Perfusion of beating false tendons of the dog heart with ionic lanthanum produced drastic but reversible modifications of the excitability and the transmembrane action potential of Purkinje cells. Ultrastructural examination of these cells revealed the appearance of a fine extracellular precipitate detectable on unstained sections. In addition, specimens perfused with La+++ showed a striking increase in the contrast of the sarcolemma, particularly in gap junctions and in pinocytic vesicles. La+++ deposits were restricted to the cytoplasmic leaflets of the sarcolemma; no precipitates were found at the plasma membrane of fibroblasts, endothelial and smooth muscle cells, or unmyelinated nerve fibers present in the same specimens. A selective deposition of La+++ was also observed in the sarcolemma of atrial and ventricular cells of dog, rabbit, and cat hearts, as well as in the membrane of the transverse tubular system of ventricular cells. Both the electrophysiological effects and the ultrastructural membrane deposits produced by La+++ disappeared when the specimens were subsequently perfused with phosphate-containing tyrode solution. These results tend to demonstrate that a distinctive feature of the sarcolemma of mammalian cardiac cells is the presence of regions with a high surface density of binding sites for polyvalent cations.

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