Uptake of relaxin in the uterus and cervix of rats in vivo: influence of ovarian steroids and tolerance.

The influence of oestradiol benzoate and progesterone treatment and of tolerance to relaxin on the uptake of 125I-labelled porcine relaxin by reproductive tissues was investigated in anaesthetized female rats. In ovary-intact rats, 125I-labelled relaxin uptake increased with time in reproductive tissues and in tissues concerned with metabolism and excretion. Administration of 50 micrograms unlabelled porcine relaxin before injection of 125I-labelled relaxin significantly reduced uptake of 125I-labelled relaxin into the uterus and cervix, but had no effect on uptake of 125I-labelled relaxin into other tissues, indicating that specific uptake of relaxin was occurring in the uterus and cervix. In ovariectomized rats, treatment with oestradiol benzoate or oestradiol benzoate plus progesterone for 1 day did not significantly increase uterine or cervical 125I-labelled relaxin uptake compared with corn oil-treated rats, but induced a significant increase in uterine uptake of 125I-labelled relaxin after treatment for 2 days. Induction of tolerance to relaxin by i.v. infusion of a high dose of relaxin significantly reduced uterine and cervical uptake of 125I-labelled relaxin at 3 h after termination of infusion compared with saline-infused rats. By 12 h after termination of infusion, 125I-labelled relaxin uptake in the uterus and cervix was similar in saline-infused rats and in rats given an infusion of relaxin. Infusion of glibenclamide (20 mg kg-1) did not influence uterine or cervical uptake of 125I-labelled relaxin; however, treatment with phentolamine (10 mg kg-1) significantly reduced 125I-labelled relaxin uptake in uterus, bladder and jejunum. This study demonstrates that steroid hormone treatment and tolerance modulate relaxin uptake in reproductive tissues.