Effect of atrial natriuretic factor on corticosteroid production by perifused frog interrenal slices

A perspective on the role of natriuretic peptides in amphibian osmoregulation

The natriuretic peptide (NP) system is a complex family of peptides and receptors that is primarily linked to the maintenance of osmotic and cardiovascular homeostasis. In amphibians, the potential role(s) of NPs is complicated by the range of osmoregulatory strategies found in amphibians, and the different tissues that participate in osmoregulation. Atrial NP, brain NP, and C-type NP have been isolated or cloned from a number of species, which has enabled physiological studies to be performed with homologous peptides. In addition, three types of NP receptors have been cloned and partially characterised. Natriuretic peptides are always potent vasodilators in amphibian blood vessels, and ANP has been shown to increase the permeability of the microcirculation. In the perfused kidney, ANP causes vasodilation, diuresis and natriuresis that are caused by an increased GFR rather than effects in the renal tubules. These data are supported by the presence of ANP receptors only on the glomeruli and renal blood vessels. In the bladder and skin, the function of NPs is enigmatic because physiological analysis of the effects of ANP on bladder and skin function has yielded conflicting data with no clear role for NPs being revealed. Overall, NPs often have no direct effect, but in some studies they have been shown to inhibit the function of AVT. In addition, there is evidence that ANP can inhibit salt retention in amphibians since it can inhibit the ability of adrenocorticotrophic hormone or angiotensin II to stimulate corticosteroid secretion. It is proposed that an important role for cardiac NPs could be in the control of hypervolaemia during periods of rapid rehydration, which occurs in terrestrial amphibians.

Natriuretic peptides are negative modulators of adrenocortical cell function of the eastern fence lizard (Sceloporus undulatus)

Elucidation of the role of natriuretic peptides (NPs) in vertebrate adrenal steroidogenesis has been facilitated by the use of freshly dispersed adrenocortical cells. Our recent characterization of lizard adrenocortical cells [Carsia, R.V., John-Alder, H.B., 2003. Seasonal alterations in adrenocortical cell function associated with stress-responsiveness and sex in the Eastern Fence Lizard (Sceloporus undulatus). Horm. Behav. 43, 408-420] provided the opportunity to examine the influence of atrial natriuretic peptides (ANPs) and related NPs on reptilian adrenal steroidogenesis at the cellular level. In the present report, the action of NPs on lizard adrenal steroidogenesis was investigated using freshly dispersed adrenocortical cells derived from the Eastern Fence Lizard (Sceloporus undulatus). Basal production rates of aldosterone and corticosterone and maximal angiotensin II (ANG II)-induced production rates of these corticosteroids were inhibited with high efficacy (75-90%) by rat ANP at potencies of 0.4-0.7 nM. By contrast, rat ANP had no effect on maximal production rates of these corticosteroids in response to a maximal steroidogenic concentration of adrenocorticotropin (ACTH; 1 nM). However, rat ANP inhibited aldosterone and corticosterone production rates in response to a half-maximal steroidogenic concentration of ACTH (10 pM; approximately 50 pg/ml), albeit with less efficacy ( approximately 50%) and potency (approximately 6 nM) than for ANG II. Rat and eel ANP and rat and chicken brain natriuretic peptide (BNP) were equally efficacious at inhibiting maximal ANG II-induced aldosterone and corticosterone production but with different potencies. The order of inhibitory potency was rat ANP = chicken BNP > eel ANP > rat BNP. However, a specific peptide ligand for the NP clearance receptor was without effect. This study indicates that ANP and related NPs are efficacious inhibitors of lizard adrenal steroidogenesis by acting directly at the level of the adrenocortical cell.

Comparative aspects of natriuretic peptide physiology in non-mammalian vertebrates: a review

The natriuretic peptide system is a complex family of peptides and receptors that is primarily linked to the maintenance of osmotic and cardiovascular homeostasis. A natriuretic peptide system is present in each vertebrate class but there are varying degrees of complexity in the system. In agnathans and chondrichthyians, only one natriuretic peptide has been identified, while new data has revealed that multiple types of natriuretic peptides are present in bony fish. However, it seems in tetrapods that there has been a reduction in the number of natriuretic peptide genes, such that only three natriuretic peptides are present in mammals. The peptides act via a family of guanylyl cyclase receptors to generate the second messenger cGMP, which mediates a range of physiological effects at key targets such as the gills, kidney and the cardiovascular system. This review summarises the current knowledge of the natriuretic peptide system in non-mammalian vertebrates and discusses the physiological actions of the peptides.

Immunohistochemical localization of atrial natriuretic factor and autoradiographic distribution of atrial natriuretic factor-binding sites in the brain of the cave salamander Hydromantes genei (Amphibia, Plethodontidae)

The distribution of atrial natriuretic factor (ANF)-like immunoreactivity in the central nervous system of the cave salamander Hydromantes genei (Amphibia, Plethodontidae) was investigated by using antisera raised against rat and human ANF(1-28). Concurrently, the location of ANF-binding sites was determined by autoradiography, using radioiodinated human ANF(1-28) as a tracer. In several regions of the brain, including the olfactory bulb, the preoptic area, the ventral thalamus, the tectum of the mesencephalon, and the choroid plexuses inside the ventricles, a good correlation was observed between the distribution of ANF-immunoreactive elements and the location of ANF-binding sites. Mismatching was found in the habenular nucleus, the commissura habenularis, the fasciculum retroflexus, and the interpeduncular nucleus, which contained high levels of binding sites but were devoid of ANF-immunoreactive structures. In contrast, a few other regions, such as the pineal gland and the subcommissural organ, showed a high concentration of ANF-like immunoreactivity but did not contain ANF-binding sites. This study provides the first localization of ANF-like immunoreactivity and ANF-binding sites in the brain of an urodele amphibian. The results show that the ANF peptidergic system in the cave salamander has an organization more simple than the organizations described for the brain of frog or other vertebrates. This feature is probably related to the expression of highly pedomorphic characters in plethodontids. The anatomical distribution of ANF-immunoreactive elements and ANF-binding sites suggests that ANF-related peptides may act as hypophysiotropic hormones as well as neurotransmitters and/or neuromodulators in the salamander brain.

Evidence for the involvement of nitric oxide in the control of steroid secretion by the frog adrenal gland

Nitric oxide (NO) has been found to modulate the response of rat, bovine and human adrenocortical cells to corticotropic factors. The aim of the present study was to investigate the possible involvement of NO in the control of corticosteroid secretion in the frog Rana ridibunda. Histochemical studies using the NADPH-diaphorase reaction and immunohistochemical labeling with antibodies against NO synthase (NOS) revealed that NOS is exclusively expressed in chromaffin cells. The NO donor sodium nitroprusside (SNP) and the NO synthase inhibitor Nw-nitro-L-arginine (L-NO(2)Arg) did not modify the spontaneous production of corticosterone and aldosterone by perifused adrenal slices. Similarly, L-NO(2)Arg had no effect on the secretory responses induced by ACTH, angiotensin II (AII) and endothelin-1 (ET-1). In contrast, SNP significantly inhibited the stimulatory effects of ACTH, AII and ET-1 on corticosterone and aldosterone secretion. These data provide the first evidence for a modulatory role of NO on adrenocortical cell activity in amphibians.

Immunohistochemical localization, biochemical characterization, and biological activity of neurotensin in the frog adrenal gland

The primary structure of neurotensin has been recently determined for the frog Rana ridibunda (Endocrinology 139: 4140-4146, 1998). In the present study, we have investigated the distribution and biochemical characterization of neurotensin-like immunoreactivity in the frog adrenal gland, using an antiserum directed against the conserved C-terminal region of the peptide. Neurotensin-like immunoreactivity was detected in two populations of nerve fibers: numerous varicose fibers coursing between adrenal cells, and a few processes located in the walls of blood vessels irrigating the gland. Reversed-phase HPLC analysis of frog adrenal gland extracts revealed the existence of a major peak of neurotensin-like immunoreactivity that exhibited the same retention time as synthetic frog neurotensin. The possible involvement of neurotensin in the regulation of steroid secretion was studied in vitro using perifused frog adrenal slices. For concentrations ranging from 10(-10) to 10(-5) M, synthetic frog neurotensin increased corticosterone and aldosterone production in a dose-dependent manner (EC50 = 1.2 x 10(-9) M and 5.8 x 10(-10) M, respectively). Repeated administration of neurotensin induced a reproducible stimulation of steroid output without any tachyphylaxis. Prolonged administration (3 h) of frog neurotensin caused a transient increase in corticosterone and aldosterone secretion followed by a decline of corticosteroid secretion. Neurotensin also produced a significant stimulation of corticosteroid secretion from dispersed frog adrenal cells. This study demonstrates that neurotensin is located in nerve processes innervating the adrenal gland of amphibians. The results also show that synthetic frog neurotensin exerts a direct stimulatory effect on corticosteroid output. Taken together, these data support the view that neurotensin, released by nerve fibers, may act as a local regulator of corticosteroid secretion.

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.

Immunocytochemical localization of atrial natriuretic factor and autoradiographic distribution of atrial natriuretic factor binding sites in the brain of the African lungfish, Protopterus annectens

The localization of atrial natriuretic factor (ANF)-immunoreactive elements was investigated in the brain of the African lungfish, Protopterus annectens, by using antisera raised against rat and human ANF(1-28). Concurrently, the distribution of ANF binding sites was studied by autoradiography using radioiodinated human ANF(1-28) as a tracer. In general, there was a good correlation between the distribution of ANF-immunoreactive structures and the location of ANF binding sites in several areas of the brain, particularly in the ventral part of the medial subpallium, the rostral preoptic region, the preoptic periventricular nucleus, the caudal hypothalamus, the neural lobe of the pituitary, and the mesencephalic tectum. In contrast, mismatching was observed in the pallium (which contained a high density of binding sites and a low concentration of ANF immunoreactive elements) as well as in the lateral subpallium and the medial region of the ventral thalamus, in which a low concentration of binding sites but a high density of ANF-immunoreactive fibers were detected. The present data provide the first localization of ANF-related peptides in the brain of dipnoans and the first anatomical distribution of ANF binding sites in the brain of fish. The results show that the ANF peptidergic systems of P. annectens exhibit similarities with those previously described in the frog Rana ridibunda, supporting the existence of relationships between dipnoans and amphibians. The location of ANF-like immunoreactivity and the distribution of ANF binding sites suggest that ANF-related peptides may act as hypothalamic neurohormones as well as neurotransmitters and/or neuromodulators in the lungfish brain.

Characterization of the receptor mediating the effect of calcitonin gene-related peptide in the frog adrenal gland

We have recently reported the presence of calcitonin gene-related peptide (CGRP)-containing nerve fibers in the frog adrenal gland and we have shown that CGRP is a potent stimulator of corticosterone and aldosterone secretion by adrenocortical cells. The aim of the present study was to characterize the type of receptors mediating the effect of CGRP in the frog adrenal gland. Amylin and adrenomedullin, two members of the CGRP family, induced a weak stimulation of corticosterone and aldosterone secretion from perifused frog adrenal slices. In contrast, salmon and human calcitonin had no effect on corticosteroid secretion. Administration of the type-1 CGRP receptor antagonists human CGRP-(8-37) and human CGRP-(19-37) did not significantly affect the secretory response induced by frog CGRP. Concurrently, the type-2 CGRP receptor agonist [acetamidomethyl-Cys2,7]human CGRP ([Cys(ACM)2,7]human CGRP) provoked a dose-dependent stimulation of corticosterone and aldosterone secretion (EC50 = 1.6 x 10(-7) M). Both frog CGRP and [Cys(ACM)2,7]human CGRP induced a significant increase in cAMP production by frog adrenal tissue. These data indicate that, in the frog adrenal gland, the stimulatory effect of CGRP is mediated through activation of a type-2 CGRP receptor positively coupled to adenylyl cyclase.

Pharmacological profile of the tachykinin receptor involved in the stimulation of corticosteroid secretion in the frog Rana ridibunda

It has recently been shown that the adrenal gland of the frog Rana ridibunda is densely innervated by a network of fibers containing two novel tachykinins, i.e. ranakinin (the counterpart of substance P) and [Leu3, Ile7]neurokinin A. Both ranakinin and [Leu3, Ile7]neurokinin A stimulate corticosteroid secretion from frog adrenal glands in vitro. In the present study, we have investigated the pharmacological profile of the receptors involved in the stimulatory action of ranakinin on perifused frog adrenal slices. The selective NK-1 receptor antagonists [D-Pro4, D-Trp7,9]substance P 4-11 and CP-96,345, did not affect the stimulatory action of ranakinin. The selective NK-1 agonist substance P 6-11 had no effect on corticosteroid secretion. The non-peptidic NK-1 receptor antagonist RP 67580 significantly reduced the stimulatory effect of ranakinin on corticosterone and aldosterone secretion by 57 and 55%, respectively. In addition, the dual NK-1/NK-2 receptor antagonist FK-224 significantly inhibited the effect of ranakinin on corticosterone (- 80%) and aldosterone secretion (- 95%). Finally, the amphiphilic analogue of substance P, [D-Pro2, D-Phe7, D-Trp9]substance P, had no effect on corticosteroid secretion. These data suggest that in the frog adrenal gland the stimulatory action of ranakinin on steroid secretion is mediated by a novel type of receptor which differs substantially from the mammalian NK-1 receptor subtype.

Localization and coexistence of atrial natriuretic peptide (ANP) and neuropeptide Y (NPY) in vertebrate adrenal chromaffin cells immunoreactive to TH, DBH and PNMT

Antisera specific for mammalian atrial natriuretic peptide (ANP) and neuropeptide Y (NPY) were applied to examine, in immunofluorescence, the occurrence of cells immunoreactive to ANP and NPY in the adrenal organs of mammals, birds, reptiles, amphibians, and bony fish. Catecholamine-containing cells were identified using antisera against tyrosine-hydroxylase, dopamine-beta-hydroxylase, and phenylethanolamine-N-methyl-transferase. In all vertebrates studied, immunoreactivities to ANP and NPY occurred in adrenal chromaffin cells but were absent from the cortex or its homolog, the interrenal. The majority of immunoreactivities to ANP and NPY was confined to the adrenaline cells. In mammals, the number of ANP-immuno-reactive cells (60%-80% of the total cell population) exceeded that of the NPY-immunoreactive cells (35%-45%). In birds, reptiles, and Amphibia, the numbers of ANP-immunoreactive (35%-40%) and NPY-immunoreactive (30%-35%) cells were in a similar range. The bony fish showed a density of both ANP-immunoreactive (80%-90%) and NPY-immunoreactive (35%-40%) cells. In all species studied, immunoreactivities to ANP and NPY partially coexisted. Generally, 30%-55% of the ANP-immunoreactive cells also contained NPY-immunoreactivity. In rat, coexistence amounted to almost 100% and in quail to 95%. Except for the rat, three subpopulations of chromaffin cells seemed to occur: ANP-immunoreactive non-NPY-immunoreactive, ANP-immunoreactive+NPY-immunoreactive, and NPY-immunoreactive non-ANP-immunoreactive cells. Thus, adrenal ANP and NPY share a conservative history and coexist as early as at the level of bony fish. The endocrine actions of ANP and NPY derived from medullary cells on cortical cells as found in mammals might be based on an ancestoral paracrine system. In submammalians, ANP and NPY may not only act as endocrine hormones, but also influence steroid-producing interrenal cells in a paracrine manner, and act as modulators on chromaffin cells.

Hormonal effect on the osmotic, electrolyte and nitrogen balance in terrestrial Amphibia

Two main hormones regulate water balance in amphibian. First, mesotocin (MT) acting as a diuretic agent, and second arginine vasotocin (AVT) being an anti-diuretic hormone. In addition, prolactin (PRL), aldosterone, corticosterone, angiotensin II and atriunatriuretic hormones, play a role too in regulating water and ion balance. The hormones affect the epidermis and bladder permeability to water and ions as well as the kidney through the control of the glomerular filtration rate (GFR). The main questions concern the presence and action of these hormones during the amphibian's life history. Are they present in both larval and adult stages? Are these hormones being synthesized in both aquatic and terrestrial adult phases? Under what circumstances are they being stored or released? Would the target organs (epidermis, bladder, kidney) respond in a similar way during all periods? The problem is the fact that under most circumstances an amphibian while in an aquatic environment responds physiologically differently than when on land. Only partial information concerning hormone presence, release and control of water balance is available at the moment, and even that is fragmentary and based on only a very small number of amphibian species.

In vitro study of the effect of urotensin II on corticosteroid secretion in the frog Rana ridibunda

Urotensin II is a cyclic dodecapeptide that was originally isolated from the fish urophysis, the terminus of a neurosecretory system located in the caudal area of the spinal cord. We have recently isolated and characterized urotensin II in the brain of a tetrapod, the frog Rana ridibunda. Recent reports, suggesting that urotensin II may stimulate cortisol secretion in fish, prompted us to investigate the possible effects of fish and frog urotensin II on corticosteroid secretion in amphibians. Exposure of perifused frog adrenal slices to goby (Gillichthys mirabilis) urophysis extracts induced a marked stimulation of corticosterone and aldosterone secretion. In contrast, at concentrations ranging from 10(-10) to 10(-6) M, synthetic goby urotensin II had no effect on corticosteroid production. Similarly, infusion of synthetic frog urotensin II (10(-10) to 10(-6) M) did not modify the spontaneous release of corticosterone and aldosterone. In addition, frog urotensin II had no effect on ACTH- and angiotensin II-induced secretion of corticosteroids. These results show that in frog, urotensin II does not modulate spontaneous and ACTH- or angiotensin II-evoked adrenal steroidogenesis.

Effects of atrial natriuretic factor on the renin-aldosterone system: in vivo and in vitro studies

We investigated the effect of high physiological plasma levels of human varies; is directly proportional to atrial natriuretic factor (ANF) on renin and aldosterone secretion in normal sodium deplete men. In short term infusion studies (2 or 8 h duration), ANF plasma levels as observed after sodium loading (50-70 pg/ml) lowered basal renin (PRA) and aldosterone, but had only a marginal effect on angiotensin II-stimulated aldosterone secretion. Preliminary results of a study with long term infusion (6 days) of ANF during a period of dietary sodium depletion argue against a significant tonic inhibitory effect of ANF on the renin-aldosterone system in the preceding period of sodium repletion: the plasma aldosterone response to sodium depletion was similar with and without ANF infusion. The second messenger of ANF for the direct inhibition of aldosterone secretion from zona glomerulosa cells is still unknown. To test the hypothesis, that cGMP is the second messenger of ANF, we produced a rise in intracellular cGMP in rat and rabbit zona glomerulosa cells using the unspecific phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) and the more cGMP specific phosphodiesterase specific inhibitor M + B2948 (Zaprinast). Both inhibitors simulated the action of ANF in suppressing steroid secretion and elevating cGMP levels. The results are compatible with the view that cGMP is of importance as a second messenger for ANF in adrenal zona glomerulosa cells. Selective inhibition of phosphodiesterases in combination with endopeptidase inhibition may be an interesting principle to enhance the action of endogenous and exogenous ANF.

Effects of atrial natriuretic peptide and toad heart extract on isolated toad Bufo arenarum aortic rings

The vascular effects of synthetic atrial natriuretic peptide (ANP) (rANP-99-126), and toad heart extract (THE) were examined on isolated toad aortic rings from the toad Bufo arenarum. ANP inhibited contraction produced by human angiotensin II (AT II), norepinephrine (NE), and arginine vasopressin (AVP) in isolated toad aortic rings. The present data show that a relaxant effect of ANP could be obtained also in the noncontracted aortic smooth muscle of toad if it had been previously challenged with AT II or NE and allowed to return to the original basal tension. Bufo arenarum THE was able to relax the AT II-induced contraction in toad aortic rings. In toad arteries contracted with 10(-6) M AT II, ANP produced a dose-dependent relaxation with a half-maximal inhibitory concentration IC50 of 1.2 x 10(-8) M. ANP was not effective in relaxing contraction induced by high K+. The vasorelaxant effect of ANP on AT II-induced contraction was significantly increased in Ca(2+)-free medium containing 3 mM ethylene glycol bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA-Ringer) or by pretreatment with the calcium antagonist, diltiazem (DIL). The vasorelaxant effect of ANP on basal tension after treatment with AT II was also obtained in absence of extracellular calcium (EGTA-Ringer). These results show that Bufo arenarum contains ANP-like material and that the ANP relaxant action in the toad aorta is similar to that in mammals.

Frog lymph heart synthesizes and stores immunoreactive atrial natriuretic peptide

The presence of immunoreactive atrial natriuretic peptide (irANP) and ANP gene expression in the frog lymph heart was examined by a radioimmunoassay (RIA) combined with HPLC and by Northern blot hybridization of total RNA. Serial dilution curve of the lymph heart extract was paralleled with the RIA standard curve. The lymph heart contained 153.32 +/- 35.80 pg of irANP/mg of wet tissue. The major form of irANP in the frog lymph heart was high molecular weight on reverse-phase and gel permeation high performance liquid chromatography as in the frog atria and ventricles. The frog lymph heart, as well as frog atria and ventricles, was shown to express mRNA coding for ANP. Dense core secretory granules similar to those observed in the mammalian atria were also found in the frog lymph heart. The presence of irANP and the expression of ANP gene in the frog lymph heart suggest that the lymph heart may participate in the regulation of homeostasis of lymph circulation and blood volume change through the synthesis and release of ANP.

Distribution patterns and coexistence of neurohormonal peptides (ANP, BNP, NPY, SP, CGRP, enkephalins) in chromaffin cells and nerve fibers of the anuran adrenal organ

Immunohistochemical techniques were used to study the adrenal organs of the anuran species Rana esculenta, Caldula pulchra and Bufo marinus with respect to the distribution and coexistence of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), substance P (SP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), Leu-enkephalin (Leu-ENK). Met-enkephalin-Arg-Phe (MEAP) and dynorphin A 1-17 (DYN). Antisera against enzymes involved in catecholamine synthesis, i.e., dopamine-beta-hydroxylase (DBH) and tyrosine hydroxylase (TH), were used for the identification of chromaffin cells. ANP-immunoreactive (-IR) cells occurred in high densities (30%-70% of the total cell population) in all species investigated. In C. pulchra and B. marinus, BNP-IR cells constituted a population of non-DBH-IR and non-TH-IR cells that were different from the ANP-IR cells. A large proportion of the adrenal cells (10%-55%) were immunoreactive to Leu-ENK, and a minority (2%-5%) showed MEAP-immunoreactivity. DYN-immunoreactivity was not observed. The anurans studied exhibited small numbers of SP-IR, CGRP-IR and NPY-IR cells. Immunoreactivities for ANP + Leu-ENK and Leu-ENK + MEAP were shown to coexist. In C. pulchra and B. marinus, immunoreactions for ANP + NPY, ANP+SP and SP + CGRP were also colocalized. Except for DYN, all neurohormonal peptides also occurred in intra-adrenal nerve fibers. SP-IR fibers also displayed CGRP-immunoreactivity and some Leu-ENK-IR fibers contained MEAP-immunoreactivity. In C. pulchra, NPY-IR fibers were found that also showed ANP-immunoreactivity.

Atrial natriuretic factor (ANF) binding sites in frog kidney and adrenal

Atrial natriuretic factor (ANF) binding sites were localized and quantified in kidney and adrenal of the frog Rana temporaria by quantitative in vitro autoradiography. [125I]-rat ANF(99-126) binding was present in kidney glomeruli and in the outer layer of interrenal tissue in the adrenal gland. ANF binding exhibited positive cooperativity with a half-maximal binding concentration (EC50) of 102 +/- 16 pM in glomeruli and 93 +/- 19 pM in interrenal tissue (n = 8). The corresponding maximal binding capacities (Bmax) were 1.33 +/- 0.16 and 1.21 +/- 0.36 fmol/mm2. [125I]-Rat ANF(99-126) binding was competitively displaced by unlabeled ANF analogues with an intact disulfide bridge showing a lower affinity than the iodinated ligand. The presence of ANF binding in glomeruli and steroidogenic interrenal cells suggests physiological functions of ANF for the osmomineral regulation in the frog by influencing glomerular filtration rate and adrenal steroid secretion.

Effects of atrial natriuretic factor on corticosteroid and catecholamine secretion by the adrenals of Xenopus laevis

The effects of atrial natriuretic factor (ANF) on the adreno-corticosteroid and catecholamine secretion of Xenopus laevis were studied in vitro and in vivo. In vitro, the effects of rANF(99-126), from 0.1 to 50 nM, on corticosteroid secretion was investigated using a perifusion system. The basal secretion of aldosterone but not corticosterone was dose dependently decreased. A prolonged perifusion with 1 nM rANF(99-126) alternated ACTH(1-28) stimulation of secretion of both corticosteroids. Only ANF analogues with intact disulfide bridges (rANF(99-126), hANF(99-126), Atriopeptin II, frogANF(21)), and an extract of Xenopus laevis hearts significantly inhibited aldosterone release; the N-terminal (99-109) and the C-terminal ANF(116-126) fragments had no effects. In vitro norepinephrine (NE) and epinephrine (E) were released but dopamine (D) was not detected. rANF(99-126) at concentrations up to 1 microM affected neither basal nor acetylcholine stimulated catecholamine secretion. In vivo, a single injection of 3 nmol rANF(99-126) per 100 g body weight was given and the serum concentrations of corticosterone, aldosterone, D, NE, and E were determined 1, 3, 6, 12, and 24 hr later. Both steroids decreased after 12 hr, whereas the catecholamine concentrations were not significantly changed. ANF is concluded to act on steroidogenic but not chromaffin cells in Xenopus laevis.