Uptake of calcium by the endoplasmic reticulum of the frog photoreceptor.

Abstract

We studied retinal photoreceptors of Rana pipiens by using techniques designed to investigate calcium localization . Particularly useful were methods in which intracellular sites of calcium uptake were detected by incubation of saponin-treated isolated retinas in calciumcontaining media, with oxalate present as a trapping agent . With these procedures, cell compartments accumulate deposits, which can be shown to contain calcium by x-ray microanalysis . Calcium accumulation was prominent in the rough endoplasmic reticulum in the myoid region . In addition, deposits were observed in agranular reticulum and in certain Golgiassociated compartments of the myoid region, in mitochondria, in axonal reticulum, and in agranular reticulum of presynaptic terminals . Calcium was also detected in the endoplasmic reticulum of retinas fixed directly upon isolation, by a freeze-substitution method . The factors influencing accumulation of calcium in the endoplasmic reticulum were evaluated by a semiquantitative approach based on determining the relative frequency of calcium oxalate crystals under varying conditions. Calcium accumulation was markedly enhanced by ATP. Studies with a nonhydrolyzable ATP analogue (adenylyl-imidodiphosphate) and with inhibitors of the sarcoplasmic reticulum Ca t+-Mg2+ ATPase (mersalyl and tetracaine) indicated that this ATP-dependent calcium uptake reflects an energy-dependent process roughly comparable to that in the sarcoplasmic reticulum .Calcium ions play regulatory roles in a variety of cell types . Most extensively studied is skeletal muscle, in which Ca" regulates the contraction-relaxation cycle (21, 69) . In skeletal muscle (30,67,69) and also in cardiac (17,23,25,81,98) and smooth muscle (18, 71), the sarcoplasmic reticulum, which is a specialized form of the smooth endoplasmic reticulum, is largely responsible for the control of concentrations of calcium ions free in the sarcoplasm . By means ofan energydependent Ca"-Mg 2+ ATPase, the sarcoplasmic reticulum can transport calcium ions into its interior (38,85) where the ions are sequestered by binding to calsequestrin and other binding proteins.Much less is known about the regulation of calcium concentration in cells of the nervous system . There is reason to suspect that the situation may resemble that in muscle, at least to some extent. Though few details are understood, there is good evidence that Ca 2+ plays a focal regulatory part in neurons, participating, for example, in regulation of transmitter release (20,40,47,57) . Neuronal mitochondria can THE JOURNAL OF CELL BIOLOGY " VOLUME

Calcium content and calcium exchange in dark‐adapted toad rods.

“…For example, in one series of measurements from a dark-adapted control retina, the outer segment 40Ca signal in sections floated on water was 814 + 227 (mean+ S.D., n = 250), while that from sections picked up in air was 783+194 (n = 50). This is in accord with previously published observations (see Schr6der, Frings & Stieve, 1980;Ungar, Piscopo, Letizia & Holtzman, 1984). Substantial Ca can be lost, however, if the sections are floated for longer periods of time or with much thinner sections (see Schr6der, Frings & Stieve, 1980;Ornberg & Reese, 1980).…”

“…There is simply too little Ca in the endoplasmic reticulum of their rods to account for our results, no matter how one imagines the Ca to have been redistributed. We suggest as a more likely alternative for the discrepancy between our results and theirs that they have somehow lost the Ca from their outer segments during the dissection or preparation of their tissue, or during the long (1 h) interval between the dissection and cryofixation, and that some of this Ca has accumulated in the endoplasmic reticulum of their cells as a result of a Ca sequestration mechanism known to be localized to this part of the receptor (Ungar, Piscopo, Letizia, & Holtzman, 1984).…”

Calcium regulation in photoreceptors

“…ER and its specializations are found in all parts of the inner segment, including the soma and the synaptic terminal (149)(150). Ca 2+ is sequestered into both rough and smooth ER (151)(152)(153). The density of the ER structures is highest in the subellipsoid space (the myoid), from which other organelles are effectively excluded.…”

“…The myoid ER is involved in synthesis of OS and IS proteins (149). It is thought to be continuous with the smooth ER which has access into synaptic terminals (149,151). ER often appears in the form of the so called subsurface cisternae, which are localized within 30 nm of the plasma membrane.…”

Ultrastructural localization of endogenous calcium in the teleost retina

“…Using phosphate buffered glutaraldehyde as primary and osmiate/bichromate as secondary fixative produces finer deposits than are produced by other methods (Fishman et aI., 1977;Ripps et al, 1979;van Reempts et al, 1982;Ungar et al, 1984;Staple et al, 1987). Therefore endogenous calcium could be localized more precisely.…”