trans

308 HELMINTH SURFACES
re-establish osmotic equilibrium is not known,
but their well-developed protonephridial system
may play a role (see below). However,
there is no direct evidence that the protonephridial
system responds to ionic or osmotic
stress. Osmoregulation could also be mediated
by active transport of ions across the tegument.
In S. mansoni, the Na concentration in
extracellular fluid is approximately equal to
that in the bathing medium over a wide range.
The Na concentration in the tegumental syncytium,
however, is maintained at a constant
level that is much lower than that in the extracellular
fluid or the external medium.
Transport of inorganic ions
The apical tegumental membrane of schistosomes
and fasciolids has electrophysiological
properties like those of other ion-transporting
epithelia. The tegument is more permeable to
K than to Na or Cl . This is a common feature
in transporting epithelia of vertebrates
and invertebrates. K efflux contributes the
hyperpolarized ground state of the tegumental
electrical compartment. In both S. mansoni
and F. hepatica, high K concentrations are
restored and low Na concentrations (i.e. relative
to the external environment) maintained
in the tegument, in part, by Na /K -ATPase.
Collectively, these biophysical properties contribute
extensively to the maintenance of an
electrochemical gradient which, in S. mansoni,
results in a membrane potential of 60 mV. In
addition, the schistosome tegument contains
at least one voltage-dependent ion channel
that is cation selective (conducts Na and K ,
but not Cl ) and exhibits an unusually large
unitary conductance of 295 pS.
Excretion
Trematodes excrete metabolic end-products,
such as lactate, amino acids, NH 4 and H .
Both the tegument and protonephridial organs
(see below) may serve important roles in the
excretion of these molecules, though no direct
physiological or biochemical evidence for this
function has been reported. However, Na levels
within the tegumental syncytium become
elevated after acidification of the external
medium. Amiloride and low Na medium
interfere with recovery from an acid load, suggesting
the existence of a Na –H exchanger in
the tegument that may serve an important role
in the elimination of H .
Signal transduction
Evidence for a role for the schistosome
tegument in sensory processing and signal
transduction comes from anatomical and histochemical
studies. Nerve fibers contact both
ciliated and non-ciliated cells in the tegument
that are believed to serve sensory functions,
and two or three types of sensory papillae are
localized within the teguments of schistosomes
and fasciolids. Antibodies to inducible
nitric oxide synthase (iNOS) localize at the surface
of schistosomes, with NADPH-diaphorase
staining being most prominent on the tubercles
and papillae of male worms. Nitric oxide
produced through iNOS in the tegument could
serve a number of roles in sensation or locomotion,
including the modulation of activity
in underlying muscle fibers that form electrical
connections with the tegument. G proteins
have been isolated from the brush border
of F. hepatica, and it is possible that they
transduce signals involving chemicals in the
environment. However, no evidence for G-
protein-coupled receptors in the tegument
has been reported.
More evidence that the tegument serves
important roles in signal transduction come
from behavioral studies on mating patterns.
Male and female adult schistosomes are permanently
paired within the vasculature of the host.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS