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Dipartimento di Fisica, Università di Genova, Italy.

1. Using the method of Hodgkin, McNaughton & Nunn (1985) for rapidly changing the extracellular medium, we analysed the effect of divalent cations on the photocurrent of isolated retinal rods of the tiger salamander. 2. When the extracellular NaCl was replaced by equiosmolar amounts of BaCl2, SrCl2, CaCl2, MgCl2 and MnCl2 the efficacy in carrying the photocurrent at early times was Ba2+ greater than Sr2+ greater than Ca2+ greater than Mg2+ greater than Mn2+. At early times Ba2+ could carry a photocurrent similar to or larger than that carried by Na+. 3. The photocurrent carried by Ba2+ increased by about 50% when [Ca2+]o was reduced from 1 to 0.1 mM. In the presence of 0.1 mM-Ca2+ in the extracellular medium the photocurrent carried by Ba2+ saturated when [Ba2+]o was close to 50 mM and was half-activated at 15 mM [Ba2+]o. 4. The photocurrent which can be carried by Sr2+ is not larger than that carried by Ba2+ and does not saturate for [Sr2+]o up to 70 mM. 5. When extracellular Na+ is replaced by the impermeant organic ion choline it is possible to observe a transient photocurrent which is carried by Ca2+. This current has a maximal value of about 11 pA and has a half-activation constant of about 50 microM. 6. Movements of Mg2+ across the light-sensitive channel can be seen only when extracellular Ca2+ is reduced below 10 microM. Under these conditions the maximal photocurrent which can be carried by Mg2+ at early times is about 8 pA and has a half-activation of about 2 mM. Under normal conditions Mn2+ is hardly permeable through the light-sensitive channel. 7. It is concluded that the selectivity of the light-sensitive channel in the low ionic concentration range is Ca2+ greater than Sr2+ greater than Ba2+ greater than Mg2+ greater than Na+.

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