Aldosterone and ADH response to heat and dehydration in cattle.

The effect of heat (35 degrees C) and dehydration under heat (30 h) on plasma levels of antidiuretic hormone (ADH) and aldosterone was studied on four nonpregnant dry Holstein cows. Heat exposure caused a rapid significant rise in plasma ADH without significant changes in hematocrit (Hct), small but significant increase in urine output and a significant reduction in total plasma protein. Dehydration under heat caused a sharp increase in ADH levels associated with a significant decrease in urine output and a significant increase in plasma protein, blood Hct, and serum osmolality. A significant reduction in plasma aldosterone level was observed after 24 h of heat exposure. This was associated with a slight rise in urinary sodium excretion and a significant reduction in serum sodium. Both serum and urinary potassium concentrations were significantly lower under heat. Dehydration resulted in a slow rise in aldosterone but did not reach thermoneutral level. This is probably due to the inhibitory effect of higher serum sodium observed during dehydration on plasma aldosterone secretion. The rise in ADH and decrease in aldosterone during heat exposure may explain why cattle are one of the few species that do not concentrate urine during heat exposure.