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. 1973 Sep 1;62(3):286–302. doi: 10.1085/jgp.62.3.286

Tetrodotoxin Desensitization in Aggregates of Embryonic Chick Heart Cells

Terence F McDonald 1, Howard G Sachs 1, Robert L DeHaan 1
PMCID: PMC2226116  PMID: 4730668

Abstract

Spontaneous beating of heart-cell aggregates from 4-day chick embryos was initially blocked by 10-5 g/ml tetrodotoxin (TTX). With continued exposure to the drug, the fraction of blocked aggregates decreased from about 80% at 15 min to about 25% at 2–3 h, at which time, beating aggregates had become desensitized to the toxin, showing no response to a fresh dose. Aggregates from 5-day hearts were more sensitive to TTX, but fewer became desensitized in its presence. Desensitization to TTX was not seen in 6- and 7-day aggregates. Inhibition of protein synthesis by cycloheximide did not affect beating or initial sensitivity to TTX of 4-day aggregates, but desensitization failed to occur. Before TTX, the mean value of maximal upstroke velocity (Vmax) of the action potentials in 4-day aggregates was 33 V/s. After desensitization Vmax was 12 V/s. Activity of desensitized aggregates in the presence of TTX was augmented by elevated calcium levels, and suppressed by presumed inhibitors of slow inward current (manganese, D600). Desensitization was reversible; upon removal of TTX 10-5 g/ml, aggregates regained their responsiveness to a fresh dose of the drug with a 2–3 h time-course similar to that of desensitization. This was prevented by continued exposure to TTX at concentrations as low as 10-8 g/ml. These data suggest that (a) desensitization involves a change in the mode of action-potential generating from one involving Na-specific, TTX-sensitive channels to one utilizing slower Mn-sensitive channels; (b) the process of desensitization occurs over a period of 2–3 h and is dependent upon the products of protein synthesis; and (c) desensitization is reversible after removal of TTX over a 2–3 h time-course similar to its onset.

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Selected References

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