On the metabolism of the rod outer segments.

Abstract

SUMMARY1. The high energy phosphate esters available for the luminescent reaction of the firefly lantern extract correspond, in the dark adapted rods of the frog, to 7 x 10-16 mole of ATP per rod, corresponding to ca. 1'4 mM. 5. The ionophore A23187 decreases the high energy phosphate esters when the extracellular free Ca concentration is greater than 10-7 M, suggesting that ATP is consumed in pumping Ca ions out of the rods, or into the disks contained in the rods, or both.

Cytochemical demonstration of guanylate cyclase activity in retinal photoreceptors with special reference to changes under light and dark adaptation.

“…This suggests that the reaction products were not due to phosphodiesterase activity. Recently light activated GTPase has been found in rod outer segments (8,44,48), but GMP-PNP used in this study as substrate is not hydrolysed by GTPase, and control studies also show that the reaction products were not due to phosphatase.…”

Cyclic GMP releases calcium from disc membranes of vertebrate photoreceptors.

“…How-ever, several lines of evidence argue against 'this possibility: (i) Exogenous GTP and ATP do not affect either calcium uptake or the cGMP effect on Ca uptake. Endogenous ATP and GTP have been reported to be hydrolyzed within seconds after isolation of the ROS (14,21). Therefore, it is difficult to imagine the development ofan energy-dependent Ca concentration gradient under the present experimental conditions (without added triphosphates).…”

Light‐activated hydrolysis of GTP and cyclic GMP in the rod outer segments.

“…The rate of hydrolysis of GTP, after a flash bleaching 10% of the rhodopsin, 2-5-3-5 n-mole/min per mg of rhodopsin, is sufficiently fast to account for the decrease of triphosphates observed in isolated rods (Robinson et al 1975: Carretta & Cavaggioni, 1976Bignetti & Cavaggioni, 1977), but small enough to have eluded some previous attempts to measure it. The complexity of this system is documented by the formation of GMP, as a product of hydrolysis of GTP, and, conversely, by the formation of GDP in the presence of kinase in our preparations.…”

“…The frog rods were isolated from the retinae of bullfrogs (Rana catesbeiana) in Ringer solution, and sedimented by the method described elsewhere (Carretta & Cavaggioni, 1976). The pellet was resuspended in 300 1I.…”

“…Previous studies have shown that (i) the illumination of isolated frog rods induces the hydrolysis of high energy triphosphates, (ii) this reaction is sensitive to stimuli that bleach a small amount of photopigment, (iii) the reaction is rapid and within 1 sec a sizeable fraction of the nucleotides is hydrolysed and (iv) after weak illuminations the level of these compounds is restored in darkness (Robinson, Yoshikami & Hagins, 1975;Carretta & Cavaggioni, 1976;Bignetti & Cavaggioni, 1977).…”