Plasma oxytocin concentrations at the start of the sampling period were in the range 35-40 pM. An increase in oxytocin concentrations was observed in the samples collected 5 or 10 sec before the rise in intramammary pressure was detected. Peak concentrations of hormone (392.4 ± 122.1 pM) were coincident with, or slightly preceded, the onset in rise of intramammary pressure. Oxytocin concentrations then fell within the time of the next one or two samples (5-10 sec) to 65.7 ± 2.7 pM. Thereafter, levels declined at a steady rate of 175.2 ± 49.8 fM per second for the remainder of the sampling period.
In contrast, i.v. relaxin suppressed reflex milk ejection for at least 25 min in 8 of 8 animals tested. No rise in intramammary pressure was observed in any of these animals, and there was no pulsatile release of oxytocin observed. Plasma oxytocin concentrations were 50.2 ± 3.6 pM at the start of the sampling period and fell throughout the period at a rate of 169.6 ± 36.2 fM per second to reach levels of 36.3 ±3.0 pM at the end of the sampling period. asthma inhalers
No difference was observed in the oxytocin profiles obtained from rats treated with biologically inert relaxin compared with rats treated with saline. In all five animals, milk ejection occurred in the middle of the sampling period. Baseline levels of oxytocin before milk ejection were 36.5 ±1.9 pM. Five to 10 sec before the onset of the rise in intramammary pressure was detected, plasma oxytocin concentrations increased sharply, and they reached a peak (404.3 ± 67.9 pM) within 2 sampling periods (10 sec). By the next sample, oxytocin concentrations had fallen to 68.5 ± 8.3 pM, and they decayed thereafter at 160.2 fM per second. These data are not significantly different from data obtained in rats treated with saline alone.