Subfractionation of sarcoplasmic reticulum from fast‐twitch and slow‐twitch rabbit skeletal muscles was performed on a sucrose density gradient. Vesicle fractions were characterized by: measurement of (Ca2+,Mg2+)‐dependent (extra) ATPase, Mg2+ ‐dependent (basal) ATPase, Ca2+ uptake characteristics, polypeptide patterns in sodium dodecylsulphate polyacrylamide gel electro‐phoreses, phosphoprotein formation and electronmicroscopy of negatively stained samples. In fast‐twitch muscle, low and high density vesicles were separated. The latter showed high activity of (Ca2+, Mg2+)‐dependent ATPase, negligible activity of Mg2+‐dependent ATPase, high initial rate and high capacity of Ca2+ uptake, high amount of phosphorylated 115000‐Mr polypeptide, and appeared morphologically as thin‐walled vesicles covered with particles of 4 nm in diameter. Low density vesicles had little (Ca2+, Mg2+)‐dependent ATPase but high Mg2+‐dependent ATPase. Although the initial rate of Ca2+ uptake was markedly lower, the total capacity of uptake was comparable with that of high density vesicles. Phosphorylated 115000‐Mr polypeptide was detectable at low concentrations. Instead, 57000 and 47000‐Mr polypeptides were characterized as forming stable phosphoproteins in the presence of ATP and Mg2+. Negatively stained, these vesicles appeared to have smooth surfaces. It is suggested that low density vesicles represent a Ca2+ sequestering system different from that of high density vesiclés and that Mg2+‐dependent (basal) ATPase as well as the 57000 and 47000‐Mr polypeptides are part of the Ca2+ transport system within the low density vesicles. According to the results from slow‐twitch muscle, Ca2+ sequestration by the sarcoplasmic reticulum functions in this muscle type only through the low density vesicles.

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The ultrastructure of photoreceptor cells of the crayfish (P. clarkii) has been examined by means of thin sections and freeze-fracturing. The study reveals that in the photoreceptor membranes there are particles associated primarily with the A faces of freeze-fracture preparations which have a mean diameter of 80-84 A and a density of 6,600 per per micrometer2. Treatment of the retina with digitonin (a substance capable of extracting visual photopigments) in Ringer's causes marked disruption of the hexagonal arrangement of the microvilli, breakdown of the microvilli into smaller segments, and gradual removal of the particles. The estimated photopigment concentration in the microvillus is 4,000 per micrometer. It is suggested that the observed particles represent the photopigment in situ.

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The membranes of the microvilli of UV- and green-photoreceptors of the ant Myrmecia gulosa have been studied with the freeze-fracture technique. Both inner fracture faces, the cytoplasmic P-face and the extracellular E-face, are covered by globular particles. The P-face particles appear to be randomly distributed, occasionally forming clusters. Their density is about 7,000/mum2, and their mean diameter is 8.5 nm. The E-face particles, however, are arranged in an ordered square pattern with a center-to-center spacing of 9 nm. The density and distribution of P- and E-face particles are the same in both the UV- and the green-photoreceptor membranes. No differences were found in the ultrastructural organization of photoreceptor membranes after dark or light adaptation. It is suggested that the P-face particles represent rhodopsin molecules.

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