Natriuretic Peptides in Antarctic Teleosts and Cardiac Receptors
207
myocardium; in addition, a receptor type with a high affinity for eel
VNP was observed in the ventricular endocardium of this fish,
suggesting the presence of a putative novel type of NPR [24]. The
linkage of these NPRs with both homologous and/or heterologous NPs
could account for some of the functional effects exerted by these
hormones on the fish cardiovascular system [3,4,6].
Cardiac NP Receptor Heterogeneity in Antarctic Teleosts
NPs have not yet been isolated from Antarctic teleosts; however, the
possibility of a hormonal modulation mediated by NPs in Antarctic
notothenioids may be indicated by the presence of NP-like material in
heart homogenates and plasma of C. hamatus and T. bernacchii [25].
Since no information is available on the presence and the nature of a
signal transduction system (i.e .. specific receptors) whereby these
hormones exert their modulatory actions on the cardiovascular function
of these fishes, the presence of NPRs in the cardiac regions of the two
endemic Antarctic notothenioids, the red-blooded T. bernacchii and the
hemoglobinless C. hamatus has been assessed. In vitro quantitative
autoradiography was applied to localize and quantify the receptors for
[125I]-rat ANP-(1-28) ([125I]-rANP) that, in the eel cardiac tissue,
specifically binds to the NP sites [24]. Transverse cardiac sections of T.
bernacchii and C. hamatus were processed as previously described [24]
with some modifications. In both teleosts the analysis of the
autoradiograms incubated in the presence of 100 pM of [125I]-rANP
indicated that ventricular endocardium and myocardium gave greater
labeling than the other regions. Interestingly, an inverse binding pattern
was observed in the bulbus arteriosus of the two fishes. Indeed, while in
C. hamatus the inner (intimal) layer was more labeled than the outer
(middle and adventitial) layer, in T. bernacchii the outer bulbar layer
gave more dense binding levels than the inner layer. This observation is
in contrast with the densest NP binding usually found in the inner layer
of other teleost species [24,26]. A search of the literature failed to
provide data that could explain the significance of this zonation. The
extent to which this is related to the presence of NPs in the pericardial
fluid of these fishes, as reported in mammals [27], requires a further
investigation.
A saturation binding study was carried on in the atrium, ventricular
myocardium and endocardium and inner and outer layer of the bulbus
arteriosus of the two Antarctic teleosts (Fig. 1). In T. bernacchii the
207
myocardium; in addition, a receptor type with a high affinity for eel
VNP was observed in the ventricular endocardium of this fish,
suggesting the presence of a putative novel type of NPR [24]. The
linkage of these NPRs with both homologous and/or heterologous NPs
could account for some of the functional effects exerted by these
hormones on the fish cardiovascular system [3,4,6].
Cardiac NP Receptor Heterogeneity in Antarctic Teleosts
NPs have not yet been isolated from Antarctic teleosts; however, the
possibility of a hormonal modulation mediated by NPs in Antarctic
notothenioids may be indicated by the presence of NP-like material in
heart homogenates and plasma of C. hamatus and T. bernacchii [25].
Since no information is available on the presence and the nature of a
signal transduction system (i.e .. specific receptors) whereby these
hormones exert their modulatory actions on the cardiovascular function
of these fishes, the presence of NPRs in the cardiac regions of the two
endemic Antarctic notothenioids, the red-blooded T. bernacchii and the
hemoglobinless C. hamatus has been assessed. In vitro quantitative
autoradiography was applied to localize and quantify the receptors for
[125I]-rat ANP-(1-28) ([125I]-rANP) that, in the eel cardiac tissue,
specifically binds to the NP sites [24]. Transverse cardiac sections of T.
bernacchii and C. hamatus were processed as previously described [24]
with some modifications. In both teleosts the analysis of the
autoradiograms incubated in the presence of 100 pM of [125I]-rANP
indicated that ventricular endocardium and myocardium gave greater
labeling than the other regions. Interestingly, an inverse binding pattern
was observed in the bulbus arteriosus of the two fishes. Indeed, while in
C. hamatus the inner (intimal) layer was more labeled than the outer
(middle and adventitial) layer, in T. bernacchii the outer bulbar layer
gave more dense binding levels than the inner layer. This observation is
in contrast with the densest NP binding usually found in the inner layer
of other teleost species [24,26]. A search of the literature failed to
provide data that could explain the significance of this zonation. The
extent to which this is related to the presence of NPs in the pericardial
fluid of these fishes, as reported in mammals [27], requires a further
investigation.
A saturation binding study was carried on in the atrium, ventricular
myocardium and endocardium and inner and outer layer of the bulbus
arteriosus of the two Antarctic teleosts (Fig. 1). In T. bernacchii the