Voltage dependence of the block by lanthanum. (A) Photocurrent elicited by a light of approximately half-saturating intensity (2.6 × 10¹¹ photons × s⁻¹cm⁻²) in a Hesse cell voltage clamped sequentially at −50, −30, and −10 mV in ASW (left) and during exposure to 100 µM La³⁺ (right). The antagonistic effects of lanthanum diminished as the membrane was depolarized. (B) Pooled data (n = 5, three Hesse and two Joseph cells) showing the average relative amplitude of the photocurrent (normalized by the response in ASW at −50 mV) in control condition (closed squares) and in the presence of lanthanum (open squares). (C) Degree of blockage by lanthanum as a function of membrane potential. The bar graph plots the relative peak amplitude of the photocurrent during superfusion with La³⁺ versus in ASW.