Abstract
The intracellular localization of calcium as an antimonate precipitate is studied in myocardial cells of a non-mammalian vertebrate. Pigeon papillary muscles are pretreated in a calcium-free potassium solution containing 60 mM KSb (OH)6, and fixed in 1 % OsO1 containing the same concentration of antimonate. Calcium is chelated by K-EDTA or K-BGTA, in part separating it from the sodium-calcium antimonate. Atomic absorption spectrophotometry is employed to study the precipitate formation when sodium and calcium ions are added to a pH controlled antimonate solution. The chelating effect of K-EDTA and K-EGTA on the precipitates is studied by the same method. Both sodium and calcium cations are heavily precipitated by the antimonate anion. More calcium ions are precipitated when sodium ions are also present in the solution. K-EDTA and K-EGTA do not redissolve more than about 50% of the calcium antimonate from a sodium-calcium antimonate precipitate. When calcium cations only are added to the antimonate solution, K-EGTA redissolves about 95% of the calcium antimonate precipitate. A direct evidence for the presence of calcium in the tissue precipitates is given by X-ray microanalyses of 2500 A thick sections. Calcium antimonate is located to the sarcoplasmic reticulum (SR), the myofibrils, the mitochondria and the nuclei. In the SR, calcium antimonate is consistently found in the subsarcolemmal cisternae of the peripheral couplings (Sommer and Johnson, 1969) and in the Z line cisternae (Saetersdal and Myklebust, 1975) or extended junctional SR (Sommer and Johnson, 1970). Along the myofibrils, calcium antimonate is found at the overlap of thick and thin filaments. In sarcomeres with short I bands, a dense antimonate precipitate consisting of large granules is found at the A-I junction. No calcium is found at the Z lines. The calcium antimonate granules along the myofilaments seem to be related to the length of the sarcomeres. The significance of these findings is discussed in relation to functional aspects of the myocardial cell
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