Experiment 1. Effects of Intravenous Relaxin on Plasma Oxytocin in Lactating Rats
Baseline plasma oxytocin concentrations were measured in three groups of lactating rats: before suckling (group 1), after the onset of suckling but before reflex milk ejection started (group 2), and after reflex milk ejection was established (group 3). Plasma oxytocin concentrations were 27.6 ± 2.9 pM in unsuckled rats. They were significantly increased in rats after the onset of suckling (34.5 ± 2.4 pM) compared with unsuckled animals. Oxytocin concentrations were increased further during reflex milk ejection (41.2 ± 2.7 pM). These data are summarized in Table 1. buy asthma inhaler
Relaxin treatment in all three groups caused a significant increase in baseline oxytocin compared with pretreatment concentrations and the effect of saline injection (Table 1). In group 1 rats, relaxin caused a significant rise in plasma oxytocin (35.3 ± 2.6 pM) compared with saline treatment alone (26.2 ±3.1 pM). In group 2 rats, treatment with relaxin caused a further, significant increase in oxytocin (40.6 ± 2.7 pM). In contrast, saline treatment had no effect on plasma concentrations compared with levels before treatment (34.7 ±2.1 pM). Last, relaxin caused a significant increase in circulating oxytocin in group 3 rats (after the onset of milk ejection). Peak concentrations of oxytocin (50.1 ± 3.7 pM) were observed in the first sample taken after relaxin treatment; thereafter concentrations fell at a rate of 1.35 fM per second by the end of the sampling period. Despite the rise in circulating oxytocin, relaxin treatment suppressed reflex milk ejection for more than 25 min in 8 of 8 tested. In contrast, saline treatment had no effect on circulating oxytocin concentrations or on the frequency and pattern of milk ejection.
TABLE 1. Effect of relaxin treatment on oxytocin concentrations in lac-tating rats.
|Treatment group||Oxytocin levels (pM) before treatment||Oxytocin levels (pM) after saline||Oxytocin levels (pM) after relaxin|
|Non-suckled||27.6 ± 2.9||26.2 ± 3.1||35.3 ± 2.6*|
|Suckled/no RMEa||34.5 ± 2.4**||34.7 ± 2.1||40.6 ± 2.7*|
|During RMEa||41.2 ± 2.7**||40.0 ± 2.9||50.1 ± 3.7*|
a RME, reflex milk ejection.
* Oxytocin concentrations significantly (p < 0.05) increased after relaxin treatment for all three groups of rats compared with pretreatment concentrations and after saline treatment.
** Oxytocin concentrations significantly (p < 0.05) increased in suckled and milk-ejecting rats compared with unsuckled rats.