Functional heterogeneity of the sarcoplasmic reticulum within sarcomeres of skinned muscle fibers

Abstract

Precipitation of Ca oxalate in the sarcoplasmic reticulum of chemically skinned rabbit psoas fibers caused an increase in light scattering which was proportional to the amount of Ca accumulated per unit fiber volume. The increase in scattering was used to measure net accumulation rates and steady-state Ca capacities of the sarcoplasmic reticulum in single fibers. The data obtained were qualitatively and quantitatively similar to those reported for isolated vesicle preparations. Under conditions in which Ca was not depleted from the medium, Ca accumulation was linear with time over much of its course. Steady-state capacities were independent of the Ca concentration; uptake rates were half-maximal at 0.5 microM Ca++ and saturated above about 1.0 microM. Both rate and capacity varied with the oxalate concentration, being maximal at oxalate concentrations greater than or equal to mM and decreasing in proportion to one another at lower concentrations, with a threshold near 0.25 mM. At the lower loads, electron micrographs showed many sarcoplasmic reticulum elements empty of precipitate alongside others that were full, whereas virtually all were filled in maximally loaded fibers. These data indicate that the Ca oxalate capacity of each fiber varies with the number and volume of elements in which Ca oxalate crystals can form at a given oxalate concentration, and that individual regions of the sarcoplasmic reticulum within each sarcomere differ in their ability to support Ca oxalate precipitation. Our working hypothesis is that this range in ability to form Ca oxalate crystals involves differences in ability to accumulate and retain ionized Ca inside the sarcoplasmic reticulum.

Ca2+-sequestering smooth endoplasmic reticulum in an invertebrate photoreceptor. II. Its properties as revealed by microphotometric measurements.

“…With a similar technique, Sorenson et al . (40) followed the uptake of Ca' into chemically skinned skeletal muscle fibers by monitoring the changes in light scattering accompanying the active Ca oxalate formation in the SR.…”

Expression of Sarco(endo)Plasmic Reticulum Ca2+‐Atpase Slow (SERCA2) Isoform in Regenerating Rat Soleus Skeletal Muscle Depends on Nerve Impulses

“…2+ uptake measurements Ca 2+ uptake by the sarcoplasmic reticulum was measured at room temperature (22-24°C) using a light-scattering method (Sorenson et al 1980;Salviati et al 1982;Midrio et al 1997). Fibres were mounted in a chamber in a relaxing solution containing (mM): potassium propionate 170, magnesium propionate 2.5, Na 2 ATP 5, K 2 EGTA 5 and imidazole buffer 10; pH 7.0, and stretched to 180 % of slack length to avoid interference in lightscattering measurements caused by actin-myosin interactions.…”

“…Fibres were then incubated in a Ca 2+ loading solution (pCa 6.4) containing (mM): potassium propionate 170, Na 2 ATP 5, magnesium propionate 2.5, K 2 EGTA 5, Ca 2+ 2.15 and imidazole buffer 10; pH 7.0. The Ca 2+ -loading activity of sarcoplasmic reticulum was measured by the increase of fibre light scattering after the addition of 5 mM oxalate (Sorenson et al 1980). The plateau level of light scattering represents the maximum capacity for Ca 2+ uptake of sarcoplasmic reticulum (Sorenson et al 1980;Salviati et al 1983).…”

“…The Ca 2+ -loading activity of sarcoplasmic reticulum was measured by the increase of fibre light scattering after the addition of 5 mM oxalate (Sorenson et al 1980). The plateau level of light scattering represents the maximum capacity for Ca 2+ uptake of sarcoplasmic reticulum (Sorenson et al 1980;Salviati et al 1983). The slope of the linear part of the curve provides a measure of the rate of Ca 2+ accumulation (Midrio et al 1997).…”

Contractions induced by sodium withdrawal in crab (Callinectes danae) muscle fibres.

“…within a single fibre. Both are known to occur in skeletal muscle (Sorenson, Reuben, Eastwood, Orentlicher & Katz, 1980).…”