X-ray analysis of Lymnaea stagnalis muscle fibres does not suggest that the sarcolemma is permeable to lanthanum ions.

The cross-striated muscle from the heart ventricle and the smooth penis retractor muscle of the freshwater snail Lymnaea stagnalis have been investigated by X-ray microanalysis to establish whether lanthanum can cross the plasma membrane, as has been reported by other investigators. Tissues were incubated in 1 mM ionic lanthanum before fixation in phosphate- or cacodylate-buffered fixative. X-ray mapping for emissions in the lanthanum energy range indicates a concentration of emissions that coincided only with the network of sub-surface transverse tubules formed by the invagination of the plasma membrane and with the plasma membrane/extracellular space interface. X-ray energy spectra were collected from various cell compartments; peak-to-background ratios were obtained and analysed statistically. Cacodylate buffer is less effective than phosphate buffer in precipitating lanthanum, but no evidence to suggest the redistribution of lanthanum in cacodylate-buffered preparations was found. Lanthanum is precipitated only in the sub-surface transverse tubules and at the plasma membrane/extracellular space interface in both heart ventricle muscle and penis retractor muscle, fixed in either phosphate or cacodylate buffer. There was no evidence of lanthanum precipitation in the background cytoplasm or on any cytoplasmic organelle. These results confirm our hypothesis that lanthanum does not cross the plasma membranes in these molluscan tissues.