trans

NEMATODES 325
a vacuolar-type H -ATPase is present in
the H-tubular cell of C. elegans (see below),
but its functional properties have not been
reported.
Direct evidence that H and organic anions
are excreted across the cuticle–hypodermis
complex comes from in vitro studies using isolated
A. suum cuticle segments in two-chamber
diffusion cells and from patch-clamp recordings
from cuticle- and muscle-facing membranes
isolated from the hypodermis. Organic
acid concentrations in the hypodermis and
muscle compartments are maintained for
extended periods in culture at levels that far
exceed those in the incubation medium. The
concentration and relative abundance of each
acid are remarkably similar within the PCF,
muscle and hypodermis compartments.
Voltage-clamp studies on isolated hypodermal
membranes reveal a large conductance, voltagesensitive,
Ca 2 -dependent Cl channel that
resembles a channel in A. suum muscle membrane.
The Ca 2 -activated Cl channel in
A. suum muscle and hypodermal membranes is
voltage-sensitive and is open at electrical
potentials recorded across the membranes of
these cells. It is an inward-rectifying channel
that closes as the membrane becomes depolarized.
Although the permeability of this channel
is greatest for Cl , larger anions, including the
organic anions excreted by A. suum, are also
conducted. Permeability across the channel is
inversely related to the Stokes’ diameter of the
anion, with 2-methyl butyrate (diameter
5.62 Å) being only about 13% as permeable as
Cl (diameter 2.41 Å). Reducing external pH
increases the probability of the channel opening
at hyperpolarized membrane potentials.
The negative electrical potentials (60 to
80 mV) recorded across hypodermal membranes
in A. suum could provide a driving force
for the extrusion of organic anions through the
Ca 2 -activated Cl channel. In fact, for each
organic anion excreted by A. suum, there is a
net outward-directed electrical driving force of
16–27 mV across the hypodermal membrane,
which could supply potential energy for
organic anion extrusion directly through the
anion-selective channels, or indirectly via facilitated
transport processes.
Other mechanisms, less studied in nematodes,
may serve important roles in organic
acid excretion. Several models have been proposed
for volatile fatty acid (VFA) transport
in vertebrates. At least some transmembrane
movement of VFAs is mediated by a family
of 12 transmembrane domain H -monocarboxylate
cotransporters. Genes encoding
these transporters have been cloned from several
organisms, and at least four homologs
have been identified in C. elegans. Parasitic
nematodes decrease the medium pH suggesting
that protons as well as organic acids are
excreted, and implicating H -monocarboxylate
cotransporters in this process. However, no
information is yet available on the expression
patterns or substrate specificity of the different
C. elegans transporters, or if homologs are present
in parasitic species.
Direct evidence for nitrogen excretion is
available only for the intestine and cuticle–
hypodermis complex of adult A. suum. However,
nitrogen, in some form, is also likely
excreted from other sites in nematodes, including
the pharyngeal and reproductive glands,
and the tubule system (see below). Depending
on species and stage, 40–90% of nitrogen is
excreted as NH 3 or NH 4 from the deamination
of amino acids. The balance is eliminated as
urea, amino acids and peptides. Since the pH in
nematode cells and PCF is maintained at
6.5–7.0, ammonia in these compartments exists
in a dynamic equilibrium that greatly favors the
ionized (NH 4 ) over the non-ionized (NH 3 )
state. NH 4 transport across intestinal or hypodermal
membranes should require a channel
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS