DEVELOPMENTAL DIFFERENCES IN MYOCYTE CONTRACTILE RESPONSE AFTER CARDIOPLEGIC ARREST

Although developmental differences in left ventricular function after cardioplegic arrest and rewarming have been postulated, whether differ ences exist at the level of the myocyte remains unexplored. This proje ct tested the hypothesis that there is a differential effect of hypoth ermic hyperkalemic cardioplegic arrest with subsequent rewarming on co ntractile function of immature compared with adult ventricular myocyte s. Myocytes were isolated from the left ventricular free wall of five immature and five adult rabbits and incubated for 2 hours in hyperkale mic modified Ringer's solution at 4 degrees C (cardioplegia) or for 2 hours in cell culture medium at 37 degrees C (normothermia). Myocytes were resuspended (''rewarmed'') in 37 degrees C cell culture medium af ter the incubation protocol. Normothermic baseline contractile perform ance was lower in immature, compared with adult, myocytes. Specificall y, myocyte shortening velocity was 62 +/- 4 mu m/sec in immature and 1 12 +/- 6 mu m/sec in adult myocytes (p < 0.01). After cardioplegia and rewarming, immature myocyte contractile function was unchanged, where as adult myocyte contractile function was significantly diminished. Fo r example, myocyte shortening velocity was 65 +/- mu m/sec in immature and 58 +/- 3 mu m/sec in adult myocytes (p < 0.01 versus normothermic ). Myocyte surface area, which reflects myocyte volume, was increased after cardioplegia and rewarming in adults (3582 +/- 55 versus 3316 +/ - 46 mu m(2), p < 0.01), but remained unchanged in immature myocytes ( 2212 +/- 27 versus 2285 +/- 28 mu m(2), p = not significant). These un ique findings demonstrate a preservation of myocyte contractile functi on and volume regulation in immature myocytes after cardioplegic arres t and rewarming. Thus this study directly demonstrates that developmen tal differences exist in myocyte responses to hypothermic hyperkalemic cardioplegic arrest with subsequent rewarming.