Oxytocic Activity in Posterior Pituitary Glands of Male Rats Measured by Two Bioassays 1

Abstract

Summary

Oxytocic activity in posterior pituitary glands of male rats at 20, 40, 80, and 200 days of age was measured using two different bioassays. The first was an in vitro assay based on the log time response of a 2 mm³ mammary cube in exuding milk to the log dose of oxytocin between 10^-1 and 10^-9 IU. The second was an in vivo assay based on the ability of the guinea pig mammary gland to increase intramammary pressure rectilinearly in the range of 0.25 to 1.0 mU of oxytocin. Oxytocic activities of posterior pituitaries by the in vitro assay were 9.1, 78.6, 150.2, and 147.9 mU at 20, 40, 80, and 200 days of age, respectively; while the in vivo assay gave values of 18.2, 96.7, 195.3, and 484.5 mU in the same order. Only the values obtained in 200-day-old rats were significantly different between the two assays (p<.05).