Potent osmoregulatory actions of homologous adrenomedullins administered peripherally and centrally in eels

Abstract

The teleost adrenomedullin (AM) family consists of three groups, AM1/AM4, AM2/AM3, and AM5. In the present study, we examined the effects of homologous AM1, AM2, and AM5 on drinking and renal function after peripheral or central administration in conscious freshwater eels. AM2 and AM5, but not AM1, exhibited dosedependent (0.01-1 nmol/kg) dipsogenic and antidiuretic effects after intra-arterial bolus injection. The antidiuretic effect was significantly correlated with the degree of associated hypotension. To avoid the potential indirect osmoregulatory effects of AM-induced hypotension, infusion of AMs was also performed at nondepressor doses. Drinking was enhanced dose-dependently at 0.1-3 pmol ⅐ kg Ϫ1 ⅐ min Ϫ1 of AM2 and AM5, matching the potency and efficacy of angiotensin II (ANG II), the most potent dipsogenic hormone known thus far. AM2 and AM5 infusion also induced mild antidiuresis, while AM1 caused antinatriuresis. Additionally, AMs were injected into the third and fourth ventricles of conscious eels to assess their site of dipsogenic action. However, none of the AMs at 0.05-0.5 nmol induced drinking, while ANG II was highly dipsogenic. AM2 and ANG II injected into the third ventricle increased arterial pressure while AM5 decreased it in a dose-dependent manner, and both AM2 and AM5 decreased blood pressure when injected into the fourth ventricle. These data suggest that circulating AM2 and AM5 act on a target site in the brain that lacks the blood-brain barrier. Collectively, the present study showed that AM2 and AM5 are potent osmoregulatory hormones in the eel, and their actions imply involvement in seawater adaptation of this euryhaline species. drinking; antidiuresis; angiotensin; intracerebroventricular administration; teleost fish TELEOST FISH maintain their body fluid osmolality at one-third of seawater, whether they are in freshwater or in seawater. To achieve body fluid homeostasis, fish regulate water and ion transport in osmoregulatory organs such as the gill, kidney, and digestive tract. Marine teleost fish suffer from water efflux and ion influx as a result of the difference in tonicity between extracellular fluid and environmental seawater, while the opposite fluxes occur in freshwater fish. For adaptation to hyperosmotic seawater, therefore, it is essential for fish to excrete excess ions and to absorb water from the environment. To this end, marine teleosts drink a large volume of seawater, which is absorbed through the digestive tract, and ions are actively excreted by the gill and kidney. Several hormones are known to be involved in the regulation of water intake in teleosts. Angiotensin II (ANG II) is a well-established dipsogenic hormone throughout vertebrates including fish (8, 38). In contrast, atrial natriuretic peptide (ANP) suppresses drinking rate (41) and also has an antidiuretic effect in teleost fish (37). In addition to these hormones, growth hormone (GH) and cortisol are known to be involved in adaptation to and survival in seawater (26). On the other hand, prolactin contr...

Potent cardiovascular effects of homologous urotensin II (UII)-related peptide and UII in unanesthetized eels after peripheral and central injections

“…Furthermore, applied URP and UII constricted the branchial and systemic arteries in eels, suggesting that the vasopressor effects of peripherally injected URP and UII are, at least in part, attributable to direct actions on the vascular smooth muscle. In previous experiments in eels using adrenomedullin 2, the cardiovascular effects were different when peptides were injected peripherally or centrally (23). However, the vasopressor effects of URP and UII were quite similar following peripheral and central injections with respect to their preferential action on branchial circulation and duration of effect.…”

“…Eight SW-adapted eels (192.0 Ϯ 2.1g) were used for this experiment. To inject peptides into the brain, a stainlesssteel cannula was surgically placed into the third ventricle, according to published protocols (23). Briefly, the optic tectum of the brain was exteriorized by making a hole in the skull and removing the dura mater.…”

Genetic basis for variation in salinity tolerance between stickleback ecotypes

“…Calcrl gene encodes receptors for calcitonin gene‐related peptide and/or adrenomedullins, and these peptides are thought to play important roles in osmoregulation (Ogoshi et al . ). Another candidate gene cldn10 encodes a component of tight junctions.…”

Central ventilatory and cardiovascular actions of calcitonin gene-related peptide in unanesthetized trout

“…In vivo studies are crucial to determine the action of peptides in cardiovascular regulation and, to our knowledge, this approach has not yet been performed for CGRP in a non-mammalian vertebrate. In the eel model, the structurally related peptides AM2 and AM5 have potent hypotensive actions when injected peripherally, but no change occurred in f H Ogoshi et al, 2008). In addition, a high dose of AM1 (5nmolkg -1 body mass) increases P DA after an initial and transient hypotension.…”