The clinical improvement of Reynaud'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\x=req-\ 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. RLX has been known since 1926 (Hisaw 1926), but its physiological and pharmacological proper¬ ties have not been fully elucidated mainly because of difficulties in its extraction and the low purity of the available preparations.In recent years, the purification of RLX from the ovary of the pregnant pig, rat and shark (Schwabe 1983) and the recognition of the similar¬ ity of its structure with the insulin family (James et al. 1977; Schwabe et al. 1977) have aroused the interest of more researchers in the RLX field.It has been demonstrated that RLX is produced not only during pregnancy but also in non-preg¬ nant females and in the male (Bryant-Greenwood 1982; Yki-Järvinen et al. 1983a,b) and is secreted not only by the corpus luteum but also by several other sources (Bigazzi et al. 1980.Besides the well-known actions of relaxin in the pubic symphysis enlargement and the inhibition of uterine contractions, in recent years many different roles have been demonstrated or postu¬ lated for RLX both as a 'systemic' and as a 'local' hormone (Bryant-Greenwood 1982): it may be involved in important steps of the physio-patho¬ logy of reproduction and in the proliferation and differentiation of various components of the mammary glands (Bani et al. 1983(Bani et al. , 1984. It is possible that RLX acts on the blood vessels, since we have recently observed an intense vasodilata¬ tion among the various effects produced by RLX on its target tissues. Casten et al. (1960) reported clinical ameliorations of peripheral arteriopathy following the administration of partially purified porcine RLX. In this study we present evidence that RLX is capable of modifying the local microcirculation of the rat mesocaecum when topically applied, influ¬ encing the venular vessels and the capillary, even in the presence of other vasoactive agents.
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