Quantitative analysis of Na⁺ and Ca²⁺ current contributions on spike initiation in the newt olfactory receptor cell

The contribution of Na⁺ (I(Na)) and T-type Ca²⁺ (I(Ca,T)) currents on the action potential initiation in newt olfactory receptor cells (ORC) was investigated quantitatively. Since both the Na⁺ channel and T-type Ca²⁺ channel show rapid inactivation, it was questioned how much I(Na) and I(Ca,T) are activated during the gradually depolarizing initial phase of the receptor potential. Because the conventional voltage clamp technique does not allow direct current measurement during voltage changes, we estimated the amplitude of I(Na) and I(Ca,T) in an ionic current model. The model was constructed based on voltage clamp experiments. The estimated ratio of I(Ca,T)/I(Na) during the action potential induced by ramp current injection was 0.59±0.03 (mean±SD) under control Ringer's solution. This ratio was more than twice as large as that (0.25) obtained from the previous voltage clamp experiment using step commands. This result suggests that the T-type Ca²⁺ channel carries a more significant amount of current to generate the action potentials than expected previously.