Half-life, clearance and production rate for oxytocin in cattle during lactation and mammary involution I

Abstract

Basal concentration, production rate, metabolic clearance rate and T 1 2 for oxytocin were measured in Holstein cows at three lactational stages and during mammary involution. At each lactational stage, oxytocin was given intravenously at .5 IU/min or 1.0 IU/min for 60 min. Infusions were preceded by priming. During involution, the high dose was used. Mean basal concentrations of oxytocin ranged from 8.7 to 21.4 uU/ml. Mean basal values at early, middle, late lactation and involution were 17.57, 12.33, 15.15, 21.13 uU/ml, respectively and differed significantly. The mean “rapid” T 1 2 was 3.87 ± .1 min. Early, middle, late lactation and involution T1 2 s were 4.2, 3.7, 4.0 and 3.5 min, respectively. The rapid T 1 2 was not affected by lactational stage. The mean “slow” T 1 2 was 25.53 ± 1 min. Early lactation and dry period means differed significantly. The overall mean oxytocin clearance rate was 8.41 ± .1 ml/kg ·min. Clearance rate declined through lactation and into the dry period. Mean values of 9.3, 8.8, 7.8 and 7.1 ml/kg ·min were obtained at early, middle, late lactation and involution, respectively. Clearance rates at late lactation and involution differed significantly from one another and from the early and middle stages. Mean entry rates for oxytocin at early, middle, late lactation and involution were 168.79, 106.03, 111.1 and 146.8 uU/kg ·min, respectively. Measurements at early lactation and involution were greater than values for middle and late lactation. To summarize, basal oxytocin concentrations can be measured in cows that are lactating or undergoing mammary involution and changes in concentrations during lactation are related to hormone production (entry rate) and metabolic clearance rates.