Introduction Relaxin (RLX) is a peptide hormone secreted mainly by the corpus luteum during pregnancy [1], with well established effects on the female reproductive organs. Cardiomyocytes from rat atria have been shown to secrete detectable amounts of RLX suggesting the cardiovascular system as a physiolog- ical target [2]. In fact, RLX has a powerful vasodilatatory action on rat uterus and mesocaecum, mouse mammary gland and pigeon crop sac [3], and decreases blood pressure in spontaneous hypertensive rats [4]. In some vascular districts, such as the coronary arteries of the guinea pig, the RLX induced increase in coronary flow is accounted for by the generation of nitric oxide (NO) [5]. The NO donors, sodium nitroprusside and glyceryl tri- nitrate, down regulate the immunological and non-immuno- logical release of histamine from sensitised tissues of the guinea pig [6]. RLX shares, with the NO donors the ability to down regulate the immunological and non-immunological release of histamine from isolated rat and guinea pig mast cells [7]. Cardiac anaphylaxis in vitro is a reliable model of immune responses [8] in which the changes in myocardial functions are attributed to the release of pro-inflammatory and vasoactive mediators from cardiac mast cells. Therefore, we were interested to extend previous investigations [7] con- cerning the interactions between RLX and the immune response to the isolated heart. Our present aim was to cor- relate the effects of RLX on cardiac anaphylaxis with the generation of NO in the isolated heart of the guinea pig. Materials and methods Guinea pig hearts, taken 18–25 days after sensitisation by intraperito- neal and subcutaneous injections of egg albumin (1 ml of 1 % solution), were perfused with Tyrode’s solution at 37°C in a Langendorff ap- paratus and challenged with egg albumin (0.1 ml of 1 % solution into the aortic cannula). Records were taken of the strength of contraction, rate and coronary outflow. RLX was added to the perfusion fluid 60 min before antigen challenge. Histamine released in the perfusates and re- Inflamm. res. 50, Supplement 2 (2001) S 122 – S 123 1023-3830/01/02S122-02 $ 1.50+0.20/0 © Birkhäuser Verlag, Basel, 2001 Inflammation Research Relaxin generates nitric oxide and provides protection against cardiac anaphylaxis J. F. Ndisang 1 , R. Baronti 1 , G. Cecere 1 , E. Masini 1 , D. Bani 2 and P. F. Mannaioni 1 1 Department of Preclinical and Clinical Pharmacology, Viale G. Pieraccini n° 6, 50139 Florence, Italy, Fax : +39 055 427 1280, e-mail: mannaion@pharm.unifi.it 2 Department of Anatomy, Histology and Forensic Medicine, University of Florence, Viale G. Pieraccini 6, 50139 Florence, Italy Correspondence to: P. F. Mannaioni tained in the heart was detected fluorimetrically. In cardiac homo- genates, cyclic GMP levels were detected by radioimmunoassay using 125 I-labelled cyclic GMP. Tissue Ca 2+ levels were determined by means of atomic absorption. Results and discussion We have previously shown that RLX increases both the coronary flow in the isolated heart of the guinea pig and the amount of nitrite that appears in the perfusates. Both effects were significantly reduced by pretreatment with the nitric oxide synthase inhibitor N-monomethyl-L-arginine, and were comparable with those obtained by the endothelium- dependent vasodilator acetylcholine as well as by the endo- thelium independent vasodilator, sodium nitroprusside [5]. The present experiments show that in the presence of RLX the pathophysiological responses to antigen challenge were fully abated (Fig. 1, panel a). Consistent eg with these observations, the release of histamine induced by antigen was also diminished in the presence of RLX (Fig. 1, panel b). Moreover, RLX significantly increased cGMP levels and reduced the antigen-induced increase in cardiac calcium (data not shown). It should be noted that in the presence of RLX, antigen challenge evoked a less sustained coronary constriction and a more pronounced coronary dilatation (data not shown). The results of these experiments show that RLX, at a concentration as low as that measured in plasma under physiological conditions, provides protection against cardiac anaphylaxis, in that the responses to antigen are fully abated and the release of histamine substantially reduced. These effects are concomitant with the increase in cardiac cGMP and with the corresponding decline in the immunological rise of cardiac calcium. It is conceivable that the protection af- forded by RLX on cardiac anaphylaxis could be accounted for by the generation of NO. In fact, the immunological and non-immunological release of histamine from isolated mast cells is similarly reduced by RLX in a NO-dependent fashion. The inhibition of mast cell histamine release by RLX is consistent with the formation of NO, which actually occurs from rat mast cells in the presence of RLX [7], as well as