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The modulation of the ionic selectivity of the light‐sensitive current in isolated rods of the tiger salamander.
Author(s) -
Cervetto L,
Menini A,
Rispoli G,
Torre V
Publication year - 1988
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1988.sp017375
Subject(s) - ibmx , photocurrent , extracellular , chemistry , divalent , biophysics , analytical chemistry (journal) , materials science , biochemistry , forskolin , chromatography , biology , optoelectronics , receptor , organic chemistry
1. 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.