[Na] in the subsarcolemmal ‘fuzzy’ space and modulation of [Ca]i and contraction in cardiac myocytes

The strength of the heart beat depends on the amplitude and time course of the transient increase in Ca²⁺] in the myocytes with each cycle. Na⁺]_i modulates cardiac contraction through its effect on the Ca²⁺ flux through the Na/Ca exchanger. Cardiac excitation–contraction coupling has been postulated to occur in a microdomain or ‘fuzzy’ space at the junction of the T-tubules and the sarcoplasmic reticulum. This ‘fuzzy’ space is well described for the Ca²⁺ fluxes and the interaction between the L-type Ca²⁺ channel, the Ca²⁺ release channel of the sarcoplasmic reticulum and the Na/Ca exchanger. Co-localization of the Na⁺ transporters, in particular the Na/K pump and the Na⁺ channel, within this ‘fuzzy’ space is not as well established. The functional and morphological characteristics of the ‘fuzzy’ space for Na⁺ and its interaction with the Ca²⁺ handling suggest that this space is not strictly co-inciding with the Ca²⁺ microdomain. In this space Na⁺] can be several-fold higher or lower than Na⁺] in the bulk cytosol. This has implications for modulation of Ca²⁺]_i during a single beat as well as during alterations in Na⁺ fluxes seen in pathological conditions.