An intracellular recording from a sinoatrial node pacemaker cell shows a maximum diastolic potential near -60 mV and a phase 0 upstroke velocity far slower than that of a ventricular myocyte. What accounts for the slow upstroke in these cells?
- A Inward current through L-type voltage-gated calcium channels, since fast sodium channels are largely inactivated at this resting potential ✓
- B Outward current through delayed rectifier potassium channels activating slowly
- C Inward current through hyperpolarization-activated cyclic nucleotide-gated channels
- D Calcium-induced calcium release from the sarcoplasmic reticulum depolarizing the sarcolemma
Correct answer: A. Inward current through L-type voltage-gated calcium channels, since fast sodium channels are largely inactivated at this resting potential
Explanation
At a maximum diastolic potential of about -55 to -60 mV, most fast sodium channels remain inactivated, so phase 0 depends on the relatively slow inward L-type calcium current. This produces a slow-response action potential with conduction velocities under 0.1 m/s. Option C describes the funny current responsible for phase 4 depolarization, not the upstroke, while options B and D do not generate the rising phase of the pacemaker potential.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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