Adrenal Function in the Cow. Diurnal Changes and the Effects of Lactation and Neurohypophyseal Hormones

Using indwelling jugular vein cannulas, blood samples were collected at 30-min. intervals during a 24-hr, period to determine the effect of differing lactation regimens and diurnal variation on plasma corticoids in nine cows (three non-lactating, three milked and three suckled). Plasma corticoids were measured by the protein-binding method. Corticoid levels (mean±S.E.) were 4.5±0.04, 6.8±0.05 and 9.4±0.09 ng/ml for non-lactating, milked and suckled groups, respectively. Levels for non-lactating and suckled groups were significantly different (P<.05). Data from all nine animals were pooled for consideration of diurnal effects. Values (mean±S.E.) were 7.34±0.53, 7.51±0.68 and 5.31±0.40 ng corticoid/ml for 0200–1000, 1000–1800 and 1800–0200 hr., respectively. The 5.31 value for 1800–0200 hr. was significantly lower than the other two time periods (P<.05). A significant (P<.01) rise in plasma corticoids occurred during and for 15 min. after milk removal (suckling or milking). The blood level was 8.3 ng/ml during this stimulatory period and 5.4 ng/ml before and after this period. Intravenous administration of two synthetic oxytocin preparations and vasopressin at dose levels of 2, 5 and 10U did not cause a significant rise in plasma corticoids except with the 10U dose of vasopressin. The 10U dose of vasopressin caused a three-fold rise in plasma corticoids that lasted for 20 to 25 minutes.