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
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