206
M.C. Cerra et al.
teleosts (13-71 ml kg-I [5]). This hypervolemic condition is associated
with cardiac enlargment that makes the icefish heart size comparable to
that of small mammals. This heart acts as a volume pump since it is able
to displace large stroke volumes (c. hamatus: 4.02 ml kg-I) against low
ventral aorta pressure (c. hamatus: 3.0/2.0 kPa) [15].
This chapter will provide information on the presence of NP receptors
in the heart of two nototheniids, Trematomus bernacchii and the
channichthyid Chionodraco hamatus. These two species, which differ
for the presence and absence of hemoglobin in their blood, show similar
ecotype being endemic and sluggish bottom dwellers.
NPs' Structure and Receptors
In several species of teleosts at least three types of NPs have been
isolated and the sequence determined. Two of them, A-type NP (ANP),
and C-type NP (CNP), are structurally very similar to the NPs identified
in other vertebrates, namely ANP, B-type NP (BNP) and CNP [16]. The
third type ofNP found in fish is mainly synthesized by the ventricle and
is called ventricular NP (VNP). It is longer than the other natriuretic
hormones but it shares a high sequence homology with all the other
vertebrates' NPs [3]. It remains unclear whether this peptide is an
additional type ofNP or it belongs to the BNP family. In mammals, BNP
is synthesized by the ventricle and this synthesis overrides the atrial one
in some cardiovascular adjustments or pathologies, such as hypertension
and/or cardiac hypertrophy [17].
As in other vertebrates [18], the NP system of teleost fish exhibits
heterogeneity not only at the ligand level but also at the receptor level as
demonstrated by the presence of multiple types of NP receptors (NPRs)
[16]. The similarities with the mammalian NPRs and the binding
potencies shown by NPRs described in fish suggested a classification
into NPR-B and NPR-C [19,20]. NPR-B, coupled to a guanylate-cyclase,
has low affinity for ANP and higher affinity for the other homologous
and heterologous NPs. NPR-C binds with similar affinity to all NP types
and is a guanylate-cyclase free receptor [19-21]. The identification of
these types of NPRs in organs such as the gills, kidney, and intestine
provided a molecular target for the osmoregulatory functions of NPs in
fish [19,22,23]. Two NPRs have been recently described in the cardiac
chambers of the eel [23,24]. Using homologous ligands, an NPR-B-like
receptor was observed in the bulbus arteriosus of the eel, while an NPRC-like receptor was found in the' atrial tissue and the venticu1ar
M.C. Cerra et al.
teleosts (13-71 ml kg-I [5]). This hypervolemic condition is associated
with cardiac enlargment that makes the icefish heart size comparable to
that of small mammals. This heart acts as a volume pump since it is able
to displace large stroke volumes (c. hamatus: 4.02 ml kg-I) against low
ventral aorta pressure (c. hamatus: 3.0/2.0 kPa) [15].
This chapter will provide information on the presence of NP receptors
in the heart of two nototheniids, Trematomus bernacchii and the
channichthyid Chionodraco hamatus. These two species, which differ
for the presence and absence of hemoglobin in their blood, show similar
ecotype being endemic and sluggish bottom dwellers.
NPs' Structure and Receptors
In several species of teleosts at least three types of NPs have been
isolated and the sequence determined. Two of them, A-type NP (ANP),
and C-type NP (CNP), are structurally very similar to the NPs identified
in other vertebrates, namely ANP, B-type NP (BNP) and CNP [16]. The
third type ofNP found in fish is mainly synthesized by the ventricle and
is called ventricular NP (VNP). It is longer than the other natriuretic
hormones but it shares a high sequence homology with all the other
vertebrates' NPs [3]. It remains unclear whether this peptide is an
additional type ofNP or it belongs to the BNP family. In mammals, BNP
is synthesized by the ventricle and this synthesis overrides the atrial one
in some cardiovascular adjustments or pathologies, such as hypertension
and/or cardiac hypertrophy [17].
As in other vertebrates [18], the NP system of teleost fish exhibits
heterogeneity not only at the ligand level but also at the receptor level as
demonstrated by the presence of multiple types of NP receptors (NPRs)
[16]. The similarities with the mammalian NPRs and the binding
potencies shown by NPRs described in fish suggested a classification
into NPR-B and NPR-C [19,20]. NPR-B, coupled to a guanylate-cyclase,
has low affinity for ANP and higher affinity for the other homologous
and heterologous NPs. NPR-C binds with similar affinity to all NP types
and is a guanylate-cyclase free receptor [19-21]. The identification of
these types of NPRs in organs such as the gills, kidney, and intestine
provided a molecular target for the osmoregulatory functions of NPs in
fish [19,22,23]. Two NPRs have been recently described in the cardiac
chambers of the eel [23,24]. Using homologous ligands, an NPR-B-like
receptor was observed in the bulbus arteriosus of the eel, while an NPRC-like receptor was found in the' atrial tissue and the venticu1ar