Further investigations on the atrial natriuretic factor (ANF)-induced inhibition of the growth and steroidogenic capacity of rat adrenal zona glomerulosa in vivo

Natriuretic and antialdosterone actions of chronic oral NEP inhibition during progressive congestive heart failure

BACKGROUND

Neutral endopeptidase (NEP) degrades atrial natriuretic peptide (ANP) that via cyclic guanosine monophosphate (cGMP) is natriuretic and aldosterone-inhibiting. We hypothesized that chronic oral NEP inhibition (NEPI), initiated in early experimental congestive heart failure (CHF), would delay onset of decreases in sodium excretion during the progression of CHF and, in the severe phase, suppress aldosterone activation and reduce the magnitude of sodium retention. We also hypothesized that chronic NEPI during progressive CHF (PCHF) would improve the natriuretic response to acute volume expansion.

METHODS

In a novel canine model that progresses over 38 days from early to moderate and finally severe CHF, we defined the actions of chronic NEPI (candoxatril, 10 mg/kg, orally, twice a day) upon cardiorenal and neurohumoral function as well as the clinical well being of treated and untreated dogs in CHF.

RESULTS

From baseline through the moderate phase of CHF, NEPI maintained sodium excretion. In contrast, in moderate CHF, sodium excretion was reduced compared to the early phase in the controls. In severe CHF, sodium excretion was higher with NEPI compared to control. Chronic NEPI also resulted in lower plasma aldosterone as compared to controls. In severe CHF, the natriuretic response to acute saline volume expansion was enhanced with oral NEPI as compared to control.

CONCLUSION

This study supports the conclusion that chronic oral NEPI delays the onset of reduction in sodium excretion during the transition from early to severe CHF in this model of PCHF. This therapeutic strategy also improved the natriuretic response to acute volume expansion in severe CHF while enhancing ANP and suppressing aldosterone activation. Thus, these studies demonstrated a selective renal and adrenal action of chronic NEPI in heart failure indicating a therapeutic potential.

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency

More than 90% of cases of congenital adrenal hyperplasia (CAH, the inherited inability to synthesize cortisol) are caused by 21-hydroxylase deficiency. Females with severe, classic 21-hydroxylase deficiency are exposed to excess androgens prenatally and are born with virilized external genitalia. Most patients cannot synthesize sufficient aldosterone to maintain sodium balance and may develop potentially fatal "salt wasting" crises if not treated. The disease is caused by mutations in the CYP21 gene encoding the steroid 21-hydroxylase enzyme. More than 90% of these mutations result from intergenic recombinations between CYP21 and the closely linked CYP21P pseudogene. Approximately 20% are gene deletions due to unequal crossing over during meiosis, whereas the remainder are gene conversions--transfers to CYP21 of deleterious mutations normally present in CYP21P. The degree to which each mutation compromises enzymatic activity is strongly correlated with the clinical severity of the disease in patients carrying it. Prenatal diagnosis by direct mutation detection permits prenatal treatment of affected females to minimize genital virilization. Neonatal screening by hormonal methods identifies affected children before salt wasting crises develop, reducing mortality from this condition. Glucocorticoid and mineralocorticoid replacement are the mainstays of treatment, but more rational dosing and additional therapies are being developed.

Up-regulation of adrenal cortical and medullary atrial natriuretic peptide and gene expression in rats with deoxycorticosterone acetate-salt treatment

Our previous study demonstrated that human adrenal medulla is a site of atrial natriuretic peptide (ANP) synthesis. To further evaluate the role of adrenal ANP in body fluid homeostasis, we investigated the changes in adrenal ANP in rats receiving deoxycorticosterone acetate (DOCA)-salt treatment. In situ hybridization and immunohistochemical study showed that adrenal ANP messenger RNA (mRNA) and ANP-like immunoreactivities (ANP-LI) were mainly localized in the zona glomerulosa and medulla of vehicle-treated rats. DOCA-salt treatment activated ANP mRNA and peptide expression in all adrenal zones, especially in the zona fasciculata/reticularis from 12 h to the entire 8-day study period. Using a semiquantitative RT-PCR technique, the relative quantities of ANP mRNA in the adrenals of the DOCA-salt-treated group were significantly increased from 1 to 8 days, whereas the adrenal weights of DOCA-salt-treated rats were significantly decreased from day 2 to day 8. Our results are the first to indicate that ANP is synthesized not only in the adrenal medulla but also in the adrenal cortex and their syntheses are markedly increased in DOCA-salt-treated rats. These results imply that adrenal ANP may participate in the intraadrenal regulation of adrenal function on water-electrolyte homeostasis in an autocrine or paracrine manner.

Natriuretic peptides receptors in human aldosterone-secreting adenomas

Atrial natriuretic peptide (ANP) and B-type or brain natriuretic peptide (BNP) inhibit aldosterone secretion in humans both in vitro and in vivo. Unresponsiveness of aldosterone-secreting adenomas (aldosteronomas) to ANP in vitro and in vivo, might be due to reduced expression of the biologically-active natriuretic peptide receptor type A (NPr-A) and/or increased expression of the clearance receptor for natriuretic peptides (NPr-C). Therefore, we have analyzed NPr gene expression and ANP binding sites in human adrenals and aldosteronomas. Using reverse transcription and polymerase chain reaction, we cloned and characterized cDNAs for NPr-A, NPr-C, and the receptor (NPr-B) for the C-type natriuretic peptide (CNP). Total RNA from three normal human adrenals (obtained at surgery from patients with renal cancer) and five aldosteronomas were used for Northern analysis. NPr-A mRNA (approximately 4 kb) and NPr-B mRNA (approximately 4 kb) were expressed without significant differences in adrenals and in aldosteronomas except in an aldosteronomas that contained only very low amounts of NPr mRNAs. The gene expression of NPr-C was barely detectable both in adrenals and in aldosteronomas. ANP binding sites were analyzed by autoradiography with 125I-labeled ligand in other six aldosteronomas. Only one of the adenomas analyzed showed ANP binding sites with density of granules similar to nonadenomatous glomerulosa, whereas the others had significantly reduced densities. In summary, aldosteronomas express the genes encoding for NPr but mainly NPr-A, similarly to control adrenals. On the contrary, the binding sites for ANP are greatly reduced in most aldosteronomas. A somatic mutation or a post-transcriptional defect that reduces ANP binding sites might be present in some aldosteronomas.

Ontogenic expression of natriuretic peptide mRNAs in postnatal rat brain: implications for development?

The central natriuretic peptide system is composed of at least three structurally homologous and uniquely distributed peptides and receptors which are thought to be involved in the central regulation of cardiovascular and autonomic function and more recently been shown to affect cellular growth and proliferation, processes pertinent to mammalian development. As such, following our initial mapping of preproatrial natriuretic peptide (ppANP) mRNA in adult brain [M.C. Ryan, A.L. Gundlach, Anatomical localization of preproatrial natriuretic peptide mRNA in the rat brain by in situ hybridization histochemistry: in olfactory regions, J. Comp. Neurol., 356 (1995) 168-182], it was of interest to determine the ontogenic expression of natriuretic peptide mRNAs in the developing rat brain. Using in situ hybridization histochemistry of specific [35S]- or [33P]-labeled oligonucleotides, ppANP and preproC-type natriuretic peptide (ppCNP) mRNAs were detected in the developing rat brain from postnatal day 4 to day 60 (adult). PpANP mRNA was observed in many hindbrain, but only some forebrain, regions at postnatal day 4. Regional differences in the temporal expression of ppANP mRNA were apparent with ppANP mRNA detected in the medial preoptic area, mammillary nuclei and medial habenular nucleus at postnatal day 4 and in other areas including the arcuate and dorsomedial hypothalamic nuclei and in olfactory and limbic regions at postnatal day 10. A number of regions also exhibited transient expression of ppANP mRNA such as the bed nucleus of the stria terminalis and the medial cerebellar nucleus. In contrast, ppCNP mRNA was detected at relatively high levels in several regions on postnatal day 4 including olfactory nuclei, the hippocampus and particularly the pontine nucleus. The level of expression appeared to increase markedly in most regions including forebrain olfactory and hippocampal areas and in brainstem regions including the pontine nucleus, the parvocellular and lateral reticular and spinal trigeminal nuclei by postnatal days 10 and 13, but decreased from this peak to equivalent to adult levels by postnatal day 28. The differential and transient expression of the natriuretic peptides during postnatal development, together with previous reports of the ontogenic regulation of natriuretic peptide receptor expression and binding patterns, further suggests their involvement in developmental processes in the rat CNS and provides information relevant to the likely functional development of natriuretic peptide-utilizing pathways.

Morphological and functional studies of the paracrine interaction between cortex and medulla in the adrenal gland

Within the last years it has become evident that besides the hypothalamo-pituitary-adrenal axis, extrapituitary mechanisms exist that regulate the activity of the adrenal cortex. In this context, intra-adrenal regulatory mechanisms play an important role. Several secretory products from adrenomedullary cells are able to influence adrenocortical steroidogenesis. Since the main blood flow within the adrenal is directed centripetally from the cortex to the medulla, chromatin cells should act on cortical cells in a paracrine manner. The morphological prerequisite for this regulatory pathway is seen in the close apposition of the two tissues. Within the mammalian adrenal, the two endocrine tissues are interwoven to an astonishing degree with cortical cells located within the medulla and vice versa. It is concluded from morphological and functional studies that paracrine interactions between cortex and medulla play an important role in the regulation of adrenocortical steroidogenesis.

Calcitonin gene-related peptide depresses the growth and secretory activity of rat adrenal zona glomerulosa

The bolus ip. injection of rat calcitonin gene-related peptide (CGRP) (5 pm. kg-1) significantly lowered plasma aldosterone concentration (PAC) in rats, despite a mild rise in plasma renin activity. Natremia, kalaemia and the blood levels of ACTH or corticosterone were not affected. Similar results were obtained after prolonged (5 days) sc. infusion of rats with CGRP (1 pm. kg-1. h-1). Moreover, CGRP infusion caused a notable atrophy of the zona glomerulosa (ZG) and its parenchymal cells, as well as a clearcut reduction in the surge of PAC evoked by a bolus injection of a high dose of angiotensin-II (100 micrograms. kg-1). From these results it is suggested that CGRP exerts an inhibitory effect on the growth and secretory activity of ZG in rats.

Effects of neuropeptide-Y and substance-P on the secretory activity of dispersed zona-glomerulosa cells of rat adrenal gland

Neuropeptide-Y (NPY) and substance-P (SP), two peptides contained in the chromaffin granules of adrenal medullary cells, were found to partially inhibit both basal ACTH-stimulated release of aldosterone and 18-hydroxy-corticosterone by isolated rat zone-glomerulosa cells, without affecting the overall post-pregnenolone yield or basal progesterone output. Conversely, the exposure to both peptides increased 11-deoxy-corticosterone and corticosterone secretion. These data indicate that NPY and SP are able to exert a direct suppression of 18-hydroxylase activity in rat zona-glomerulosa cells, without conceivably altering the earlier steps of aldosterone synthesis. The possible physiological implications of these findings are discussed in light of previous studies suggesting a net adrenoglomerulotrophic effect of NPY and SP in vivo.

Effects of prolonged infusion with endothelin-1 on the function and morphology of rat adrenal cortex

A week of SC infusion with endothelin-1 (ET-1) (0.2 microgram.kg-1.hr-1) lowered PRA and raised plasma aldosterone (A) concentration in rats. Kalaemia and the plasma levels of ACTH and corticosterone (B) were not affected. Prolonged ET-1 administration caused a notable hypertrophy of zona glomerulosa (ZG) and its parenchymal cells, without inducing any apparent change in zona fasciculata. Stereology showed that ZG cell hypertrophy was mainly due to the increase in the volume of the mitochondrial compartment and to the proliferation of smooth endoplasmic reticulum (i.e., the two organelles in which the enzymes of steroid synthesis are contained). Isolated ZG cells from ET-1-infused animals evidenced a notable enhancement in their basal production of A and B. The secretory responses of ZG cells to the maximal effective concentrations of their three main stimulators (ACTH, angiotensin-II and K+) displayed comparable increases. These findings indicate that ET-1, when chronically administered, is able to specifically enhance the growth and steroidogenic capacity of rat ZG, and suggest that the mechanism underlying this ET-1 effect involves stimulation of the de novo synthesis of both the steroidogenic enzymes and the membrane framework in which they are located.

Long-term effects of neuropeptide-Y on the rat adrenal cortex

Adult female rats, intact or dexamethasone suppressed, were i.p. injected twice a day for 4 consecutive days with 0.5 ug neuropeptide-Y (NPY). In intact rats, NPY lowered serum ACTH and aldosterone levels, and had no effects either on serum and adrenal corticosterone or the morphology of the adrenal cortex. In dexamethasone-treated rats, NPY again decreased serum aldosterone concentration. Moreover, it caused a small but significant drop both in the volume of zona fasciculata cells and the adrenal content of corticosterone. These findings seem to suggest an inhibitory effect of NPY on the function of rat zona glomerulosa and perhaps zona fasciculata.

Effects of acute and chronic treatments with atrial natriuretic factor (ANF) on the Leydig cells of the rat testis

Acute ANF bolus administration (40 micrograms.kg-1) did not affect secretory activity and morphology of rat Leydig cells. Prolonged (7-day) ANF infusion (20 micrograms.kg-1.h-1), on the contrary, elevated both basal and hCG-stimulated testosterone blood concentration, and caused a notable hypertrophy of rat Leydig cells. Leydig-cell hypertrophy was due to increases in the volume of all the organelles involved in cholesterol and testosterone synthesis (i.e. mitochondria, smooth endoplasmic reticulum and peroxisomes). These findings suggest that ANF, when chronically administered, is able to stimulate the growth and steroidogenic capacity of rat Leydig cells.

Atrial natriuretic peptide: water and electrolyte homeostasis

In the few years since its identification, a clear role for ANP in the regulation of water and electrolyte balance has emerged (Figure 3). The peptide is released in response to blood volume expansion, both acutely and gradually during changes in dietary sodium intake. Similarly, plasma levels are elevated in pathophysiological conditions such as cardiac and renal failure. It has become apparent that ANP has natriuretic, diuretic and vasorelaxant properties. Many of the original studies employed what we now know to be pharmacological doses of the peptide. However, recent reports have confirmed that small, sustained elevations in plasma ANP within or marginally above the 'normal' physiological range produce similar effects. A number of recent studies have tried to specifically address the physiological relevance of ANP. Although undoubtedly release by atrial distension and effective when infused to similar concentrations, atrial distension also has other effects via neural pathways. Thus, the demonstration that excretion of saline is impaired by atrial appendectomy (Benjamin et al, 1988) does not imply that this is only due to the absence of an atrial hormone. Goetz et al (1986) demonstrated that in the denervated heart, although ANP is still released, the excretion of a saline load is impaired. Similarly, in man, Richards et al (1988a) needed to infuse ANP to much higher plasma levels than those achieved by a saline load in order to reproduce the natriuresis. Although these experiments can be criticized, they confirm that ANP is not the sole mechanism for excreting a volume load, or for the natriuresis following atrial distension, but that these effects are likely to reflect the balance between ANP, AVP, the renin-angiotensin and autonomic nervous systems. In rats immunized against ANP (Greenwald et al, 1988), although the ability to excrete an acute saline load was impaired, long-term sodium balance was normal, suggesting that the rats were able to compensate for the absence of ANP. Many of the actions of ANP can be explained by antagonism of the renin-angiotensin-aldosterone system. Teleologically, it seems appropriate that a natriuretic hormone should counterbalance the major pressor and antinatriuretic hormones within the body. There is good evidence for cellular interactions between angiotensin, AVP, aldosterone and ANP at a number of discrete sites which are additional to the straightforward physiological antagonism of systems with opposing actions. ANP inhibits aldosterone secretion directly and may also reduce renal renin release. In the vascular tree there is evidence that ANP specifically blocks the vasoconstrictor actions of angiotensin II and possibly AVP.(ABSTRACT TRUNCATED AT 400 WORDS)