Dexamethasone concentrations in plasma and milk of cows following the injection of long- and short-acting dexamethasone esters.

A radioimmunoassay has been developed for the measurement of dexamethasone in plasma and milk of cows injected with long- and short-acting dexamethasone esters. Dexamethasone antiserum was produced by injecting cows with a dexamethasone-21-hemisuccinate-human serum albumin complex. The antisera was highly specific for dexamethasone, cross-reacting less than 0.7% for all endogenous steroids tested. Plasma concentrations of dexamethasone in cows injected intramuscularly with either 20 mg dexamethasone-21 trimethyl acetate (n = 2) or the tributyl derivative (n = 2) reached a peak level of 0.6-1.1 ng/ml in 2-6 days then declined to undetectable levels (less than 0.15 ng/ml) and 14 days after injection. In general, dexamethasone concentrations in milk were 0.3-0.5 times the plasma concentrations but showed the same pattern of values. Plasma dexamethasone concentrations were also determined in three lactating dairy cows injected intramuscularly with tritiated dexamethasone-21 trimethyl acetate. In these cows plasma dexamethasone concentrations, as determined by isotopic dilution, reached maximal levels of 1.1-1.6 ng/ml in 1-3 days then declined to levels of around 0.05 ng/ml within 30 days. The concentrations of dexamethasone in milk of two of these cows were, in general, similar to those found in plasma. In three cows injected intramuscularly with 20 mg dexamethasone sodium phosphate the concentrations of dexamethasone in plasma rose sharply to maximum levels of 24-70 ng/ml within 2-20 min after injection and fell to undetectable levels (less than 0.15 ng/ml) after 72 hr.