Abstract
Euryhaline teleost fish are characterized by their ability to tolerate a wide range of environmental salinities by modifying the function of osmoregulatory cells and tissues. In this study, we experimentally addressed the age-related decline in the sensitivity of osmoregulatory transcripts associated with a transfer from fresh water (FW) to seawater (SW) in the euryhaline teleost, Mozambique tilapia, Oreochromis mossambicus. The survival rates of tilapia transferred from FW to SW were inversely related with age, indicating that older fish require a longer acclimation period during a salinity challenge. The relative expression of Na+/K+/2Cl− cotransporter 1a (nkcc1a), which plays an important role in hyposmoregulation, was significantly upregulated in younger fish after SW transfer, indicating a clear effect of age in the sensitivity of branchial ionocytes. Prolactin (Prl), a hyperosmoregulatory hormone in O. mossambicus, is released in direct response to a fall in extracellular osmolality. Prl cells of 4-month-old tilapia were sensitive to hyposmotic stimuli, while those of >24-month-old fish did not respond. Moreover, the responsiveness of branchial ionocytes to Prl was more robust in younger fish. Taken together, multiple aspects of osmotic homeostasis, from osmoreception to hormonal and environmental control of osmoregulation, declined in older fish. This decline appears to undermine the ability of older fish to survive transfer to hyperosmotic environments.
Highlights
The maintenance of body fluid osmolality and cellular sensitivity to osmotic and endocrine stimuli are fundamental components of homeostasis that become compromised with age in vertebrates
In a separate trial, when 4-mo-old tilapia were transferred from fresh water (FW) to SW over a 1, 3, and 12-h period, 0, 70, and 100% of the fish survived by 24 h, respectively (Figure 1B)
The survival rates of fish aged >24 mo transferred from FW to SW over a 1, 3, and 12-h period were 10, 20, and 90% by 24 h, respectively (Figure 1C)
Summary
The maintenance of body fluid osmolality and cellular sensitivity to osmotic and endocrine stimuli are fundamental components of homeostasis that become compromised with age in vertebrates. Osmoreception, the first step in osmoregulation, involves the integration between cellular mediators of osmotic changes, and effectors, typically hormones, capable of acting systemically to restore salt and water balance (Grau et al, 1994; Bourque and Oliet, 1997; Seale et al, 2012b). A number of studies have shown age-related differences in thirst, hypothalamic cellular sensitivity, and osmoregulatory endocrine responses, especially those related to vasopressin release (Robertson and Rowe, 1980; Baylis, 1987; Mack et al, 1994; Lee et al, 1998; Thunhorst et al, 2010; Thunhorst et al, 2011). Cellular senescence in osmoregulatory tissues, such as the kidney, is accelerated due to high interstitial NaCl concentration
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