The local administration of relaxin induces changes in the microcirculation of the rat mesocaecum

The clinical improvement of Raynaud's disease during pregnancy has been attributed to the increased relaxin (RLX) level in blood. Therefore we investigated the effect of topical porcine RLX (NIH-RXN-Pl) on the microcirculation of the mesocaecum of Wistar male rats, under direct microscope observation as judged by two observers. The hormone was applied locally to avoid systemic interferences either alone or after norepinephrine (NE) and promethazine (PM). The drugs were coded and the results were recorded independently by two observers. Porcine RLX induced rapid dilatation of the veins of the rat mesocaecum in a dose-related manner. The arteriolas and capillary flows were unchanged, while the venular flow was progressively reduced. The observed effects were sustained and disappeared with tissue wash-out. The RLX effects were reversed by addition of NE; conversely high doses of RLX were able to oppose the NE and PM vasospasm. We conclude that the local administration of RLX influences the microcirculation, possibly through an action on the smooth muscle of the venulae. The effects seems antagonistic with those of NE and PM.