Target Tissues for Relaxin in the Rat: Tissue Distribution of Injected125I-Labeled Relaxin and Tissue Changes in Adenosine 3′, 5′-Monophosphate Levels afterin VitroRelaxin Incubation*

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Abstract
Possible target tissues for relaxin in estrogenprimed ovariectomized rats were identified by determining 1) the tissue distribution of injected [125I]relaxin in vivo and 2) the changes in cAMP levels in several tissues after incubation with relaxin in vitro. Porcine relaxin was radioiodinated with 125I-labeled Bolton-Hunter reagent (N-succinimidyl 3-(4-hydroxyphenyl) propionate). Relaxin retained its biological activity after modification with unlabeled Bolton-Hunter reagent. Of the tissues examined, the thyroid, kidney, liver, spleen, lung, and uterus (horns and cervix) had higher concentrations of radioactivity than blood during the first 4 h after the injection of 5 μCi (∼16 ng) [125I]relaxin. Only in the uterus could the higher levels of radioactivity be significantly displaced by the simultaneous administration of 15,000 ng unlabeled relaxin (P < 0.05). Samples of uterine horns, cervix, liver, kidney, spleen, adrenal, diaphragm, lung, ileum, bladder, ureter, and arterial vascular wall were incubated with Krebs-Ringer bicarbonate buffer (pH 7.4; 37 C; gas phase, 95% O2-5% CO2 containing 5.5 mm glucose and 5 mm theophylline). After 15 min of incubation with 4 μg/ml porcine relaxin, only the uterine horns and cervix showed significant increases in cAMP levels over controls (P < 0.001) as measured by RIA. The increases in cAMP levels within the uterine horns were time and hormone dose dependent. It appears that the uterine horns and cervix are target tissues for relaxin in the estrogen-primed ovariectomized rat. Relaxin may exert its biological effects, at least in part, through cAMP. (Endocrinology 106: 1203, 1980)

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