Dexamethasone Concentrations in Plasma and Milk of Cows Following the Injection of Long- and Short-Acting Dexamethasone Esters

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Abstract
A radioimmunoassay was 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. Antisera was highly specific for dexamethasone, cross-reacting < 0.7% for all endogenous steroids tested. Plasma concentratons of dexamethasone in cows injected i.m. with 20 mg dexamethasone 21-trimethyl acetate (n [no.] = 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 (< 0.15 ng/ml) by 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 3 lactating dairy cows injected i.m. with 3H- 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 .apprx. 0.05 ng/ml within 30 days. Concentrations of dexamethasone in milk of 2 cows were in general similar to those found in plasma. In 3 cows injected i.m. with 20 mg dexamethasone sodium phosphate, concentrations of dexamethasone in plasma rose to maximum levels of 24-70 ng/ml within 2-20 min after injection and fell to undetectable levels (< 0.15 ng/ml) after 72 h.

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