What is the equilibrium potential for calcium (ECa)?

What is the equilibrium potential for calcium (ECa)?

• A. -96 mV
• B. +52 mV
• C. +134 mV
• D. 0 mV

Answer: (C)

Calcium equilibrium potential is set by the Nernst equation for Ca2+ a divalent ion, so the membrane potential that would balance diffusion is very positive. Using E = (RT/zF) ln([Ca2+]o/[Ca2+]i) with z = +2, at body temperature (RT/F ≈ 26.7 mV) gives ECa ≈ (26.7/2) ln([Ca2+]o/[Ca2+]i) ≈ 13.35 mV × ln([Ca2+]o/[Ca2+]i). With typical extracellular Ca2+ around 2 mM and intracellular free Ca2+ around 0.0001 mM (0.1 μM), the ratio is about 20,000 and ln ≈ 9.9, yielding ECa ≈ 130–135 mV. So the equilibrium potential for calcium is highly positive, around +134 mV. The other values don’t fit this large positive driving force for Ca2+.

Calcium equilibrium potential is set by the Nernst equation for Ca2+ a divalent ion, so the membrane potential that would balance diffusion is very positive. Using E = (RT/zF) ln([Ca2+]o/[Ca2+]i) with z = +2, at body temperature (RT/F ≈ 26.7 mV) gives ECa ≈ (26.7/2) ln([Ca2+]o/[Ca2+]i) ≈ 13.35 mV × ln([Ca2+]o/[Ca2+]i). With typical extracellular Ca2+ around 2 mM and intracellular free Ca2+ around 0.0001 mM (0.1 μM), the ratio is about 20,000 and ln ≈ 9.9, yielding ECa ≈ 130–135 mV. So the equilibrium potential for calcium is highly positive, around +134 mV. The other values don’t fit this large positive driving force for Ca2+.