Which ion channel primarily mediates the fast upstroke (phase 0) in non-nodal cardiac cells?

• A. Ca++ channels
• B. Na+ channels
• C. K+ channels
• D. Cl- channels

Answer: (B)

The fast upstroke in non-nodal cardiac cells is produced by a rapid inward Na+ current through fast voltage-gated sodium channels (Nav1.5). When these cells depolarize from a highly negative resting potential, these channels activate quickly, allowing a large influx of Na+ that drives a steep, rapid rise in membrane potential (phase 0) and sets the pace for conduction velocity. Calcium channels mainly carry the plateau phase and, in nodal tissue (where the upstroke is slower), calcium influx through L-type (and T-type) channels can drive phase 0. Potassium channels mediate repolarization, not the depolarizing upstroke, and chloride channels play little role in phase 0.