Development of an homologous radioimmunoassay for secreted prolactin from the California ground squirrel (Spermophilus beecheyi).

A sensitive homologous radioimmunoassay was developed for secreted prolactin from the California ground squirrel (Spermophilus beecheyi). S. beecheyi plasma and pituitary extracts displaced 125I-labeled S. beecheyi prolactin in a parallel manner with S. beecheyi prolactin (sbPRL). Mean minimum sensitivity of the assay was 0.21 ng/ml, and mean intra- and interassay coefficients of variation were 4.1% and 14.5%, respectively. The assay was used to measure basal prolactin concentrations in male and female ground squirrels at various stages of their annual reproductive cycles. Mean concentrations in nonpregnant, nonlactating young females, pregnant females, and lactating females were 1.63, 11.35, and 10.86 ng/ml, respectively. Mean concentrations in nonreproductive and breeding males were 1.50 and 9.81 ng/ml, respectively. Finally, the assay was used to evaluate cross-reactivity between sbPRL and prolactins and growth hormones from other rodent species. Of the tested hormones, only hamster prolactin showed any cross-reactivity with sbPRL (about 0.03%).