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Early induction by crotoxin of biphasic frequency changes and giant miniature endplate potentials in frog muscle
Author(s) -
Hawgood Barbara J.,
Smith I. Christopher H.,
Strong Peter N.
Publication year - 1988
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1988.tb11587.x
Subject(s) - egta , phospholipase a2 , acetylcholine , phospholipase , biophysics , chemistry , phospholipase a , calcium , grenouille , phospholipid , medicine , endocrinology , anatomy , biology , biochemistry , membrane , enzyme , salientia , xenopus , gene , organic chemistry
1 Following the addition of crotoxin (250 n m ) at the frog neuromuscular junction, there was an initial fall in frequency of miniature endplate potentials (m.e.p.ps), followed by a secondary rise which was characterized by the appearance of large spontaneous potentials (giants, g.m.e.p.ps) and an occasional large potential of the burst type. 2 In the presence of 2‐(4‐phenylpiperidino)cyclohexanol (AH5183, vesicamol), an inhibitor of vesicular acetylcholine uptake, the frequency of g.m.e.p.ps induced by crotoxin was reduced. 3 The characteristic changes in m.e.p.p. frequency and amplitude distribution were absent with crotoxin in Sr‐EGTA Ringer. In the presence of high concentrations of Mn (3.6 or 5.4 mM with 0.9 mM Ca), the crotoxin‐induced initial fall and the onset of the secondary rise in m.e.p.p. and g.m.e.p.p. frequencies were slower. The timing of these phases was unaffected by Ca concentrations ranging from 6.3 to 0.9 mM. 4 High concentrations of Mn ions partially inhibited the phospholipase A 2 activity of crotoxin on artificial phospholipid membranes. This also supports the involvement of the Ca‐dependent phospholipase A 2 subunit in both phases of the physiological action of the toxin. 5 G.m.e.p.ps were associated with a moderate increase in m.e.p.p. frequency (2–3 s −1 ) and were of a time‐course similar to that of m.e.p.ps. They persisted after washing with medium lacking Ca ions and in the presence of Ca‐Mn Ringer that blocked evoked responses. 6 It is concluded that crotoxin, acting through its phospholipase A 2 subunit, produces specific disturbances of synaptic exocytosis and vesicle formation in the axolemma of the motor nerve terminal which lead to biphasic changes in m.e.p.p. frequency and the onset of large spontaneous potentials.