trans

318 HELMINTH SURFACES
mec-8 or sym-1 leads to arrest during embryonic
elongation, and to defects in attachment
of somatic muscle to the basement membrane
of the cuticle.
The high internal pressure within the pseudocelom
of nematodes is also essential for ingestion.
Nematodes lack circular muscles, and the
gut normally collapses under the hydrostatic
pressure of the pseudocelom. Ingested food is
pumped caudally against this pressure by the
pharynx, which is the only muscle in the digestive
tract except for those in the rectum that
regulate defecation. The internal pressure,
which collapses empty regions of the gut and
thereby prevents rapid passage of digesta, may
contribute to the efficiency of nutrient absorption
across the intestine.
Nutrient absorption
Transcuticular absorption of glucose, amino
acids and other nutrients has been demonstrated
in all filarial nematodes examined. In
some cases, the cuticle is probably the principal
site of nutrient absorption, with very little
occurring across the intestine. This conclusion
is supported by several observations. For example,
microfilariae of most species possess a
non-functional gut, yet absorb nutrients. In
O. volvulus, significant atrophy of the gut occurs
as the adult matures, coincident with thickening
of, and increases in enzyme activity in, the
hypodermis. The filarial hypodermis contains
proteins associated with amino acid transport,
whereas the intestine often does not. Also,
direct measurement of nutrient transport
across the cuticle/hypodermis complex in some
filarial nematodes has demonstrated saturability,
stereoselectivity and competitive as well
as non-competitive inhibition, indicating the
presence of specific carrier systems.
Evidence for transcuticular transport of physiologically
relevant quantities of nutrients in
intestinal parasites is less compelling than for
filarial species. Most studies have used
A. suum, which absorbs little glucose in vitro,
and instead catabolizes stored glycogen, which
is abundant in muscle cell bodies. The fact
that some transcuticular absorption of glucose
occurs in vitro, however, suggests that the
process may be more important in vivo, where
most glucose absorbed is converted quickly to
glycogen. Other species, such as H. contortus
and Trichostrongylus colubriformis, absorb and
metabolize large quantities of exogenous glucose
in vitro. The importance of the cuticle in
glucose absorption is suggested by studies that
demonstrate that metabolism of exogenous
glucose by H. contortus is not altered when the
pharynx is paralyzed with ivermectin.
How amino acid transport in nematodes is
regulated is not yet understood. Significant
transport of amino acids across the alimentary
tract has not been demonstrated for any species,
but this process has received little attention.
Histochemical and enzymological data demonstrate
that -glutamyl transpeptidase is present
in the cuticle–hypodermis complex of A. suum
in considerably greater abundance than in the
intestinal epithelium. This enzyme is part of
an amino acid transport system found in many
organisms, including vertebrates. The cuticle–
hypodermis complex may be more important
than the intestine for uptake of some amino
acids.
Transport of inorganic ions
Inorganic ions play many critical roles in cell
function. Most important for homeostatic
maintenance in cells are Na , K , Cl , Ca 2 and
Mg 2 . Intracellular concentrations of these
ions, particularly Ca 2 , are usually maintained
within very narrow limits. Inorganic ion concentrations
are regulated by an array of integral
membrane proteins, including specific channels,
energy-dependent pumps (e.g. Na /K -
ATPase, Ca 2 -ATPase, Na /H -ATPase) and
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS