Prolonged fasting increases the response of the renin-angiotensin-aldosterone system, but not vasopressin levels, in postweaned northern elephant seal pups

The 8- to 12-week postweaning fast exhibited by northern elephant seal pups (Mirounga angustirostris) occurs without any apparent deleterious effects on fluid and electrolyte homeostasis. However, during the fast the role of vasopressin (AVP) has been shown to be inconclusive and the involvement of the renin-angio-tensin-aldosterone system (RAAS) has yet to he examined. To examine the effects of prolonged fasting on these osmoregulatory hormones, 15 postweaned pups were serially blood-sampled during the first 49 days of their fast. Fasting did not induce significant changes in ionic or osmotic concentrations, suggesting electrolyte homeostasis. Total proteins were re duced by day 21 of fasting and remained depressed, suggesting a lack of deh ydration. Aldosterone and plasma renin activity exhibited a correlated, lin ear increase over the first 49 days of the fast, suggesting an active RAAS. Aldosterone exhibited a parabolic trend over the fast with a peak at day 3 5, suggesting a shift in the sensitivity of the kidney to aldosterone later in the fast. AVP was elevated at day 49 only, but concentrations were rela tively low. RAAS was modified during the postweaning fast in pups and appea rs to play a significant role in the regulation of electrolyte and, most li kely, water homeostasis during this period. (C) 2000 Academic Press.