ENDOCARDIAL VERSUS EPICARDIAL DIFFERENCES OF INTRACELLULAR FREE CALCIUM UNDER NORMAL AND ISCHEMIC CONDITIONS IN PERFUSED RAT HEARTS

Transmural heterogeneity of myocardial metabolism and function are pre sent in the left ventricle under normal and ischemic conditions. To de termine if endocardial versus epicardial differences of [Ca2+]i are al so present, perfused rat heart studies using indo-1 fluorescence as an index of [Ca2+]i were performed in the left ventricular epicardium an d endocardium. Hearts were studied during control conditions and low-f low ischemia. Results demonstrated the following: 1) At a pacing rate of 1.5 Hz, endocardial levels of diastolic and systolic (Ca2+]i (470+/ -40 and 1,240+/-170 nM) were higher than epicardial levels (290+/-30 a nd 920+/-150 nM). 2) At a more physiological pacing rate of 5 Hz, endo cardial levels of diastolic and systolic [Ca2+]i (680+/-50 and 1,230+/ -70 nM) were also higher than epicardial levels (390+/-20 and 950+/-60 nM). 3) During low-flow ischemia, endocardial levels of diastolic [Ca 2+]i rose to a greater degree (from 680+/-50 to 1,050+/-70 nM at 10% o f control coronary flow) compared with epicardial levels (from 390+/-2 0 to 580+/-40 nM at 10% of control flow), suggesting that the endocard ium is more susceptible to low-flow ischemia. 4) The amplitude of the [Ca2+]i transient was the same at the endocardium (540+/-50 nM) and ep icardium (560+/-50 nM) and did not change during low-flow ischemia, de spite marked contractile dysfunction. These findings are consistent wi th the hypothesis that endocardial versus epicardial differences of [C a2+]i exist under normal and low-flow ischemic conditions and may, in part, account for the previous observations of transmural metabolic an d functional gradients in the left ventricle of the whole heart. Furth ermore, the contractile failure associated with low-flow ischemia is n ot due to a decrease of the [Ca2+]i transient amplitude.