Decoupling of heart muscle cells: Correlation with increased cytoplasmic calcium activity and with changes of nexus ultrastructure

Purkinje fibers of the sheep heart were exposed to (a) 0.1 mm dihydro-ouabain (DHO), followed by (b) 0.1 mm DHO in Na-free solution or to (c) 1 mm dinitrophenol (DNP). The degree of electrical decoupling was characterized in terms of the inside longitudinal resistance r_i as measured with a 3-microelectrode voltage-clamp technique. Procedure a increased r_i by a factor of 3.7±1.1 (mean±sd), b by a factor of 9.8±2.2, whereas in c incomplete voltage control indicated nearly complete uncoupling. Intracellular calcium activity (aCa_i) was monitored with a microelectrode system. At control conditions aCa_i was below 0.1 μm. The procedures listed above increased aCa_i to (a) 4±1.5 μm, (b) 8±2 μm, and (c) 36±12 μm. The increase of aCa_i was in good correlation with the changes in core resistance. Effects on nexus ultrastructure, investigated with freeze-fracture techniques, are shown in histograms. At control conditions, the particle diameter distributed around a single peak (8.3±0.5 nm). Procedures b and c induced a second population at 10.8 nm; increased decoupling reduced the control population in favor of the 10.8 nm population. Decoupling enlarged the width of the nexus gap by a factor of 1.6; again, the control population decreased in favor of a new population. In the decoupled state the height of the particle was smaller. Pits on the E-face displayed a more regular array and a nearly unchanged center-to-center spacing. Separation into several peaks was not possible due to scatter of the data. We interpret the findings to mean that elevated aCa_i induces a conformational change of the nexus subunits which corresponds to a transition from an open to a closed state. The conformational change can be formally described by a particle contraction which disrupts the continuity with the particle of the adjacent membrane. Purkinje fibers exposed to DNP for 1 hr showed thinned (7.7±0.5 mm) and elongated particles. We suggest that this is a secondary event and not a precursor of functional uncoupling.