Excess cortisol interferes with a principal mechanism of resistance to dehydration in Bos indicus steers

This study investigated the effects of excess cortisol on physiological mechanisms that resist dehydration in Bos indicus steers (n = 31, 2 yr of age, 193 ± 21.47 kg mean BW) during a 90-h period. Steers were assigned randomly to one of four groups: 1) no water/no cortisol (n = 8), 2) water/no cortisol (n = 8), 3) no water/cortisol (n = 8), and 4) water/cortisol (n = 7). Animals allocated to cortisol treatment groups were given 0.1 mg·kg BW^-1·h^-1 of hydrocortisone suspended in isotonic saline for the duration of the study. Total body water, osmolality, hematocrit, urine output, feed and water intake, and plasma concentrations of arginine vasopressin (AVP), angiotensin II (AII), electrolytes, total protein, and albumin were determined at 24-h intervals for 90 h. In the presence of excess plasma cortisol, total body water was maintained in the presence of a water deprivation insult for 90 h, whereas hydration indices, such as total plasma protein and albumin, did not change, supporting the body water data. However, plasma osmolality increased for the water-deprived groups from 24 h (P = 0.008). Hematocrit did not reflect dehydration in any group. Water deprivation induced an increase in endogenous plasma cortisol concentrations after 60 h of the study (P = 0.028). Plasma concentrations of AVP increased with water deprivation (P = 0.006). Excess cortisol decreased the plasma concentration of AVP at 72 h only (P = 0.027) and suppressed plasma concentrations of AII at 24 and 72 h (P < 0.001 and P = 0.036, respectively). Animals treated with excess cortisol maintained urinary output for 48 h before decreasing at 72 h (P = 0.057), although there was no effect on water or feed intake. Water deprivation increased plasma sodium concentrations (P < 0.05) until 72 h, whereas potassium decreased under the influence of excess plasma cortisol (P = 0.001) at 24 h. Water deprivation increased plasma chloride concentration at 72 and 90 h (P = 0.051 and P = 0.026, respectively). Plasma phosphorus decreased at 24 h (P = 0.001) and remained at lesser concentrations for the duration of the study (P = 0.05). These results highlight the complexity of endocrine interactions associated with water balance in Bos indicus steers. We accept our hypothesis that, in the presence of excess cortisol, the renin-angiotensin-aldosterone axis is suppressed; however, homeostasis is achieved through other physiological systems.