title: The ionic selectivity and calcium dependence of the light-sensitive pathway in toad rods.
abstract: A new method is described for determining the effects of rapid changes in ionic concentration on the light-sensitive currents of rod outer segments. Replacing Na with another monovalent cation caused a rapid change in current followed by an exponential decline of time constant 0.5-2 s. From the magnitude of the initial rapid change in current we conclude that Li, Na, and K and Rb ions pass readily through the light-sensitive channel in the presence of 1 mM-Ca, whereas Cs crosses with difficulty and choline, tetramethylammonium and tetraethylammonium not at all. The effect of reducing Ca in the external medium indicates that the residual inward current recorded for a few seconds when Na is replaced by an impermeant ion is carried largely by Ca ions. With 1 microM-Ca in the external medium the relative ability of monovalent cations to carry light-sensitive current is Li:Na:K:Rb:Cs = 1.4:1:0.8:0.6:0.15. The same order applied in the physiological region but the values are less certain. Large transient inward currents are seen if external Ca is raised form 1 microM to 5 mM or more; these currents which are maximal in an isotonic Ca solution are presumably carried by Ca. The effect of monovalent cations on the number of open light-sensitive channels was tested by adding the cation to a solution containing 55 mM-Na. Na ions open light-sensitive channels with a delay, probably by promoting Na-Ca exchange; K and Rb close channels by inhibiting exchange; Li and Cs seem inert in the exchange mechanism. The rate at which inward current declines in low [Na]o or high [Ca]o is accelerated by weak background lights and slowed by 3-isobutyl-1-methylxanthine (IBMX), which inhibits the hydrolysis of cGMP. On returning to Ringer solution after a period in low [Na]o the current recovers with a delay of about 1 s which decreases as the Ca concentration of the low [Na]o medium is reduced. We conclude that intracellular Ca has a strong effect on the number of open light-sensitive channels. None the less, several observations are inconsistent with channel closure being dependent simply on combination with internal Ca.
authors.forename: A L
authors.initials: AL
authors.lastname: Hodgkin
authors.forename: P A
authors.initials: PA
authors.lastname: McNaughton
authors.forename: B J
authors.initials: BJ
authors.lastname: Nunn
mesh_terms: D000200:Action Potentials; D000818:Animals; D002024:Bufo marinus; D002118:Calcium / Q000502:physiology*; D002414:Cations, Monovalent / Q000378:metabolism; D002463:Cell Membrane Permeability; D002794:Choline / Q000494:pharmacology; D004553:Electric Conductivity; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D008027:Light; D010786:Photoreceptor Cells / Q000502:physiology*; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D012374:Rod Cell Outer Segment / Q000502:physiology*; D012964:Sodium / Q000502:physiology; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
chemical_list: D002414:Cations, Monovalent; D007473:Ion Channels; D000644:Quaternary Ammonium Compounds; D013757:Tetraethylammonium Compounds; D019789:Tetraethylammonium; D012964:Sodium; C027917:tetramethylammonium; D002794:Choline; D002118:Calcium