The present study examined cardiac myocytecontractile and
Ca2+ Transient responses to insulin duringsimulated
ischemia/reperfusion
(I/R) and furtherinvestigated the role of protein kinase B (Akt) in the insulin-induced inotropic effect. Ventricular myocytes wereenzymatically isolated from adult Sprague-Dawley rats and perfused with Tyrode solution while electrically field-stimulated. Simulated I/R was induced by perfusing the cells with chemical anoxic solution including sodiumcyanide-sodium lactate for 15 min followed by reperfusionwith normal oxygenated Tyrode solution with or withoutinsulin. It is found that insulin only at concentration as high as 10 IU/L could increase cell shortening (16±5%, P < 0.05) in normal myocytes, whereas it concentration-dependently (0.01—10 IU/L) increased the contraction,the velocity ofshortening/releng- thening and
Ca2+ Transient in I/Rmyocytes. In addition, insulin treatment (1 IU/L) increasedAkt phosphorylation of I/R cardiomyocytes by 2.4-foldcompared with that of the control (P < 0.01). Mostimportantly, pretreatment with LY 294002, a specificinhibitor of phosphatidylinositol 3′-kinase (PI3-kinase), significantly inhibited both Akt phosphorylation and thepositive inotropic response to insulin in the I/Rcardiomyocytes. These results suggest that insulin exertsdirect positive inotropic effect by increasing Ca2+ transient of cardiomyocytes, which is enhanced in the pathologicalcondition of I/R. Akt activation plays an important role inthe insulin-induced improvement of myocyte contractile function following I/R.