Osmoregulation of vasopressin secretion and thirst during the estrous cycle of pigs

The influence of the reproductive cycle on ingestive behaviors, osmotically induced fluid intake, and peripheral blood levels of several hormones involved in fluid electrolyte balance was investigated in young adult female pigs. Food, water, and salt intakes and plasma aldosterone levels were significantly lower during estrus compared with the luteal and follicular phases, whereas plasma sodium (PNa) was higher. Plasma renin activity and lysine vasopressin (LVP) levels did not vary with the cycle. Regression analyses of the relationship between plasma LVP and PNa in unanesthetized, unstressed animals infused for 2 h with intravenous 5% NaCl revealed no significant differences in terms of the osmotic threshold and sensitivity of LVP release at different stages of the reproductive cycle. In contrast, when osmotic loading was carried out with continuous access to water, the osmotic threshold for drinking was found to be significantly higher and the rate of drinking lower during estrus. The results indicate that in the adult female pig the stage of the reproductive cycle has an influence on ingestive behaviors and the osmoregulation of thirst. Osmoregulation of vasopressin secretion, on the other hand, appears to be independent of the estrous cycle in this species.