Structural complexes in the squid giant axon membrane sensitive to ionic concentrations and cardiac glycosides

Abstract

Giant nerve fibers of squid Sepioteuthis sepioidea were incubated for 10 min in artificial sea water (ASW) under control conditions, in the absence of various ions, and in the presence of cardiac glycosides. The nerve fibers were fixed in OsO, and embedded in Epon, and structural complexes along the axolemma were studied. These complexes consist of a portion of axolemma exhibiting a three-layered substructure, an undercoating of a dense material (~0.1 #m in length and ~70-170 ,/~ in thickness), and a narrowing to disappearance of the axon-Schwann cell interspace. In the controls, the incidence of complexes per 1,000 #m of axon perimeter was about 137. This number decreased to 10-25% when magnesium was not present in the incubating media, whatever the calcium concentration (88, 44, or 0 raM). In the presence of magnesium, the number and structural features of the complexes were preserved, though the number decreased to 65% when high calcium was simultaneously present. The complexes were also modified and decreased to 26-32% by incubating the nerves in solutions having low concentrations of sodium and potassium. The adding of 10 -5 M ouabain or strophanthoside to normal ASW incubating solution decreased them to 20-40%. Due to their sensitivity to changes in external ionic concentrations and to the presence of cardiac glycosides, the complexes are proposed to represent the structural correlate of specialized sites for active ion transport, although other factors may be involved.

Freeze-fracture studies on the giant axon and ensheathing Schwann cells of the squid

“…After freeze-fracturing, the arrangement of the various cell layers in the squid giant fibre is consistent with that observed in thin sections (Villegas & Villegas, 1968, 1976Hodge & Adelman, 1980), but membrane faces, hitherto inaccessible, are revealed in the replicas. The peripheral endoneurial cell membranes show multiple fractures through the intracellular invaginations and inpushings of the matrix that they exhibit in thin sections, while the cleavage planes reveal an extensive population of IMPs on the P-face, with the usual less IMP-enriched E-face.…”

“…The *To whom correspondence should be addressed. 0300-4864/87 $03.00 + .12 9 1987 Chapman and Hall Ltd. possibility of axo-glial coupling and specialized zones of interaction or mechanical attachment between the giant axon plasma membrane and the overlying Schwann cell membranes has been advanced (Villegas, 1972;Villegas & Villegas, 1976) and it seemed appropriate, therefore, to also analyse these structures by the freeze-fracture procedure. A preliminary account of these results has been published in abstract form and in a review account of squid axon ultrastructure .…”

Cell-to-cell transfer of glial proteins to the squid giant axon: The glia- neuron protein transfer hypothesis

“…Isolated giant fiber refers to a giant fiber from which most of the adhering small fibers have been cleaned. The giant fiber contains the giant axon which is surrounded by an inner layer of adaxonal glial cells (Schwann cells) and an outer layer of connective tissue (44,66,67; see also Fig. 1).…”

“…The glial cell processes are in close proximity with the axon, and the plasma membranes of the glial cells and the axon are separated by a narrow cleft of only 5-10 nm (66,67). Adaxonal glial cells of the giant fiber have many of the morphological characteristics associated with certain types of protein-secreting cells, including a large number of vesicular profiles (66,67).…”