| Objective: AMP579 is a novel adenosine agonist with a long half-life period. Administration of AMP579 prior to or during myocardial ischemia significantly reduces infarct size in rabbit and pig, showing its stronger cardioprotective effect than adenosine dose. So far, however, no reports about the effects of AMP579 on ionic channels of cardiac myocytes could be found. The aim in our study is to observe the effects of AMP on ionic channels of ventricular myocytes of rat; to compare with the effects of adenosine in the same condition. This study will benefit to explore the ionic mechanisms underlying the cardioprotective effect of AMP579 and to predict its utility in treatment at conditions involving myocardial ischemia-reperfusion injury. Materials and methods: AMP579 is a gift from department of cardiothoracic surgery research laboratory, Emory University School of Medicine. The object of study is myocytes dispersed enzymatically from adult rat and guinea-pig. Ionic currents are recorded by path clamp technique in whole-cell configuration.Results: 1. Effects on ICa-L in isolated rat ventricular myocytes. Adenosine has no effect on ICa-L at the concentration ranges from 10nml/L to 50μmol/L, however in the condition that the isoproterenol which can increase the ICa-L at 10nml/L was previously administrated, adenosine shows inhibitory effect on ICa-L in concentration dependent manner (Tab1,2 , Fig1,2). The IC50 of adenosion on ICa-L is 13.06μmol/L.The same as adenosion, AMP579 also shows inhibitory effect on increased ICa-L in the condition of presence of isoproterenol, the inhibitory rates of AMP579 and adenosine in the same dose (10μmol/L) on isoproterenol-induced ICa-L are 11.1% and 5.2%, respectively, being stronger effect in AMP579. It has inhibitory effect on ICa-L as well without isoproterenol in concentration dependent manner (Tab1,3, Fig3,4). The IC50 of AMP579 on Ica-L was 1.17μmol/L. Infusion of PD116948, a adenosine A1 receptor inhibitor at 30μmol/L, did not show any influence on the inhibitory effect of AMP579, demonstrating the inbibitory effect of AMP579 on ICa-L is not related with adenosine A1 receptor (Tab5). However, AMP579 lost its inhibitory effect on ICa-L in the condition of previous administration of GF109203X, a PKC special blocker (Tab6). Infusion of GF109203X at 0.4μmol/L can also abolish the inhitory effect AMP579 on ICa-L (Tab7, Fig5), indicating that inhibitory effect of AMP579 on ICa-L is induced through activating PKC.2. Effects on Na+/Ca2+ exchange current in isolated ventricular myocytes. Consistent with previous report , adenosine had no effect on Na+/Ca2+ exchange current at the concentration range from 10nmol/L to 50μmol/L (Tab18, Fig16). AMP579 significantly enhanced both outward and inward Na+/Ca2+ exchange currents with concentration dependent (Tab18, Fig17). Infusion of PD116948, a adenosine A1 receptor antagonist, at 30μmol/L, had no influence on outward and inward Na+/Ca2+ exchange current increased by AMP579 at 10μmol/L (Tab19,20, Fig18). Infusion of DMPX, a adenosine A2 receptor antagonist at 10μmol/L, had also no effect on the outward and inward Na+/Ca2+ exchange currents increased by AMP579 (Tab21,22, Fig19). The results same as PD116948 and DMPX were obtained from RT5720, a PKA special antagonist and GF 109203x, a PKC special antagonist (Tab23-26, Fig20,21), suggesting that AMP579 possibly possesses a direct pharmacological effect in activating Na+/Ca2+ exchange current.3. Effect on Ito current in isolated ventricular myocytes. Both adenosine and AMP579 show activated effect on Ito current in isolated ventricular myocytes (Tab 8). EC50 of adenosine and AMP579 are 2.33μmol/L vs 8.32μmol/L (p<0.05), adenosine has more powerful effect. The mechanism on Ito current activated by two drugs is the same ,that effect can be eliminated by PD116948, a adenosine A1 receptor blocker (Tab 9,10,13,14, Fig 6,8), demonstrating that effect is mediated by adenosine A1 receptor. 4. Effect on voltage gated sodium current in isolated rat ventricular myo...
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