Hydrins

Definition
Hydrins 1 and 2 have been isolated from the pituitary glands of Xenopus laevis and Rana esculenta, respectively. It is assumed that the hydrins could be involved in the water-electrolyte regulation of amphibians.

Discovery
Earlier reports show that neurohypophysial extracts of anurans contain a third class of peptides. Isolation of neurohypophysial peptides by high pressure liquid chromatography (HPLC) from a number of anuran Amphibia has revealed the presence of peptides that is not detected in other non-mammalian tetrapods. These peptides were sequenced and named “hydrins” in 1989.

Structural Characteristics
Hydrins seem to be derived from the pro-vasotocin-neurophysin precursor. The differential maturation of provasotocin leads to processing-arrested intermediates, namely vasotocinyl-Gly (Hydrin 2) in virtually all anuran amphibians and vasotocinyl-Gly-Lys-Arg (Hydrin 1) in Xenopus laevis. These intermediates result from a down regulation of the alpha-amidating enzyme or carboxypeptidase E, respectively. 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 amphibians1.

Mode of Action
Distinct hydroosmotic receptors for the neurohypophyseal peptides vasotocin and Hydrins: A comparative study was conducted to examine 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. It has been shown that Hydrins are as active as 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. Further, 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 has been suggested 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 animal2.

Functions
Effects of hydrin2 on cutaneous electrical properties of Rana pipiens -
In a study, electrical properties were measured in isolated skin patches from leopard frogs (Rana pipiens) to explore the possibility that hydrin 2 (vasotocinyl-gly) may regulate cutaneous ion transport. Hydrin 2 is a neurohypophysial peptide commonly implicated in rehydration of anurans. Experimental frogs were given either one or two intraperitoneal (i.p.) injections of hydrin 2 in saline solution, each calculated to achieve a final blood concentration of 81 ng/ml. After a 2-h equilibration period, the frogs were euthanized and abdominal skin removed and used for study. Results of this study confirm that exposure of R. pipiens to hydrin 2 promotes a highly significant increase in transepithelial potential (TEP) and short-circuit current (Isc).In a dose–response study, 10-fold increases in serosal hydrin 2 concentration triggered significant changes in TEP and Isc, with maximal responses observed at 10 ng/ml. Because hydrin 2 is known to facilitate “cutaneous drinking” in anurans, this study suggests a correlation between ion and water transport across the skin3.

A fluorescent analogue of hydrin 1: a new probe for vasotocin receptors - In a study, deamino and fluorescein analogues of hydrin 1 were used to characterize their physiological action in the urinary bladder of the toad, Bufo marinus. It was shown that 1-Deamino-hydrin 1 (d-hydrin) was more potent than vasotocin in stimulating osmotic water flow across intact bladders and more potent than vasotocin in displacing tritium-labeled vasopressin [(3H] AVP) from cell membranes. Furthermore, 1-Deamino-[11-lysine (fluorescein)]-hydrin 1 (flu-hydrin) was found to be the most potent fluorescent vasotocin receptor probe synthesized to date. Flu-hydrin increased osmotic water flow across bladders with a half-maximal effective dose (ED50) value of 6 x 10(-10) M and displaced [3H]AVP from membranes with a half-maximal concentration (IC50) value of 3 x 10(-9) M. This study shows that d-hydrin can serve as a foundation molecule to which reporter groups, such as fluorescent residues, can be attached with better preservation of hydrosmotic activity than is possible with similar modifications of vasotocin4.

References
1.Rouille Y, Michel G, Chauvet MT, Chauvet J, Acher R (1989). Hydrins, hydroosmotic neurohypophyseal peptides osmoregulatory adaptation in amphibians through vasotocin precursor processing. Proc. Natl. Acad. Sci, 86:5272–5275.
2.Rouille Y, Ouedraogo Y, Chauvet J, Acher R (1995). Distinct hydroosmotic receptors for the neurohypophyseal peptides vasotocin and hydrins in the frog Rana esculenta. Neuropeptides, 29:301–307.
3.Ford NA, Robinson GD (2003). Effects of hydrin 2 on cutaneous electrical properties of Rana pipiens. General and Comparative Endocrinology, 134 (2):103-108.
4.Eggena P, Ma CL, Lu MQ, Buku A (1990). A fluorescent analogue of hydrin 1: a new probe for vasotocin receptors. Am J Physiol Endocrinol Metab, 259:524-528.