It has been hypothesized that the light-evoked rod hyperpolarization (the receptor potential) initiates the light-evoked decrease in extracellular potassium ion concentration, [K+]o, in the distal retina. The hypothesis was tested using the isolated, superfused retina of the toad, Bufo marinus; the receptor potential was recorded intraceilularly from red rods, and [K+]o was measured in the photoreceptor layer with K+-specific microelectrodes. In support of the hypothesis, variations in stimulus irradiance or duration, or in retinal temperature, produced qualitatively similar effects on both the receptor potential and the decrease in [K+]o. A mechanism for the relationship between the receptor potential and the decrease in [K+]o was suggested by Matsuura et al. (1978. Vision . In the dark, the passive efflux of K + out of the rod is balanced by an equal influx of K + from the Na+/K + pump. The light-evoked rod hyperpolarization is assumed to reduce the passive efflux, with little effect on the pump. Thus, the influx will exceed the efflux, and [K+]o will decrease. Consistent with this mechanism, the largest and most rapid decrease in [K+]o was measured adjacent to the rod inner segments, where the Na+/K + pump is most likely located; in addition, inhibition of the pump with ouabain abolished the decrease in [K+]o more rapidly than the rod hyperpolarization. Based upon this mechanism, Matsuura et al. (1978) developed a mathematical model: over a wide range of stimulus irradiance, this model successfully predicts the timecourse of the decrease in [K+]o, given only the time-course of the rod hyperpolarization.

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A technique has been developed for embedding alumina particles 0.05 micrometer in size in the surface of a polyurethane film laid on glass. This abrasive surface is used for rapid, precise, and reliable beveling of Pyrex micropipettes with tip diameters at least as small as 0.1 micrometer. In the snapping turtle retina the beveled electrodes give much better cell penetration and intracellular response stability than unbeveled electrodes of considerably higher proach to this problem.

Intracellular recording in outer segments of red rods of the toad retina showed that increasing Ca2+ concentration in the perfusate mimicked certain aspcts of light adaptation. Light sensitivity was reduced, the amplitude of light responses was reduced, the time course of light responses was altered by shortening the delay to the peak and increasing the decay rate, and the resting membrane potential was generally increased. These results provide further support for the hypothesis that Ca2+ acts as an internal transmitter that is subject to light-induced release from the rod saccules. The Recently, much interest has focused upon the role of ions in generating light responses in the outer segments of vertebrate photoreceptors. Yoshikami and Hagins (1) have proposed that Ca2+ plays a critical role in rods by being concentrated in the saccules of the outer segment and then being released to the cytoplasm after a light-induced conformational change of rhodopsin in the saccule membrane. The released Ca2+ then diffuses to the outer membrane and decreases the membrane conductance for Na+, thus giving rise to the hyperpolarizing light responses recorded intracellularly in vertebrate photoreceptors. Although supported by several lines of evidence (1-4), there are outstanding questions (4), especially concerning the light-induced release of sufficient Ca2+ at low light intensities (4).Bellhorn and Lewis (5) have recently shown by secondary ion mass spectroscopy that barium is much more concentrated in outer and inner segments of photoreceptors than at other sites in the cat retina. This suggests that Ba2+ may also play a crucial role in the normal physiology of rod outer segments. Hence, we examined the effects of both Ca2+ and Ba2+ in the perfusate while recording intracellularly from rod outer segments. METHODSPreparation. Studies were conducted in the isolated and inverted retina of the toad Bufo marinus. A glass contact lensThe costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. 1587with the convex surface upward was sealed into a perfusion chamber mounted on a microscope stage, and the retina was placed on the glass contact lens. Well-focused stimuli were delivered from below through the-condensing lens of the microscope. The receptor surface was viewed continuously by using infrared illumination from below and a Zeiss X40 water-immersion lens. The magnified image was focused upon a silicon diode image tube, which acted as an infrared image converter, in a high-resolution TV camera whose output was displayed upon a monitor. Microelectrodes were introduced at an angle of about 250 from the retinal surface, and contact of an electrode with the receptor surface could be visualized by a slight movement of the tip of the outer segment initially contacted. Scanning electron microscopy has shown that rod outer segments are the only structures availabl...