Relaxin : a new cardiovascular hormone in humans? : comparative potency and mechanisms of action

INTRODUCTION: The focus of this MD thesis has been relaxin, a member of the insulin family, which is a protein composed of two disulphide linked chains of approximately 6000 Daltons. Relaxin has been traditionally recognised as a hormone of parturition, though more recently it has been postulated that relaxin may be involved in cardiovascular regulation. We used concentrations similar to those found in the plasma in physiological (non-pregnant, pregnancy) and pathophysiological (chronic heart failure) states. Firstly, we characterised the effects of relaxin in small human resistance arteries ex vivo using wire myography obtained from gluteal biopsies taken from patients with coronary heart disease (CHD) and normal left ventricular systolic function. We also studied the same effects in larger calibre arteries (internal mammary) and veins (saphenous) using standard organ bath techniques. The effect of relaxin in veins has not previously been described. Internal mammary arteries and saphenous veins were obtained from patients undergoing coronary artery bypass surgery. Small pulmonary arteries were obtained from patients undergoing thoracotomy for bronchial carcinoma. In addition, we wished to determine if a transcardiac or transpulmonary gradient of relaxin could be measured to suggest either pulmonary or cardiac secretion or clearance of the hormone. Relaxin secretion in heart failure has previously been described. Lastly, we wished to determine whether an increased relaxin plasma concentration in patients with chronic heart failure (CHF), is of prognostic importance. METHODS AND RESULTS i)comparative potency of relaxin compared to other vasodilators: Small resistance arteries were obtained from biopsies taken from patients with CHD. Each set of vessels was preconstricted with noradrenaline. Thereafter, cumulative concentration response (relaxation) curves (CRCs) were constructed with known vasodilators 25 atrial natriuretic peptide (ANP), epoprostenol, substance P and relaxin (n=8). Relaxin was found to be a more potent vasodilator than ANP and equipotent to epoprostenol. ii) mechanism of vasorelaxation: CRCs to relaxin (as above) were constructed to identify the importance of the endothelium – following the removal of the endothelium by the established method of intraluminal rubbing with a human hair. We found that relaxin is endothelium dependent. iii) interaction of relaxin with nitric oxide and other possible mechanisms of vasodilation and importance of ACE inhibitor treatment: We identified the importance of the effect of ACE inhibitor treatment on the action of relaxin in human resistance arteries. Relaxin’s vasodilatory action was significantly reduced in those patients on ACE inhibitors (n=28) compared with those patients not on ACE inhibitors (n=30). In patients treated with an ACE inhibitor, we found that manipulation of prostanoids is important. Indomethacin, (a cyclooxygenase inhibitor) (n=8) blocked relaxin’s vasodilatory action. Manipulation of the cAMP second messenger system, with milrinone, (a cAMP phosphodiesterase inhibitor) (n=6) is also important as relaxin’s vasodilatory action was enhanced. Manipulation of cyclic GMP second messenger system is also important. ODQ, (a guanylate cyclase inhibitor) (n=10) reduced relaxin’s action while zaprinast, (a cGMP phosphodiesterase inhibitor) (n=7) enhanced relaxin’s action. Manipulation of nitric oxide with L-NAME (n=8) and L-NOARG (n=10), nitric oxide synthase (NOS) inhibitors and EDHF with apamin and charybdotoxin (potassium channel blockers) (n=7) had a curious effect causing the opposite action to that expected, by enhancing relaxin’s vasodilatory action. In patients not treated with an ACE inhibitor, we found that manipulation of nitric oxide with L-NAME (n=8) and LNOARG (n=8), is important, as both reduced relaxin’s vasodilatory action. Manipulating the cGMP second messenger system with ODQ (n=8) greatly reduced relaxin’s action. but zaprinast (n=9) did not. Manipulation of EDHF with apamin and charybdotoxin (n=8) had no effect on relaxin’s action. Manipulation of prostanoids with indomethacin (n=10) reduced relaxin’s action but manipulation of cAMP with milrinone (n=8), had no effect. 26 iv)relaxin and small human pulmonary arteries: We determined, using wire myography, that relaxin is not a vasodilator of small pulmonary resistance arteries (n=5). v)relaxin and large calibre vessels: We determined, using the organ bath technique, that relaxin is not a vasodilator of larger calibre arteries i.e. internal mammary arteries removed from patients during coronary artery bypass surgery (n=5). Relaxin is not a venodilator studying saphenous veins removed from patients during coronary artery bypass surgery (n=5). vi)transmyocardial and transpulmonary gradient of relaxin: Plasma relaxin concentrations were measured using a validated assay. Samples were taken from patients undergoing CABG surgery, from the aorta, coronary sinus, pulmonary artery and pulmonary vein. We found that in 20 patients with normal left ventricular function that there was no transpulmonary gradient but there was a transcardiac gradient suggesting net cardiac extraction of relaxin. vii)prognostic value of relaxin in patients with chronic heart failure: Relaxin was compared with N-terminal pro brain natriuretic peptide to determine whether relaxin is of prognostic importance. Plasma concentrations of the hormones were measured in 87 patients admitted with CHF. These patients were followed up for a year during which time hospitalisations due to CHF and death were recorded. While NT-proBNP was found to be a powerful and independent predictor of outcome in these patients, relaxin was not. CONCLUSION. In addition to its established role in pregnancy, relaxin has many other actions. In particular, its antihypertensive, antithrombotic and vasodilatory properties suggest that relaxin may have a central role in cardiovascular regulation.