Cardiomyocytes from hearts with left ventricular dysfunction after ischemia-reperfusion do not manifest contractile abnormalities

OBJECTIVES: This study evaluated contractile function in cardiomyocytes isolated from hearts with global left ventricular dysfunction following ischemia-reperfusion. BACKGROUND: Ischemia followed by reperfusion is associated with transient contractile dysfunction, termed 'stunning.' It is not clear whether this phenomenon is primarily due to intrinsic cardiomyocyte contractile dysfunction. METHODS: Global contractile dysfunction was induced in isolated perfused rat hearts (n = 8) using a model of transient global ischemia (20 min) followed by reperfusion (20 min). Hearts perfused uninterrupted for 40 min were used as controls (n = 8). Cardiomyocytes were isolated using enzymatic digestion and were studied under varying degrees of inotropy (using increasing extracellular calcium [Ca²⁺]o) and loading conditions (varying extracellular perfusate viscosity). Mechanical function was studied with video edge detection and intracellular calcium ([Ca²⁺]i) kinetics using fura-2 AM. RESULTS: Global ischemia-reperfusion increased left ventricle (LV) end diastolic pressure (450% vs. 33%, p < 0.01) and reduced LV developed pressure (9% rs. 33%, p < 0.01), LV positive (3% rs. 26%, p < 0.01) and negative (5% vs. 33%, p < 0.01) dP/dt. However, cells isolated from these hearts did not manifest contractile dysfunction. In fact, cell shortening (p < 0.0001) and peak rate of cell shortening (p < 0.05) and increase in [Ca²⁺]i with each contraction (p < 0.024) were higher in these cells during stimulation with [Ca²⁺]o of 1 to 10 mmol/liter. The EC₅₀ values, for calcium dose response and the slope of the relation between change m [Ca²⁺ and change in cell length were no different between the groups. Cell loading (with increasing superfusate viscosity from 1 cp to 300 cp) also did not reveal any abnormalities in cells from the hearts subjected to ischemia- reperfusion. CONCLUSIONS: Cardiomyocytes isolated from hearts with ischemia- reperfusion-induced LV dysfunction or 'stunning' have normal contractile function and normal [Ca²⁺]i transients, when studied both in the unloaded and loaded state. Our data suggest that nonmyocyte factors such as abnormalities in extracellular matrix or abnormal myocyte-interstitial tissue coupling may be important for the genesis of cardiac contractile failure in the stunned heart.