Oxytocin levels and disappearance rate and plasma follicle-stimulating hormone and luteinizing hormone after oxytocin infusion in men.

Plasma oxytocin (OT), FSH, and LH were measured by specific RIA in eight healthy adult males before, during, and after stopping iv infusions of OT. With a constant infusion of 132 mU OT/min for 60 min, plasma OT reached a steady state concentration of 228-241 pg/ml at 30-60 min. When the dose of oxytocin infused was doubled every 15 min, plasma OT increased from 81.0 +/- 17.9 pg/ml (mean +/- SE) with 32 mU/min to reach a steady state concentration of 378 +/- 73.4 pg/ml with 256 mU/min (1 muU = 2 pg OT). The curve of disappearance of plasma OT could be resolved into a single exponential curve in all of the subjects, with a mean calculated half-life of 10.3 +/- 1.6 min (range, 5.3-17.3 min). The mean MCR of OT was 21.5 +/- 3.3 ml/kg.min, and the mean apparent volume of distribution was 305 +/- 46 ml/kg. Plasma FSH and LH showed no significant change throughout OT infusion and for up to 60 min after stopping the OT infusion. The findings demonstrate that in man 1) plasma OT concentration achieved is closely related to the infusion rate, 2) OT infusion does not affect plasma FSH and LH, and 3) the apparent volume of distribution of OT suggests that infused OT is distributed into space or spaces other than the circulating plasma volume.