Abstract
The patch-clamp technique was used to examine single channel currents activated by
the anthelmintics levamisole and pyrantel in muscle vesicles from Ascaris suum. Cellattached and isolated inside-out patches were used. Levamisole (1-90 μM), applied to
the extracellular surface, activated channels which had apparent mean open-times in the
range 0.12-2.23 ms and linear current / voltage relationships with conductances in the
range 19-46 pS. Pyrantel (0.03-100 μM) activated cation selective channels with linear
current/voltage plots and mean open times in the range 0.22-4.14 ms. With pyrantel
there were at least two conductance levels: main conductance 41± 2.04 pS (mean±s.e.,
n=28), smaller conductance 22.4 ± 0.34 pS (mean±s.e., n=8). Ion-replacement
experiments for both agonists showed the channels to be cation selective. The kinetics
of the channels were analysed. Generally open- and closed-time distributions were best
fitted by two and three exponentials respectively, indicating the presence of at least two
open states and at least three closed states. The distributions of burst-times were bestfitted by two exponentials. Channel open- and burst-times were voltage-sensitive: at
low levamisole (1-10 μM) or pyrantel concentrations (0.1-10 μM), they increased with
hyperpolarisation. At higher concentrations of levamisole (30 μM & 90 μM) and
pyrantel (100 μM), flickering channel-block was observed at hyperpolarised potentials.
Using a simple channel-block model, values for the blocking dissociation constant, KΒ
were determined as: 123 μM at -50 mV, 46 μM at -75 mV and 9.4 μM at -100 mV for
levamisole; and 37 μM at -50 mV and 20 μM at -75 mV for pyrantel. At the higher
concentrations of levamisole (30 μM & 90 μM) and pyrantel (100 μM) long closedtimes separating "clusters" of bursts were observed, at both hyperpolarised and
depolarised membrane potentials and this was interpreted as desensitisation.
Although the simple channel-block model was used to describe the data, there were
limitations in the use of this model: for example, the burst durations did not increase
with concentration as forecast by the simple channel-block model. Limitations of this
model are discussed
It was concluded that levamisole and pyrantel act in Ascaris suum by opening
nicotinic ACh channels but in addition they produced open channel-block.
The patch-clamp technique was also used to examine the effect of intracellular
levamisole on nicotinic ACh receptors in muscle vesicle preparations from Ascaris
suum. Initial experiments were performed where channels were activated with
levamisole (2 μM) applied in the patch-pipette, isolated inside-out patches were used.
In addition, levamisole (30-926 μM) was added to the cytoplasmic membrane surface
via the bath solution. In 8 out of 9 experiments, addition of levamisole to the bath
solution resulted in an increase in channel activity; a voltage-sensitive open channelblock and desensitisation. The open channel-block occurred at hyperpolarised
potentials, an observation consistent with levamisole (a cationic substance) blocking the
channel from the extracellular surface. Thus it was concluded that levamisole crossed
from the cytoplasmic side of the membrane, via the lipid phase, to the extracellular
surface of the patch. In the presence of high cytoplasmic concentrations of levamisole
open channel-block was not observed at depolarised potentials suggesting channel
asymmetry.
