Fig. 2.

Dependence of somatic Ca²⁺ responses on extracellular Ca²⁺ concentration. A, The action potential-induced intracellular Ca²⁺ rise is independent of external Ca²⁺ concentration from 500 to 3000 μm (data points and solid line). Thedashed line illustrates the expected results of this experiment under the assumption that the intracellular Ca²⁺rise was the direct result of Ca²⁺ influx from voltage-gated Ca²⁺ channels. B, Only when external [Ca²⁺] is lowered below 50 μm does the action potential-induced Ca²⁺ rise become diminished.C, Dependence of somatic Ca²⁺ responses on extracellular lanthanum concentration. La³⁺ suppressed the action potential-induced intracellular Ca²⁺ rise but with a much higher IC₅₀ (13 μm) than expected. An extracellular [Ca²⁺] of 1.5 mm was present throughout. The dashed line again represents the expectation based on the assumption of the Ca²⁺ rise coming directly from influx via voltage-gated Ca²⁺ channels, modeled on data from the effect of La³⁺ on individual voltage-gated Ca²⁺ channels, with an IC₅₀ of 1.7 μm. The solid line is the best fit to the data with the form: A =aIC₅₀/([La³⁺] + IC₅₀), where IC₅₀ = 13 μm, anda = 262 nm.