seem to be derived from the pro-vasotocin-neurophysin precursor. Hydrin 1, found in Xenopus, has been identified as vasotocin C-terminally extended with the Gly-Lys-Arg sequence; hydrin 2, found in Rana, has been identified as vasotocin C-terminally extended with glycine. Hydrin 2 has been detected in several Ranidae (R. esculenta, Rana temporaria, Rana pipiens) and Bufonidae (Bufo bufo, Bufo ictericus) and appears to have a large distribution in terrestrial or semiaquatic anurans. Hydrins, in contrast to vasotocin, are not active on rat uterus or rat blood pressure. They are absent from other vasotocin-bearers such as birds and could be involved specifically in water-electrolyte regulation of amphibians.

8-BrcAMP-induced capacitance and transport of H2O and Na in skin and urinary bladder of urodele amphibians

The effects of 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) on capacitance (C), osmotic water flow (Jv), and amiloride-sensitive short-circuit current (INa) were studied in bladder and skin derived from the tiger salamander (aquatic and postmetamorphosed terrestrial phase). 8-BrcAMP-dependent increases in C, measured from the transepithelial voltage response to constant current pulses, occurred in aquatic (delta C = 44%) and terrestrial (delta C = 61%) bladders and terrestrial skin (delta C = 19%). Jv (200-mosM gradient, mucosal side hypotonic) was observed in the bladders and was further enhanced by addition of 8-BrcAMP [10(-3) M; delta Jv = 0.42 microliter.min-1.microF-1 (aquatic) and 0.32 microliter.min-1.microF-1 (terrestrial)]. The aquatic and terrestrial skins were relatively impermeable to water, but the terrestrial skin showed a small response to 8-BrcAMP (delta Jv = 0.04 microliter.min-1.microF-1). 8-BrcAMP-mediated natriferic responses were observed in aquatic bladder (delta INa = 62%) and terrestrial skin (delta INa = 105%). Antidiuretic hormone (ADH)-induced Jv was also observed in the aquatic bladder (delta Jv = 0.33 microliter.min-1.microF-1) and was similar to the 8-BrcAMP-mediated Jv measured in this tissue. The terrestrial bladder displayed a more vigorous response to 8-BrcAMP than to ADH (delta JvADH = 0.09 microliter.min-1.microF-1 and delta Jv8-BrcAMP = 0.32 microliter.min-1.microF-1), suggesting that diminished sensitivity to ADH accompanies the transition from water to land in this species.

Effects of arginine vasotocin on water balance of three leiopelmatid frogs

The effects of arginine vasotocin (AVT) on water movements in three frogs of the anatomically primitive genus Leiopelma were compared in vivo and in vitro. Treatment with AVT in vivo (7 x 10(-13) mol/g body weight) led to significant water retention in the terrestrial species L. archeyi and L. hamiltoni, but not in the semiaquatic species L. hochstetteri. All three species showed increasing water retention over some part of the dose range 7 x 10(-15)-7 x 10(-11) mol/g. Isolated pelvic skin from L. archeyi and L. hamiltoni increased in osmotic water flow (OWF) in response to AVT, whereas that from L. hochstetteri did not. Dorsal skin from all three species, as well as pectoral skin from L. archeyi and L. hochstetteri, showed no hydroosmotic response to AVT. Isolated bladders of all species are small and showed small increases in OWF in response to AVT. Glomerular filtration rate in vivo was reduced significantly by AVT in L. archeyi and L. hamiltoni, but not in L. hochstetteri. These results show that water balance responses to AVT among the three species are correlated with habitat, and indicate that the absence of a cutaneous hydroosmotic response in L. hochstetteri is not a characteristic, primitive feature of the entire genus.

Water balance of five amphibian species at different stages and phases, as affected by hypophysial hormones

Five amphibian species were studied for the effect of hypophysial hormones on their water balance. The species were three anurans, Rana ridibunda, Bufo viridis and Pelobates syriacus, and two urodeles Salamandra salamandra and Triturus vittatus. In the first four species different stages of development were studied, in the newt both the terrestrial and aquatic phases of the adult were examined. The hormones used were oxytocin (OXY), arginine vasotocin (AVT) and prolactin (PL). Oxytocin caused most water retention when compared with the other hormones, especially responding were juveniles of Rana and Bufo, but also the terrestrial phase of the adult newt Triturus. Arginine vasotocin affected mostly juvenile Pelobates. Prolactin caused water retention in juvenile Rana and in the terrestrial phase of Triturus. In general the hormones affected the juvenile stages more than either larvae or adults.

Natriferic and hydrosmotic effects of neurohypophysial peptides and their analogues in augmenting fluid uptake by Bufo melanostictus

1. The effect of intramuscular injection of 8-arginine vasotocin, 8-arginine vasopressin, 8-lysine vasopressin, oxytocin, 8-ornithine oxytocin and 8-ornithine vasopressin on fluid uptake across the skin was studied in the live toad, Bufo melanostictus, bathed either in distilled water or in NaCl solution (0.1 g/100 ml.).2. When the bathing solution was distilled water, 8-arginine vasotocin was the most potent, 0.14 nmole/kg augmenting the rate of fluid uptake by 50%. Compared with it the others had relative potencies of: 8-arginine vasopressin 0.8, 8-lysine vasopressin 0.8 x 10(-3), oxytocin 0.8 x 10(-3), 8-ornithine oxytocin 0.8 x 10(-2), 8-ornithine vasopressin < 1.4 x 10(-4).3. When the bathing solution contained 0.1% NaCl, 8-arginine vasotocin was again the most potent, 0.06 nmole/kg augmenting the rate of fluid uptake by 50%. Compared with it the others had relative potencies of: 8-arginine vasopressin 0.3, 8-lysine vasopressin 0.3 x 10(-3), oxytocin 0.3 x 10(-2), 8-ornithine oxytocin 0.8 x 10(-2), 8-ornithine vasopressin < 0.6 x 10(-4).4. Dose-response curves for each peptide showed that in the case of 8-arginine vasopressin, 8-lysine vasopressin and 8-ornithine vasopressin the augmentation of rate of fluid uptake did not differ in the absence or in the presence of NaCl in the bathing solution; whereas in the case of 8-arginine vasotocin, oxytocin, and 8-ornithine oxytocin the augmentation was greater in the presence of sodium chloride.5. Support has been found for the postulate of a binary action of some neurohypophysial peptides on amphibian skin, arginine in position 8 being correlated with hydrosmotic effect, and isoleucine in position 3 with natriferic effect.