Calcium-binding sites as determined by electron microscope X-ray microanalysis in the electrocytes of the electric organ of Torpedo marmorata.

Application of the Oschman and Wall technique to slices of non-stimulated electric organs of Torpedo marmorata reveals electron-dense deposits along the uninnervated and innervated membranes and in mitochondria of the electrocytes as well as in synaptic vesicles of the nerve terminals. As identified by X-ray microprobe analysis, the area containing these deposits reveal higher concentrations of calcium than the other probed regions in which they are lacking. The deposits generally occur in the form of rounded corpuscles, of about 10--20 nm in diameter in the plasma membranes to 30 nm in synaptic vesicles. In mitochondria, they can reach 100 nm or more. These results are briefly discussed with regard to previous data concerning calcium involvement in cholinergic transmission and its possible role in ionic regulation of the electrocyte.