Intracellular distribution of calcium in smooth muscle: facts and artifacts. A correlation of cytochemical, biochemical and X-ray microanalytical findings.

Cytochemical methods, based on the use of oxalate and pyroantimonate as Ca markers, show Ca accumulation in the sarcoplasmic reticulum (SR), mitochondria and nuclei of smooth muscle cells from the guinea-pig taenia coli. A good correlation was found between the Ca concentrations measured in subcellular components in situ by X-ray microanalysis and the endogenous Ca concentrations estimated in subcellular fractions by atomic absorption spectrophotometry. Our findings suggest that the in situ Ca concentrations (mmol/kg dry weight) are: about 100 in the SR and mitochondria, and about 60 inside the nucleus, significantly higher than in cytoplasm.