trans

CESTODES 301
Cestodes have at least two choline uptake
systems. In H. diminuta one predominates
at pH 7, is highly sensitive to Na and
HCO 3 , and is inhibited by hemicholinium-3
and amiloride, known Na transport blockers.
The other functions at pH 5, via a Na -
independent mechanism, and is inhibited by
benzamil, verapamil and 130 mM Na .
Amino acids are absorbed by adult cestodes
through multiple carriers that are saturable
and sensitive to temperature and a variety
of inhibitors; accumulation typically occurs
against steep concentration gradients. Cestode
amino acid carrier systems are biochemically
similar to those in mammals. There are separate
carriers for acidic and basic amino acids
and multiple carriers for the neutral amino
acids, which overlap in their specificities. Unlike
mammals, however, which show higher affinities
for L-amino acids, cestode transporters are
not stereoselective. Also, amino acid absorption
in some cestodes is not dependent on
cotransport of inorganic ions; replacement of
Na with choline or K does not affect transport
kinetics.
Cestodes synthesize pyrimidines de novo,
whereas purines must be absorbed from host
fluids across the tegument. Both types of
nitrogenous base are absorbed in vitro, mediated
in part in H. diminuta by at least three
distinct carriers, two of which contain multiple
substrate-binding sites. A passive diffusional
component also contributes to both
purine and pyrimidine absorption.
Endocytosis does not appear to play a major
role in the absorption of proteins, though the
capacity of cestodes to absorb macromolecules
has not been systematically explored. Horseradish
peroxidase and ruthenium red appear
to be endocytosed by the pseudophyllidean
cestodes Lingula intestinalis and Schistocephalus
solidus and by Hymenolepis nana.
The apparent lack of endocytosis detected in
most studies may be attributable to the fact
that only mature proglottids were examined.
Transport of inorganic ions
Homeostatic regulation of inorganic ions is a
key function of all transporting epithelia. Close
orthologs of proteins that serve this function in
vertebrates are present in cestodes. Na /K -
ATPase, a key regulator of intracellular levels of
these cations in all metazoa, has been demonstrated
in the tegument of Echinococcus granulosus,
but has not yet been reported in other
cestodes. However, ouabain, a specific inhibitor
of this enzyme, blocks amino acid absorption by
H. diminuta, suggesting that Na /K -ATPase is
present in other species. The cestode tegument
also contains a Ca 2 -ATPase activity which, by
analogy with better characterized systems,
maintains intracellular Ca 2 at low concentrations.
Two forms of the enzyme occur in the
outer tegumental membrane of H. diminuta,
one of which is calmodulin-dependent. Though
no direct electrophysiological measurements of
an ion channel in a cestode tegument have been
reported, at least two large conductance cation
channels have been isolated and partially characterized
from protoscoleces of E. granulosus.
One is a 244 pS channel that resembles some
Ba 2 -sensitive, Ca 2 -activated K channels in
vertebrates. The other is a 107 pS channel with a
very high open probability under a broad range
of recording conditions, resembling a cation
channel that has also been characterized in
the apical membrane of S. mansoni tegument.
Neither the location nor physiological roles of
these channels has been determined.
Osmotic/volume regulation
Control of hydrostatic pressure gradients across
the body wall and across cell membranes in
cestodes has been linked to bulk ion transport.
Some cestodes are osmoconformers, adjusting
internal osmotic pressure by varying fluid
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS