Neostigmine alters the shape of the response of the
rat anococcygeus muscle to field stimulation, causing
a 'shoulder' to appear during the relaxation phase
(Figure 1).
Anococcygeus muscles from male Wistar rats
(220-260 g) were suspended in Kreb's solution at
37°C gassed with 5% CO2 in 02. Field stimulation
was applied via parallel platinum wire electrodes at
30 pulses/s, 1 ms and supramaximal voltage for 30 s
every 6 min, and responses recorded isometrically.
The size of the 'shoulder' was dose dependent in
the range 5 x Io-7 M to 5 x 10-6 M neostigmine,
and was abolished by 5 x 10-8 M atropine, but
was unaffected by 10-6 M (+ )tubocurarine. Physostigmine
(10-s M) produced a 'shoulder' which
was abolished by atropine, whereas iso-OMPA
(5 x 10-6_10-4 M) and 1,5-bis (4-trimethylammoniumphenyl)
pentan-3-one diiodide (3W 62 c 47;
10- 7-s x I0-5 M) were without effect.
Total cholinesterase (ChE) activity of rat anococcygeus
muscle was measured colorimetrically (Ellman,
Courtney, Andres & Featherstone, 1961), and found
to be 1.65 + 0.13 pmol min-' g-' tissue. The slope
of the plot of % inhibition of ChE against log concentration
neostigmine was significantly different
(P < 0.001) from the plot of the % maximum response
against log concentration neostigmine for the neostigmine 'shoulder'. The ED50 for neostigmine was
1.4 x lo- M for ChE inhibition, but 9.6 x 10-' M
for the neostigmine 'shoulder'.
Preliminary experiments indicate that neostigmine
enhances [3H]-noradrenaline overflow from the rat
anococcygeus muscle during field stimulation. Neostigmine
might elicit repetitive firing of motor nerve
terminals in skeletal muscle, by an effect unrelated to
ChE inhibition (Riker, Roberts, Standaert & Fujimori,
1957; Blaber & Bowman, 1963; Blaber &
Christ, 1967). The effect of neostigmine in the rat anococcygeus
muscle could be elicited in an analogous
way. Studies of the [3H]-noradrenaline overflow from
the rat anococcygeus muscle during field stimulation
are being undertaken but have so far failed to yield
conclusive results.
