Biochemical and cytochemical analyses of BNP-stimulated guanylate cyclase in frog choroid plexus

Biochemical activity and multiple locations of particulate guanylate cyclase in Rhyacophila dorsalis acutidens (Insecta: Trichoptera) provide insights into the cGMP signalling pathway in Malpighian tubules

In insect renal physiology, cGMP and cAMP have important regulatory roles. In Drosophila melanogaster, considered a good model for molecular physiology studies, and in other insects, cGMP and cAMP act as signalling molecules in the Malpighian tubules (MTs). However, many questions related to cyclic nucleotide functions are unsolved in principal cells (PC) and stellate cells (SC), the two cell types that compose the MT. In PC, despite the large body of information available on soluble guanylate cyclase (sGC) in the cGMP pathway, the functional circuit of particulate guanylate cyclase (pGC) remains obscure. In SC, on the other side, the synthesis and physiological role of the cGMP are still unknown. Our biochemical data regarding the presence of cyclic nucleotides in the MTs of Rhyacophila dorsalis acutidens revealed a cGMP level above the 50%, in comparison with the cAMP. The specific activity values for the membrane-bound guanylate cyclase were also recorded, implying that, besides the sGC, pGC is a physiologically relevant source of cGMP in MTs. Cytochemical studies showed ultrastructurally that there was a great deal of pGC on the basolateral membranes of both the principal and stellate cells. In addition, pGC was also detected in the contact zone between the two cell types and in the apical microvillar region of the stellate cells bordering the tubule lumen. The pGC signal is so well represented in PC and, unexpectedly in SC of MTs, that it is possible to hypothesize the existence of still uncharacterized physiological processes regulated by the pGC-cGMP system.

Atrial natriuretic peptide: its putative role in modulating the choroid plexus-CSF system for intracranial pressure regulation

Evidence continues to build for the role of atrial natriuretic peptide (ANP) in reducing cerebrospinal fluid (CSF) formation rate, and thus, intracranial pressure. ANP binds to choroid plexus (CP) epithelial cells. This generates cGMP, which leads to altered ion transport and the slowing of CSF production. Binding sites for ANP in CP are plentiful and demonstrate plasticity in fluid imbalance disorders; however, specific ANP receptors in epithelial cells need confirmation. Using antibodies directed against NPR-A and NPR-B, we now demonstrate immunostaining not only in the choroidal epithelium (including cytoplasm), but also in the ependyma and some endothelial cells of cerebral microvessels in adult rats (Sprague-Dawley). The choroidal and ependymal cells stained almost universally, thus substantiating the initial autoradiographic binding studies with 125I-ANP. Because ANP titers in human CSF have previously been shown to increase proportionally to increments in ICP, we propose a compensatory ANP modulation of CP function to down-regulate ICP in hydrocephalus. Further evidence for this notion comes from the current finding of increased frequency of "dark" epithelial cells in CP of hydrocephalic (HTx) rats, which fits our earlier observation that the "dark" choroidal cells, associated with states of reduced CSF formation, are increased by elevated ANP in CSF. Altogether, ANP neuroendocrine-like regulation at CSF transport interfaces and blood-brain barrier impacts brain fluid homeostasis.

Structural and functional evolution of the natriuretic peptide system in vertebrates

The natriuretic peptide (NP) system consists of three types of hormones [atrial NP (ANP), brain or B-type NP (BNP), and C-type NP (CNP)] and three types of receptors [NP receptor (R)-A, NPR-B, and NPR-C]. ANP and BNP are circulating hormones secreted from the heart, whereas CNP is basically a neuropeptide. NPR-A and NPR-B are membrane-bound guanylyl cyclases, whereas NPR-C is assumed to function as a clearance-type receptor. ANP, BNP, and CNP occur commonly in all tetrapods, but ventricular NP replaces BNP in teleost fish. In elasmobranchs, only CNP is found, even in the heart, suggesting that CNP is an ancestral form. A new guanylyl cyclase-uncoupled receptor named NPR-D has been identified in the eel in addition to NPR-A, -B, and -C. The NP system plays pivotal roles in cardiovascular and body fluid homeostasis. ANP is secreted in response to an increase in blood volume and acts on various organs to decrease both water and Na+, resulting in restoration of blood volume. In the eel, however, ANP is secreted in response to an increase in plasma osmolality and decreases Na+ specifically, thereby promoting seawater adaptation. Therefore, it seems that the family of NPs were originally Na(+)-extruding hormones in fishes; however, they evolved to be volume-depleting hormones promoting the excretion of both Na+ and water in tetrapods in which both are always regulated in the same direction. Vertebrates expanded their habitats from fresh water to the sea or to land during evolution. The structure and function of osmoregulatory hormones have also undergone evolution during this ecological evolution. Thus, a comparative approach to the study of the NP family affords new insights into the essential function of this osmoregulatory hormone.