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Calcium dependent action potentials produced in leech Retzius cells by tetraethylammonium chloride.

1. Retzius cells of leech segmental ganglia were exposed to tetraethylammonium chloride (TEA) presented both extracellularly, dissolved in the perfusing fluid, and intracellulary, by iontophoresis from a microelectrode. 2. Extracellular TEA, 10 and 25 mM, greatly prolonged the cells' action potentials, and the higher concentration increased their amplitude as well. At 10 mM the characteristic changes developed gradually over a period of about half an hour, while at 25 mM they appeared much more rapidly. However, at both concentrations the changes were reversible within minutes, even after long soaks in drug‐containing solution. It is therefore probable that the drug acted at the outer surface of the membrane. 3. Intracellular TEA also prolonged the action potentials but there were several differences from the response produced by extracellular application. The changes developed gradually, and for a time, each firing of the cell was a complex event consisting of several early, brief depolarizations followed by a single much larger and more prolonged one. The large, late depolarization eventually obliterated the early ones; its gradual development suggested that it was produced only after TEA diffused to some extrasomatic portion of the cell. Intracellular TEA always caused progressive depolarization; this and the changes in the action potential were both irreversible, suggesting that the site of action was on the inner surface of the membrane. 4. Manipulations of external Na and Ca provided evidence that (a) in the absence of TEA, Retzius cell action potentials were exclusively Na‐dependent, (b) that the early depolarizations in the complex action potentials produced by intracellular TEA were Na‐dependent, while the later, large depolarization was Ca‐dependent and (c) that the prolonged action potentials produced by extracellular TEA contained a large Ca‐dependent component. 5. We conclude that TEA, acting from either side of the membrane, caused a voltage‐sensitive, slowly activated Ca current to become a major contributor to the inward current of the action potential, probably by blocking the outward K current which ordinarily counteracts it. However, we cannot rule out the possibility that TEA enabled a Ca current by some means independent of its presumed action on K conductance. 6. Data resembling ours in some respects have been obtained from studies of the action of TEA on frog dorsal root ganglion cells, frog neuromuscular junction, and squid stellate ganglion. No clear counterpart of our findings has been reported form experiments on squid and amphibian axons, molluscan neurones, or frog skeletal muscle fibres.