| There are abundant insulin receptors on the membrane of myocytes, which renders cardiomyocytes typical target cells of insulin. Accumulating evidence has indicated that abnormal insulin level is an important predisposing factor hi cardiovascular disorders and contributes to the development of cardiovascular diseases, such as coronary heart disease and heart failure. It has been shown that insulin reduces infarct size in experimental animals subjected to myocardial ischemia/reperfusion (I/R). Our previous study showed that in vivo treatment with insulin reduced postischemic myocardial apoptotic death by activating the phosphatidylinositol 3-kinase-Akt-endothelial nitric oxide synthase (PI3-kinase-Akt-eNOS) signaling pathway, which might further contribute to the prolonged improvement of cardiac performance following I/R.I/R leads to the depression of myocardial systolic/diastolic function. Previous evidence of our experiments has shown that insulin improved postischemic myocardial contracile function in vivo. However, to date, the effects of insulin on I/R hearts, especially the direct effects on I/R cardiac myocytes mechanical properties are not clear.In the present study, we evaluated the direct effects of insulin on systolic/diastolic function in isolated hearts and calcium transient in cardiac myocytes subjected to I/R. A potential signaling pathway involved in the insulin-induced improvement of contractile function was also investigated.The results are as follows: 1. Effect of insulin on isolated perfused I/R heartsIsolated perfused I/R hearts were developed. A highly compliant fluid-filled latex balloon connected to a pressure transducer was inserted into the left ventricle for measuring left ventricular systolic pressure (LVSP) and maximal rate of left ventricular pressure development (+LV dP/dt). A 5/0 silk suture was placed around the left coronary artery, close to its origin and the hearts were then allowed to stabilize for 10 min. Regional ischemia was induced by tightening the silk suture around the coronary artery for 20 min followed by 40 min of reperfusion. LVSP, +LV dP/dtmax and HR were recorded before ischemia, 20 min of ischemia, onset of reperfusion, reperfusion for 5, 10, 20 and 40 min respectively. The results showed that LVSP at reperfusion was not significantly influenced, whereas insulin (1 IU/L)significantly augmented LVSP, V dP/dtmaxat 40 min of reperfusion (n=5, P<0.01), the HR also decreased compared with that of contol (n=5, P<0.05). These results suggested the improvement of insulin on postischemic cardiac contractile function. 2. Effect of insulin on I/R cardiac myocytesThe present study examined cardiac myocyte contractile and Ca2+ transient responses to insulin in simulated I/R and further investigated the role of protein kinase B (Akt) in the insulin-induced inotropic effect. Ventricular myocytes were enzymatically isolated from adult Sprague-Dawley rats and perfused with Tyrode solution while field stimulated at a frequency of 0.5 Hz, 5 ms duration. Simulated I/R was induced by perfusing the cells with chemical anoxic solution including sodium cyanide-sodium lactate for 15 min followed by reperfusion with normal oxygenated Tyrode solution with or without insulin.Ventricular myocytes were perfused with Tyrode solution for 5 min while field stimulated at a frequency of 0.5 Hz, 5 ms duration. Following this equilibration period the myocytes were exposed to chemical hypoxic solution for 15 min. The magnitude of cell contraction was gradually decreased to 30+8% compared with that of preischemia (n=10, P<0.01). Upon reperfusion, cell contraction rapidly returned to baseline of preischemic level, with continued reperfusion the contraction gradually declined to 85+9%.In our experiments, it was found that insulin only at concentration as high as 10 IU/L could increase cell shortening by 16+5% in normal myocytes ( n=10, P<0.05), whereas insulin at 1 IU/L augmented I/Rcell shortening by 62+8% (n=10, P<0.01). Insulin (0.01-10 IU/L) con...
|
PDF Full Text Request