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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    The journal of membrane biology 88 (1985), S. 263-275 
    ISSN: 1432-1424
    Keywords: vision ; rod photoreceptors ; ionic channels
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The permeability properties of the plasma membrane of intact rod outer segments purified from bovine retinas (ROS) were studied with the aid of the optical probe neutral red as described in the companion paper. The following observations were made: (1) Electrical shunting of ROS membranes greatly stimulated Na+ and K+ transport, suggesting that this transport reflects Na+ and K+ currents, respectively. The dissipation of a Na+ gradient across the plasma membrane occurred with a half-time of 30 sec at 25°C. (2) The Na+ permeability was progressively inhibited when the external Ca2+ concentration was raised from 1 μm to 20mm. A similar Ca2+ dependence was observed for H+ and Li+ transport. The Na+ permeability was not affected when the total internal Ca2+ content of ROS was varied between 0.1 mol Ca2+/mol rhodopsin and 7 mol Ca2+/mol rhodopsin, or when the free internal Ca2+ concentration was varied between 0.1 and 50 μm. (3) The K+ permeability was progressively stimulated when the external Ca2+ concentration was raised from 0.001 to 1 μm, whereas a further increase to 20mm was without effect. A similar Ca2+ dependence was observed for Rb+ and Cs+ transport. (4) At an external Ca2+ concentration in the micromolar range the rate of transport decreased in the order: Na+〉K+=H+〉Cs+〉Li+. (5) Na+ fluxes depended in a sigmoidal way on the external Na+ concentration, suggesting that sodium ions move in pairs. The concentration dependence of uniport Na+ transport and that of Na+-stimulated Ca2+ efflux (exchange or antiport transport) were very similar.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    The journal of membrane biology 108 (1989), S. 91-102 
    ISSN: 1432-1424
    Keywords: photoreceptors ; vision ; ion channels ; silver ions ; sulfhydryl modification ; Ca2+ transport
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Micromolar concentrations of silver ion activate large Ca2+ fluxes across the plasma membrane of intact rod outer segments isolated from bovine retinas (intact ROS). The rate of Ag+-induced Ca2+ efflux from intact ROS depended on the Ag+ concentration in a sigmoidal manner suggesting a cooperative mechanism with a Hill coefficient between 2 and 3. At a concentration of 50 μm Ag+ the rate of Ca2+ efflux was 7×106 Ca2+/outer segment/sec; this represents a change in total intracellular Ca2+ by 0.7mm/outer segment/sec. Addition of the nonselective ionophore gramicidin in the absence of external alkali cations greatly reduced the Ag+-induced Ca2+ efflux from intact ROS, apparently by enabling internal alkali cations to leak out. Adding back alkali cations to the external medium restored Ag+-induced Ca2+ efflux when gramicidin was present. In the presence of gramicidin, Ag+-induced Ca2+ efflux from intact ROS was blocked by 50 μm tetracaine orl-cis diltiazem, whereas without gramicidin both blockers were ineffective. Bothl-cis diltiazem and tetracaine are blockers of one kinetic component of cGMP-induced Ca2+ flux across ROS disk membranes. The ion selectivity of the Ag+-induced pathway proved to be broad with little discrimination between the alkali cations Li+, Na+, K+, and Cs+ or between Ca2+ and Mg2+. The properties of the Ag+-induced pathway(s) suggest that it may reflect the cGMP-dependent conductance opened in the absence of cGMP by silver ions.
    Type of Medium: Electronic Resource
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