Biology Reference
In-Depth Information
during pre- and early prometamorphosis serves to reduce the free fraction of
TH in blood thus limiting bioavailability. Conversely, hormone-binding
proteins can serve as a reservoir for hormone in the blood; TTR could
therefore help to sustain increasing plasma TH concentrations prior to
the acceleration of thyroid gland activity induced by rising plasma
thyroid-stimulating hormone (TSH) titers. The TTR concentration in
the blood declines at metamorphic climax when plasma TH concentration
is maximal. The continued rise in TH synthesis by the thyroid gland, paired
with a decline in TTR, could result in an increase in the free hormone frac-
tion in the blood. At the same time, the rate of clearance of T
3
from the cir-
culation would likely increase. However, because the thyroid synthetic rate is
high at metamorphic climax, total plasma T
3
concentration continues to rise.
Thus, one would predict that, not only does the hormone production rate
increases at metamorphic climax but also does the proportional availability
of T
3
to the target tissues. To my knowledge, T
3
or T
4
clearance rates have
not been calculated in tadpoles at different stages of development. Based on
TTR expression profiles, one would predict that clearance rates would be
lower during prometamorphosis compared with premetamorphosis or meta-
morphic climax. Further, given the lower affinity of TTR for T
4
compared
with T
3
, one would predict that the clearance rate for T
4
would be higher
than T
3
.
1.1.3 Membrane TH transporters and cytosolic thyroid
hormone-binding proteins
Tadpole cells have the capacity to actively take up TH (see
Krain & Denver,
2004
), and this activity may be regulated during metamorphosis. Saturable,
carrier-mediated uptake of THs has been demonstrated in tadpole RBCs
(
Galton, Stgermain, & Whittemore, 1986; Murata & Yamauchi, 2005;
Yamauchi, Horiuchi, Koya, & Takikawa, 1989
). The genes that encode
TH transport proteins could be the important loci for the modulation of
the timing of metamorphosis.
There are three general classes of proteins that allow for active uptake of
TH by cells: the organic anion transporters (OATC), monocarboxylate
transporters (MCT), and the
L
-amino acid permeases (LAT) (
Friesema,
Jansen, Milici, & Visser, 2005; Jansen, Friesema, Milici, & Visser, 2005;
Ritchie, Peter, Shi, & Taylor, 1999; Ritchie et al., 2003; Visser,
Frieserna, Jansen, & Visser, 2008
). Orthologs of
oatc
,
mct
and
lat
genes have
been isolated from frogs and patterns of expression throughout metamor-
phosis have been described (
Connors, Korte, Anderson, & Degitz, 2010;
