AGE-RELATED CHANGES IN THE TIME COURSE OF THE CA²⁺ TRANSIENT IN CONTRACTING RABBIT MYOCYTES. † 225

The relative roles of voltage-gated Ca²⁺ channels and the Na⁺-Ca²⁺ exchanger in providing Ca²⁺ for tension generation in immature heart are not well established. Developmental changes in the mechanisms underlying Ca²⁺ influx may influence the time course of the intracellular [Ca²⁺] transient. Accordingly, we have characterized age-related changes in the time course of the rise in[Ca²⁺] during cell contraction. Single ventricular cardiac myocytes were isolated from neonatal (3D), young (14D and 30D), and adult (60D) New Zealand White rabbits. Myocytes were loaded with the Ca²⁺ sensitive fluorescent dye, Indo-1 AM (0.1 mM). The time course of [Ca²⁺] was measured using the ratio of the light emitted at wavelengths of 400 and 500 nm. We found that the time to the peak rise in [Ca²⁺], (T_p), shortened markedly with development. T_p was 462±48 ms (8) for 3D, 262±52 ms (8) for 14D, 82±18 ms (5) for 30D and 50±5 ms (7) for 60D myocytes [values represent mean±SEM (n)]. Thus, unlike adult heart, T_p in neonates roughly corresponds to the duration of the cardiac action potential. In neonatal cells, T_p shortened to 313±64 ms (6) when extracellular Ca²⁺ was increased from 1.8 to 5 mM. Similarly, T_p shortened to 306±58 ms (5) when cells were loaded with a lower concentration of Indo-1 AM (0.05 mM). These effects likely reflect the shortening of the action potential seen in immature heart when intracellular Ca²⁺ is not buffered. T_p in adult cells was not significantly affected by changes in extracellular Ca²⁺ or by changes in Indo-1 loading: T_p was 61±17 ms (4) in 5 mM Ca²⁺ and 80±23 ms (3) when loaded in 0.05 mM Indo-1 AM. The prolonged time to the peak of the Ca²⁺ transient in neonatal heart demonstrates that significant changes in excitation-contraction coupling occur with development. These data suggest that the time course of the Ca²⁺ transient is closely coupled to the action potential duration in immature heart but not in mature heart. Furthermore, since voltage-gated Ca²⁺ channels rapidly inactivate, the slow, sustained rise in [Ca²⁺] in the 3D myocytes suggests that the Na⁺-Ca²⁺ exchanger may play an important role in Ca²⁺ influx in immature heart.