A Study Of Positive Inotropic Effects Induced By Genistein In Guinea Pig Myocardium

Aim: Genistein, belongs to isoflavone of phytoestrogen, and is abundant in soybean and structurally or functionally similar to estradiol. It has been reported that genistein possesses many physiological and biochemical actions, such as estrogenic, anti-atherosclerosis, anti-osteoporosis and relieving menopausal symptoms. Genistein is thought to exert protective effect on the cardiovascular system and decrease the incidence of cardiovascular diseases through direct and indirect mechanisms. Furthermore, the inhibitory effects of genistein on contractile response of vascular smooth muscle are similar to estradiol, and which has been confirmed by many experiments. But Liew et al. found that genistein exerted opposing actions on guinea pig cardial myocytes and enhanced the contraction of myocardium. Therefore, cardiovascular action of genistein is similar to P-adrenoceptor on the membrane of vascular smooth muscle and cardial myocytes. However, it has not reported whether or notβ-adrenoceptor mediated cardiovascular action of genistein. Therefore, using the change of myocardial contractility as the index and isoprenaline hydrochloride as the positive control, we studied the effect of genistein on the contractility in guinea pig myocardium, compared its action with 17β-estradiol and isoprenaline hydrochloride and also determined the possible roles ofβ-adrenoceptor, extracellular Ca²⁺ inflow, cAMP, estrogen receptor and tyrosine kinase pathway in positive inotropic actions induced by genistein in guinea pig myocardium.Methods: Regardless of male and female, adult healthy guinea pigs were chosed and killed by a heavy blow on the head and the heart was quickly removed and placed in Krebs-Henseleit (K-H) solution bubbling with 95% O₂ and 5% CO₂ (mmol. L^-1: NaCl 118, KC1 4.7, CaCl₂ 2.5, MgSO₄ 1.2, NaHCO₃ 24.9, KH₂PO₄ 1.2, glucose 11.1, ascorbic acid 0.057 and EDTA 0.027). Remnant blood in the heart were evacuated by slightly extrusion, and then put the heart into culture capsule where was filled with Krebs-Henseleit (K-H) solution. The heart was fixed by pin, left and right ventricles of heart were scissored by eye scissors, and the papillary muscles and ventricular muscles were excised from the heart. Then, the muscle preparations of the heart were suspended horizontally in 5 ml tissue chambers containing 37℃±0.5℃K-H solution. Bipolar pace-making electrode drove the specimen with the field stimulation (1Hz, 1ms, 2 times threshold). Myocardium was allowed to equilibrate for 90 min with a resting tension of 1.0 g and the solution was changed every 15 min. The myocardial contraction was measured with a force transducer and recorded with BL-420E⁺ experimental system of biological function (TME, China) by microcomputer.Results were as follows:1. Effects of genistein, isoprenaline hydrochloride and 17β-estradiol on the contractility of myocardium.Genistein (1, 10, 50, 100μmol·L^-1) had positive inotropic effects in guinea pig myocardium, which was similar to isoprenaline hydrochloride and was obviously dose-dependent, but 17β-estradiol (1, 50, 100μmol·L^-1) inhibited the contractility of myocardium. Furthermore, the maximal contraction of myocardium appeared at 24 min after incubating with 50μmol·L^-1 or 100μmol·L^-1 genistein.2. Effects of genistein on the contractility of myocardium in the presence of propranolol, atenolol, verapamil.In the presence of propranolol (1μmol·L^-1), specificβ₁-adrenoceptor inhibitor atenolol (1μmol·L^-1) and Ca²⁺ channel blocker verapamil, the enhancement of the contractility of myocardium induced by isoprenaline hydrochloride (1μmol·L^-1) was attenuated significantly, but the positive inotropic actions induced by genistein in myocardium had no obvious changes.3. Effects of genistein on the contractile activity induced by elevation of extracellular Ca²⁺ concentration.After incubating with genistein (1, 10μmol·L^-1) respectively, the excitatory effects induced by elevation of extracellular Ca²⁺ concentrations (2.5, 5 and 10 mmol·L^-1) in myocardium had no significant changes.4. Effects of genistein on the myocardial contractility in the presence of SQ22536.Adenylyl cyclase inhibitor SQ22536 (1μmol·L^-1) inhibited the positive inotropic effect of genistein significantly.5. Effects of genistein on the contractility of myocardium in the presence of tamoxifen, ICI 182, 780.The positive inotropic effect of genistein was blocked partly by pretreatment with non-steroid selective estrogen receptor modulator tamoxifen (1μmol·L^-1), but not affected by specific estrogen receptor inhibitor ICI 182,780.6. Effects of genistein on the myocardial contractility of myocardium in the presence of bpV.In the presence of bpV (1μmol·L^-1), the positive inotropic effect of genistein had no significant changes.Conelusious: (1) Genistein has the positive inotropic effect in dose-dependent manner on guinea pig myocardium. In contrast, 17β-estrodiol produces an opposite effect, decreasing the contractility of myocardium. (2)The positive inotropic action induced by genistein is not related to the activation of Ca²⁺ channel on cell membrane,β-adrenoceptor and estrogen receptor, but may involve in cAMP of intracellular signal transduction and tyrosine kinase pathway.