trans

320 HELMINTH SURFACES
current, when an external voltage is imposed
on the system.
The hypodermis is both an anatomical and
electrical syncytium, with apical (cuticle-facing)
and basal (muscle-facing) membranes separated
by a cytoplasm-filled space of variable
thickness. The transmural potential is maintained,
in part, by the separate contributions
of the cuticle-facing and pseudocelom-facing
membranes. In A. suum, the electrical potential
within the hypodermal compartment is
70 mV more negative than the external medium
and 40 mV more negative than the pseudocelomic
compartment, when tested in artificial
media containing inorganic ions at concentrations
approximating porcine intestinal fluid.
Cations do not appear to play a major role in
establishing the transmural potential. The
concentrations of Na and K in PCF from
freshly collected parasites are approximately
equal to those found in swine intestinal fluid,
consistent with the high permeability of the
body wall to these cations. When parasites are
placed in media containing elevated or reduced
levels of these cations, concentrations in the
PCF change fairly rapidly to approximate the
outside concentrations. In contrast, the concentrations
of the divalent cations Ca 2 and
Mg 2 in PCF are maintained at constant levels
in the face of large changes in their concentrations
in the incubation medium. However, due
to the low concentrations of these cations in
cytoplasm or PCF, relative to Na , K , Cl and
organic anions produced via carbohydrate
metabolism, they do not contribute significantly
to the transmural potential.
This situation contrasts markedly with that
typical of transporting epithelia in other metazoa,
where cation conductance is tightly regulated.
Both passive and active transport of
cations contribute extensively to the resting
potential of most vertebrate and invertebrate
cells, including somatic muscle cells and the
pharynx in nematodes. However, electrogenic
transport of ions contributes only about 36% of
the transmural current in A. suum. Also, the
concentrations of Cl in A. suum PCF and
porcine intestinal fluid are nearly equivalent.
Changing the external Cl concentration has
little effect on the transmural potential in
A. suum, suggesting that other anions play
more important roles in this process. Given the
high concentrations of organic anions in PCF
(80 mM), relative to porcine intestinal contents
(3–8 mM), it is likely that these species contribute
extensively to the transmural electrical
potential. A large conductance Cl channel that
can also conduct organic acids has been
described in A. suum muscle membranes, and
a similar channel has been localized and partially
characterized on both the inward- and
outward-facing membranes of the hypodermis.
This channel and other organic anion transporters
identified in some nematode species
probably play more important roles than inorganic
ion channels in maintaining transmural
electrical potential in nematodes.
Direct control of hypodermal or intestinal
membrane permeability by the nervous system
has not been demonstrated in nematodes.
However, a receptor that recognizes bombesinlike
neuropeptides, which serve an analogous
function (i.e. regulation of Cl permeability
across epithelial membranes) in some vertebrates,
has recently been partially characterized.
Bombesin-like peptides appear to be broadly
distributed among nematodes. Immunohistochemical
studies on A. suum and Dirofilaria
immitis indicate that bombesin like immunoreactivity
is localized primarily in the apical
regions of the hypodermis, where the cuticlefacing
membrane of the hypodermis appears
to overlap with the cuticle. Competition binding
assays using [ 125 I]GRP (a close structural
analog of bombesin) and membranes prepared
from A. suum and C. elegans reveal a saturable,
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS