Control of the Dark Current in Vertebrate Rods and Cones

Actions of EGTA and high calcium on the cones in the turtle retina.

“…It has been shown that increasing external Ca2+ reduces the photocurrent which can be recorded with extracellular micro-electrodes from the isolated retina of rats (Yoshikami & Hagins, 1973). In agreement with these findings, intracellular measurements in the rods of Bufo marines demonstrated that high external Ca2+ increases membrane potential in darkness and reduces the amplitude of the responses to light (Brown & Pinto, 1974).…”

“…Since Gi is the value of the conductance Gi when z1 = 0, the assumption that the conductance G1 is increased by removal of Ca2+, implies that Ca2+ and Z1 are different substances, even though both can block the same channels. The alternative interpretation, that Ca2+ and Z1 are the same substance, implies that a concentration of Z, sufficient to block 80 % of the light sensitive channels (a fraction required to account for the increase of conductance observed when the blocking particles are removed by EGTA) is already present in darkness (see Yoshikami & Hagins, 1973;Simon, Hodgkin & Lamb, 1975).…”

“…In agreement with these findings, intracellular measurements in the rods of Bufo marines demonstrated that high external Ca2+ increases membrane potential in darkness and reduces the amplitude of the responses to light (Brown & Pinto, 1974). Yoshikami & Hagins (1973) found that the light intensity required to produce half-maximal photocurrent is increased tenfold when the retina is kept in low Ca2+ while Brown & Pinto (1974) observed depolarization in darkness and corresponding increase of responses to bright lights. If indeed the blocking particles responsible for visual responses are inside disks in rods but are in the external fluids in cones, changes of outside Ca2+ concentration should have a more direct effect on cones than on rods.…”

Sodium‐calcium exchange in the outer segments of bovine rod photoreceptors.

“…These effects are generally thought to be mediated by changes in the intracellular Ca2+. The internal Caa2+ concentration has been proposed to control the light-sensitive conductance (Yoshikami & Hagins, 1971), while it has been suggested that the internal Ca2+ concentration is controlled by Na-Ca exchange (Fain & Lisman, 1981; Schnetkamp, 1981).…”

Selective photoinduced Ca2+ diffusion in rhodopsin–lipid bilayers: an internal transmitter hypothesis

“…The "dark current" seems to be carried by Na+ since the photo-response is suppressed by a complete removal of Nat from the external medium (9,10) and the "dark current" is reduced in the presence of ouabain (3,10). Most researchers agree that the effect of light, most likely, is to cause reduction of the sodium conductance ( 5 , 9 , 10, 1 I ) by means of liberation of an internal transmitter which can block sodium channels present in the plasma membrane (10,12,13). The efficiency of light is so great that a single absorbed photon can transiently reduce a rod's "dark current" by about 3 % (14).…”