Ultrastructural localization of calcium in normal and abnormal skeletal muscle

Abstract

Calcium was demonstrated ultrastructurally as a fine black reaction product with unbuffered 2% saturated potassium pyroantimonate, pH 9.4. In comparison with normal muscle, there was increased precipitate in degenerating skeletal muscle fibers and some degenerating-regenerating fibers occurring in pathologic human muscle, regardless of disease entity, and in experimentally injured rat muscle. The pathologically increased calcium was mainly within the sarcoplasmic reticulum and mitochondria. Both structures could be completely blackened. Nuclear calcium was also increased, the precipitates being localized as circular profiles within the nucleoli and heterochromatin as well as being associated with the nuclear envelope. Myofibrillar calcium was only modestly increased. When normal rat muscle was preincubated in 136 mM calcium-enhanced Hanks' medium, calcium accumulated in the muscle fibers--it was especially heavy in the mitochondria and sarcoplasmic reticulum and appeared identical with the pathologic human and rat muscle fibers. Preincubation of normal rat muscle in 0.1 M acetate buffer (pH 4.65) before calcium loading augmented myofibrillar staining, mainly in the H-zone of the A-bands excluding the M-zone and in broad irregular N1, N2, and "N3" lines of the I-bands. EMMA-4 electron probe microanalysis and EGTA (ethylene glycolbis (beta-aminoethyl ether)N,N'-tetraacetic acid) chelation prior to staining confirmed that the precipitate in the several loci was calcium antimonate. It is proposed that in skeletal muscle fibers injured by various pathologic processes, a breach of the plasmalemma barrier to calcium occurs as a very early abnormality. Extracellular calcium would then pour into the aqueous sarcoplasm of the muscle fiber, from which it would be withdrawn by and accumulated with the still active organelles normally having a great avidity for uptake of this ion, especially the mitochondria and sarcoplasmic reticulum. The resultant organellar calcification would impair function and damage the structure of proteins and phospholipids.