The slow current results from an increase in a Ca-permeable membrane conductance. (A) The photoreceptor input resistance was continuously monitored by applying a repetitive rectangular command step, 10 mV in amplitude, superimposed upon the steady holding potential (−60 mV). After the flash, the size of the current steps gradually increased (insets) concomitantly with the development of the slow current, indicating a reduction in membrane resistance. (B) To assess the permeation of individual divalent cations, photoreceptors were tested in an extracellular solution containing either 60 mM Ca²⁺ or 60 mM Mg²⁺ as the sole divalent species. The internal solution contained no sodium to prevent calcium loading via reverse operation of the Na/Ca exchanger, and the holding potential was set at −50 mV. A robust I_slow was observed in the presence of Ca²⁺ (top trace), whereas it was entirely absent with Mg²⁺ (bottom trace). Light intensity of 3.2 × 10¹⁵ photons × cm⁻² × s⁻¹. (C) I_slow does not invert with membrane depolarization to +40 mV, whereas the primary photocurrent does. The holding potential was stepped a few seconds before the recording. Although considerably reduced in amplitude, the slow current remained inwardly directed.