Acutely elevated vasopressin increases circulating concentrations of cortisol and aldosterone in fasting northern elephant seal (Mirounga angustirostris) pups

Abstract

SUMMARYThe physiological actions of vasopressin (VP) in marine mammals are not well defined. To help elucidate its hormonal and renal effects in this group of mammals, northern elephant seal (Mirounga angustirostris) pups(N=7; 99±4 kg) were first infused with 0.9% saline (control;220 ml), followed 24 h later with VP (as a 20 ng kg-1 bolus, then 2 ng kg-1 min-1 for approximately 35 min in 225±16 ml saline). During both control and VP periods, blood samples were collected prior to infusion, and 15, 30, 60, 120 min and 24 h after infusion to examine the hormonal responses of the pups to VP. Renal responses were quantified from 24 h urine samples obtained prior to infusion (control) and 24 h post-infusion. Compared to the control period, infusion of VP increased plasma concentrations of cortisol over a 120 min period and aldosterone over 30 min,while plasma renin activity (PRA) was decreased for a 120 min period. The plasma urea:creatinine ratio was elevated following infusion of VP. Urine output and osmotic clearance were increased by 69±18% (mean ± s.e.m.) and 36±10%, respectively, but free water clearance and glomerular filtration rate were not significantly altered 24 h post-infusion of VP. Solute (osmolality, Na+, K+ and Cl-) excretion and fractional excretion of electrolytes were also increased when compared to control values. The increase in cortisol concentration suggests that VP may possess corticotropin releasing hormone-like activity in elephant seals. If osmotic diuresis and natriuresis are typical consequences of elevated [VP] in fasting pups, then not increasing VP normally during the fast may serve as a protective mechanism to avoid the potential loss of Na+ induced by elevated [VP]. Therefore, under natural fasting conditions, pups may be highly sensitive to small changes in[VP], resulting in the maintenance of water and electrolyte balance.

Angiotensin II and Aldosterone Increase with Fasting in Breeding Adult Male Northern Elephant Seals (Mirounga angustirostris)

“…Mean aldosterone concentrations from nursing and postnursing adult female harp seals (300 pg/mL) were similar to those reported here during the late period ( pg/mL) and for lactating adult 254 ‫ע‬ 39 female northern elephant seals (380 pg/mL; D. E. Crocker, unpublished data). In elephant seal pups, mean aldosterone concentrations increased from 200 to 1,500 pg/mL during the first 7 wk of the postweaning fast (Ortiz et al 2000(Ortiz et al , 2002a(Ortiz et al , 2002b(Ortiz et al , 2003Houser et al 2001), representing a 7.5-fold increase, whereas the increase in fasting adult males over a comparable duration was only 1.7-fold. An age-related change in the sensitivity of aldosterone response to fasting may be but one potential explanation for the discrepancy in the degree of change of aldosterone between pups and adults.…”

“…We have previously shown that an acute intravenous infusion of AVP induced a rapid (within 15 min) increase in circulating cortisol in fasting pups, suggesting that AVP stimulated a neuroendocrine response in fasting seals (Ortiz et al 2003). In the current study, cortisol was measured simultaneously (data not reported here) to address the potential concern that the observed concentrations of AVP were the product of a neuroendocrine response induced by the sampling protocol and did not necessarily reflect an osmotic response.…”

“…Previous studies have shown that RAAS in elephant seal pups responds to various stimuli (fasting [Ortiz et al 2000], altered salinity [Ortiz et al 2002a[Ortiz et al , 2002b, AVP infusion [Ortiz et al 2003]), and in West Indian manatees (Trichechus manatus), it responds to changes in water salinity (Ortiz et al 1998). However, to date, no studies of mammals adapted to prolonged fasting have demonstrated an association between changes in some component of the RAAS and changes in some parameter of water conservation, such as reduced water efflux rate.…”

“…Only a few studies in pinnipeds have demonstrated an association between AVP (either infused as pitressin or quantified by changes in concentration) and reduced urine output or calculated free water clearance (Bradley et al 1954;Page et al 1954;Hong et al 1982;Ortiz et al 2002b). If the observed concentrations of AVP are physiologically significant, then the relatively higher concentrations of AVP in adults versus pups (Ortiz et al 1996(Ortiz et al , 2000(Ortiz et al , 2002a(Ortiz et al , 2002b(Ortiz et al , 2003Zenteno-Savin and Castellini 1998) suggest that the renal collecting ducts may be hyposensitive to AVP during the fasting period and thus necessitate higher circulating concentrations to elicit a physiological response. Thus, the lack of an increase in AVP does not necessarily preclude AVP from contributing to water and osmotic homeostasis in fasting adult seals.…”

“…0.05 commercially available kits for Ang II and AVP (Phoenix Pharmaceuticals, Belmont, CA) and aldosterone (DPC, Los Angeles). Each assay had been validated previously for use with northern elephant seal samples (Zenteno-Savin and Castellini 1998; Ortiz et al 2000Ortiz et al , 2002aOrtiz et al , 2002bOrtiz et al , 2003Houser et al 2001). For measurements of Ang II and AVP, separate 1.0-mL aliquots were extracted using C18 columns (Prep-Sep, Fisher Scientific, Fair Lawn, NJ) as previously described (Zenteno-Savin and Castellini 1998).…”