AJP - Cell Physiology, Vol 246, Issue 3 271-C276, Copyright © 1984 by American Physiological Society
Some effects of lead at mammalian neuromuscular junction
J. B. Pickett and J. C. Bornstein
The effect of lead on transmitter release was investigated in a rat phrenic
nerve-hemidiaphragm preparation using conventional microelectrode
techniques. Lead reduced the number of quanta released by a nerve stimulus
(m) in a dose-dependent fashion. As extracellular Ca2+ concentration
([Ca2+]o) was varied in the absence of lead, a linear relationship between
ln(m) and ln([Ca2+]o) was obtained. Lead shifted the relationship between
ln(m) and ln([Ca2+]o) to the right without altering the slope. This
suggested lead competed with Ca2+, which was confirmed by using a modified
Lineweaver-Burk plot. Lead inhibits Ca2+ entry into frog sympathetic
preganglionic nerve terminals, and a similar mechanism may underlie this
present finding; such a mechanism, however, could not explain all the
observed actions of lead. Lead increased the frequency of spontaneous
quantal release in a dose-dependent manner, and 10(-4) M lead doubled the
magnitude of facilitation of evoked release seen with five stimuli at 60
Hz. It is suggested that these effects result from inhibition of some, or
all, of the nerve terminal's Ca2+ sequestration mechanisms.
