Figure 7.

Synaptic transmission between VD1 and heart muscle cells in vitro. A, Effect of synthetic α2 peptide (10^–8 m) applied by pressure application on a ventricle muscle fiber (VMF). B–F, Chemical synapses between VD1 and auricle muscle fiber (AMF) and ventricle muscle fiber reform in cell culture. Simultaneous intracellular recordings from VD1 and auricle muscle fiber did not reveal electrical coupling between the cells. However, depolarizing current injected in VD1 (at filled arrow) triggered spikes in this cell, which induced 1:1 EJPs in the auricle muscle fiber. Open arrow represents hyperpolarizing current injection to determine the incidence of electrical coupling between the cells. C, Similarly, induced action potentials in VD1 induced 1:1 EJPs in the ventricle muscle fiber. D–F, Voltage dependence of the muscle cell response to VD1 stimulation. To show that the synapse between VD1 and the auricle muscle fiber is indeed excitatory, the postsynaptic potentials were tested at various different membrane potentials (–80 to –60 mV). Raising the membrane potential closer to its threshold (–60 mV; F) generated action potential in the muscle fiber. G, A heart muscle fiber cocultured with a nonsynaptic partner, RPeD1, did not develop synapses because the induced action potentials in RPeD1 failed to elicit any response in the ventricle muscle fiber despite physical contacts between the cells.