The Effect Of I_k1 Agonist Zacopride On L-thyroxine-induced Ventricular Remodeling In Rats

Objective:To examine the effect of zacopride,a specific agonist of inward rectifier potassium channel(I_K1),on L-thyroxine（T₄）-induced ventricular remodeling and the underlying mechanisms.Methods:SD rats were randomly divided into control,L-thyroxine(L-thy,1 mg · kg^-1 · d-1,ig,10 d)model,L-thy + zacopride(5,15,50 μg · kg^-1,respectively,ip),L-thy + zacopride(15 μg · kg^-1)+ chloroquine(7.5 μg · kg^-1,ip)and L-thy + captopril(100 mg · kg^-1 · d-1,drinking water)groups.Echocardiography and cardiac hypertrophic indexes were measured to confirm the establishment of the ventricular remodeling model.The morphology of myocardium and interstitial collagen deposition are monitored by HE and Masson’s staining,respectively.The changes of I_K1 and L-calcium current（ICa-L）were detected by whole cell patch clamp technique.The confocal microscopy and fluorescent indicator Fluo-4 were applied to examine the intracellular Ca²⁺ concentration([Ca²⁺]i)of isolated adult rat ventricular myocytes.The expressions of Kir2.1(the predominant subunit of I_K1),CaMKII,p-CaMKII,SERCA2,cleaved caspase-3 proteins were also detected to explore the underlying cellular and molecular mechanisms.Results:L-thyroxine induces left ventricular hypertrophy with increased ratio of heart weight（HW）to body weight（HW · BW-1）,ratio of left ventrical weight（LVW）to body weight（LVW · BW-1）,left ventricular dimension in diastole（LVIDd）,left ventricular dimension in systole（LVIDs）,interventricular septum thickness（IVS）;decreased ejection fraction（EF）,fractional shortening（FS）（P < 0.01）;increased cross sectional areas（P < 0.01）and collagen deposition（P < 0.01）.Patch clamp data suggested I_K1 was downregulated while ICa-L was upregulated（P < 0.01）.In isolated adult cardiomyocytes,L-thyroxine increased the cell area and [Ca²⁺]i（P < 0.01）.Zacopride treatment obviously alleviated cardiac remodeling,improve cardiac function,reversed the changes of I_K1 and ICa-L,and significantly attenuated intracellular calcium overload（P < 0.01）.The optimum dose of zacopride in vivo is 15 μg · kg^-1 at which the effect is compared favorably with captopril,a classical anti-remodeling agent（P > 0.05）.Low-dose I_K1 atagonist chloroquine can reverse the effect of zacopride（P < 0.01）.L-thyroxine down-regulated the expressions of Kir2.1 channel and SERCA2 significantly,increased p-CaMKII activity and activated caspase-3 in the myocardium（P < 0.05）.Zacopride(15μg · kg^-1)and L-thyroxine group reversed it（P < 0.05）and was blocked by chloroquine too（P < 0.05）.Conclusion:Via activating I_K1,zacopride decreased Ca²⁺ influx and intracellular calcium overload,inhibited calcium-dependent pro-remodeling signal thereby attenuated L-thyroxine induced ventricular remodeling.The project might identify a new therapeutic target for ventricular remodeling.