Developmental differences in the response of cytosolic free calcium to potassium depolarization and cardioplegia in cardiac myocytes

Several biochemical and functional characteristics of immature myocardium suggest a diminished capacity to regulate intracellular Ca²⁺ during stress. In particular, cellular calcium overload has been postulated as an important pathogenetic mechanism accounting for suboptimal functional recovery following cardioplegia in immature myocardium. Using intracellular Fura-2 fluorescence as Ca²⁺ indicator, we measured cytosolic free calcium ([Ca_i]) in single myocytes and cell suspensions derived from both juvenile (4 weeks post-partum) and mature (6-12 months post-partum) New Zealand white rabbits. Resting [Ca_i] in juvenile heart cells (26±3 nm) were ∼50% of that found in adult myocytes (55±5 nm). In addition, on exposure to increasing concentrations of extracellular potassium ([K_ex]), adult but not juvenile myocytes exhibited increases in [Ca_i]. These two observations underscore developmental differences in intracellular Ca²⁺ homeostasis. Of particular clinical relevance is the [Ca_i] response to cardioplegia containing 16 mm [K_ex]: neither group demonstrated the expected [Ca_i] increase in response to potassium depolarization. The lack of [Ca_i] response to cardioplegia was most likely due to the high levels of Mg²⁺ (32 mm) contained in cardioplegic solutions. We conclude that cellular calcium overload does not occur following exposure to cardioplegia alone. Accordingly, these findings do not account for recognized developmental differences in functional recovery from "myocardial protection".