Potential Binding Sites for Relaxin in Pregnant Rabbits

: The purpose of this study was to identify tissues in the day 25 pregnant rabbit that bind relaxin. First, the clearance half‐life of relaxin from the rabbit (n= 6) was determined by injecting 10 g porcine relaxin via the marginal ear vein. One‐milliliter blood samples were collected via a cannula in the central ear artery. Samples were collected at 10‐ and 5‐min pre‐relaxin and at 1‐min intervals post‐relaxin injection, and the relaxin concentrations determined by radioimmunoassay. The clearance half‐life was 4 min. Next, pregnant rabbits were infused with [125I]‐relaxin. Control rabbits (n= 3) received 10 g radio‐inert relaxin via the marginal ear vein in order to saturate endogenous receptors. Ten minutes later, 10 ng [125I]‐relaxin was similarly injected. Treated rabbits (n= 3) received only [125I]‐relaxin. After allowing sufficient time for clearance (24 min), a 1‐mL blood sample was removed via the central ear vein. Rabbits were euthanized, tissues of maternal and fetal origin excised, and cpm/mg of tissue divided by cpm/mL of blood was determined. Differences in uptake of [125I]‐relaxin between control and treated animals, using the Student paired t test, were found for the uterus (P