trans

NEMATODES 321
high-affinity binding site with a K d of 3 nM and
B max of 1 fmol/mg protein. Although the chemical
structures of bombesin-like peptides in
nematodes have not been determined, the
GRP binding data suggest the peptides are similar
to their putative vertebrate homologs. A
molecule from Panagrellus redivivus extract
that is recognized by antibodies to GRP has a
molecular weight in the 1.7 kDa range, similar
to that of vertebrate bombesin-like peptides.
Demonstrating a role for this peptide and its
receptor in the hypodermis of nematodes
awaits determination of its sequence and identification
of the gene that encodes its receptor.
Turgor pressure and osmotic/volume
regulation
Processes that regulate turgor pressure and
osmotic gradients in nematodes are distinct,
but interdependent; both are critical aspects of
the physiology of these organisms. Fluid in the
pseudocelom, which is under pressure imposed
by the limited ability of the cuticle to expand
or contract, forms a hydrostatic skeleton that
is essential to coordinated movement and to
the digestive process. As noted above, hydrostatic
pressure within the pseudocel of A. suum
at rest is maintained at about 70 mmHg above
ambient, and values may oscillate from 16 mm
to 275 mmHg during contraction of the body
wall muscle. The consequences of interfering
with turgor pressure in nematodes can be
deduced from in vitro experiments. For example,
puncturing the cuticle leads to immediate
evisceration of the intestine and ovaries, followed
shortly by uncoordinated movement
and death in A. suum and other nematodes.
Loss of turgor pressure would thus likely be
lethal to parasites in vivo. This effect can be
partially mimicked by placing adult-stage
nematodes in distilled water, which causes
them to swell over several hours and eventually
burst. Other clues to the important links
between osmotic regulation and muscle function
come from studies using C. elegans mutants
that are resistant to levamisole and appear to
lack pharmacological acetylcholine receptors.
Among the phenotypes displayed by these
mutants, referred to as ‘uncs’ because they
exhibit uncoordinated movement, is extreme
sensitivity to hypotonic shock.
Osmotic and volume regulation are tightly
linked processes in all metazoans. Marked
fluctuations in environmental solute concentrations
are encountered by parasitic species
that reside in brackish water or on damp grass
as larvae. Adults in the mammalian gastrointestinal
tract may encounter osmotic pressures
in excess of 500 mOsm. Adult stages of
parasitic nematodes are less tolerant of hyposmotic
conditions than are larvae. Adults swell
and explode within a few hours when incubated
in fresh water; however, they tolerate incubations
for up to 4 days in dilutions of artificial sea
water that range from 20–40%. During these
incubations, osmotic pressure measured in PCF
of A. suum is approximately equal to that of the
external medium. The most spectacular examples
of osmotic stress tolerance in nematodes
come from studies that examine desiccation
survival. Infective larvae of the gastrointestinal
parasite T. colubriformis survive vacuum desiccation
at 0% relative humidity for 9 hours. In
most species, anhydrobiosis must be induced
slowly for the worms to survive.
Though the mechanisms underlying anhydrobiotic
survival in nematodes are unknown,
several organic solutes, including trehalose,
glycerol and inositol, may serve as replacement
molecules for membrane-bound water.
However, Panagrellus silusae and A. suum produce
high levels of glycerol and trehalose,
respectively, but also are highly susceptible to
desiccation, suggesting that the presence of
those solutes does not alone confer tolerance
to anhydrobiosis.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS