By using the method of Hodgkin, McNaughton & Nunn (1985) for rapidly changing the extracellular medium, we analysed the effect of the organic compound IBMX (3-isobutyl-1-methylxanthine) on the movement of divalent cations through the light-sensitive channels of isolated retinal rods of the tiger salamander. 2. When the rod is treated with 0.5 mM-IBMX it is possible to observe photocurrents larger than 50 pA carried by Ba2+, Sr2+, Ca2+, Mg2+ and Mn2+. Under these conditions Ca2+, Mg2+ and Mn2+ carry photocurrents of similar amplitude, while Ba2+ and Sr2+ usually carry larger photocurrents. 3. The movement of Mn2+ through the light-sensitive channel, which is hardly detected under normal conditions, can also be observed after treating the rod for a few seconds with a solution containing 35 mM[Na+]o and 10(-7) M[Ca2+]o. Under these conditions the photocurrent carried by Mn2+ is fully saturated in the presence of 1 mM-extracellular Mn2+. 4. When the rod is pre-treated with an extracellular solution containing 0.5 mM-IBMX the maximal photocurrent which can be carried by 10 mM [Ca2+]o increases from about 10 pA to approximately 200 pA. In these conditions the half-activation of the Ca2+ current is between 1 and 10 mM, that is 20-50 times higher than in normal conditions (Menini, Rispoli & Torre, 1988). 5. When the rod is pre-treated with an extracellular solution containing 0.5 mM-IBMX the half-activation of the photocurrent which can be carried by Mg2+, Ba2+ and Sr2+ is equivalent to or greater than 10 mM. In the absence of pre-treatment with IBMX the half-activation of the photocurrent carried by Mg2+, Ba2+ and Sr2+ is less than 5 mM. 6. We conclude that the light-sensitive channel can exist in at least two distinct open states. The selectivity of the channel in the first open state is as described in a previous paper (Menini et al. 1988). Mn2+, which is hardly permeable through the light-sensitive channel in the first open state, can move through the light-sensitive channel in the second open state. Ca2+, Mg2+, Ba2+ and Sr2+ permeate more freely through the light-sensitive channel in the second open state, probably because the electrostatic interactions between these ions and the channel are less strong.
