Changes of aldosterone in blood serum of dairy cattle during estrous cycle.

Blood samples were taken daily from five lactating cows and five open heifers beginning with the 1st day postestrus and continuing until the animal was observed in standing estrus. Means of aldosterone in blood serum were 102.6 +/- 5.1 pg/ml for lactating cows and 94.1 +/- 5.1 pg/ml for open heifers. Age and lactation did not appear to be major factors affecting circulating aldosterone in the bovine species. Effect of day of the estrous cycle on aldosterone was quadratic. Concentrations of aldosterone for lactating cows started at 92.3 pg/ml on day 1 of the estrous cycle, peaked at 125.5 pg/ml on day 8, then declined steadily to a low of 43.2 pg/ml on day 20. There was no significant interaction of group by day. During the estrous cycle aldosterone appeared to be related to development of corpus luteum and progesterone. Sodium in blood serum was higher during estrus for lactating cows than open heifers for linear, quadratic, and cubic effects. Although not correlated with aldosterone, sodium exhibited similar cyclic patterns throughout the estrous cycle. Responses between sodium and aldosterone appeared delayed during estrus.