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