Lamprey Metamorphosis - Metamorphosis in Fish - page 44

LH-β and TSH-β. When this proposition is considered in conjunction
with the observations that lampreys have one functional gonadal GpH
receptor (GpH-RI; Freamat et al . , 2006) and a second functional thyroidal
GpH receptor (GpH-RII; Freamat and Sower, 2008), it seems very likely
that there is overlap between the reproductive and thyroid components
of the lamprey HP axis and it is likely to be a close approximation of the
ancestral axis (Sower et al . , 2009). Therefore, in lampreys, it is feasible that
a single GTH might function to regulate both the reproductive and thyroid
axes via gonad- and thyroid-specifi c GpH-Rs. Many of these studies have
been carried out using adult lampreys, thus, studies on these hormones
and their regulatory effects in larvae and throughout metamorphosis will
surely provide a better understanding of their role in metamorphosis in
this ancient vertebrate and the evolution of the HP axis.
2.7.4.3 Lipogenic and lipolytic hormones
The accumulation of lipids prior to the onset of metamorphosis is a key
indicator of immediately pre-metamorphic lampreys. As discussed above,
lamprey metamorphosis is characterized by a two-phase, lipid metabolism
cycle. During the lipogenic phase (larval to metamorphic stage 3/4),
elevated levels of acetyl CoA carboxylase (ACC) and diacylglycerol acetyl
transferase (DAGT) activity result in the accumulation of lipids in the
kidney and liver, primarily in the form of triacylglycerol (TG) (Kao et al . ,
1997a,1997b). The lipolytic phase coincides with decreases in the activities
of the aforementioned enzymes and an increase in triacylglycerol lipase
activity as lipid stores are depleted between stages 4 and 7 of metamorphosis
(Fig. 4; Table1; Kao et al . , 1997a,1997b).
Coincident with these changes in lipid metabolism are alterations in
key metabolic hormones such as insulin and somatostatin. Serum insulin
concentrations are elevated in stages 6 and 7 of metamorphosis relative
to earlier stages and larval levels (Youson et al . , 1994). Likewise the
concentration of somatostatin in pancreatic-intestinal tissue homogenates
begins to increase at stage 4 of metamorphosis and peak at stage 7 of
metamorphosis (Elliott and Youson, 1991). Although these changes are surely
in part related to the development and expansion of the endocrine pancreatic
tissues, the fact that this elevation coincides with the shift from the lipogenic
phase to lipolytic phase cannot be overlooked. A role for these hormones
in lipid metabolism during lamprey metamorphosis is substantiated by a
series of experiments (Kao et al . , 1998, 1999a). Intraperitoneal injections
of somatostatin-14 (SST-14) elevated plasma fatty acid levels in larval and
stage 6 metamorphosing P. marinus (Kao et al . , 1998) and injections of insulin
and alloxan (a selective pancreatic β-cell toxin) resulted in decreases and
increases, respectively, of plasma fatty acid concentrations (Kao et al . , 1999a).