The specificity of the combined oxalate-pyroantimonate (OPA) technique for subcellular localization of calcium was examined by laser microprobe mass analysis (LAMMA) of the same tissue sections. The recorded spectra strongly suggest that the cytochemically detected precipitates contained calcium. This could be confirmed by LAMMA analysis of ethylene glycol tetraacetic acid-treated sections. The detected calcium was localized mainly in the rod outer segments, more particularly in the middle part. The validity of the OPA and LAMMA methods is discussed. A combination of both techniques is found to be a valuable tool to elucidate the role of calcium in physiological mechanisms.
