Atrial Natriuretic Factor is Released into Hypophysial Portal Blood: Direct Evidence that Atrial Natriuretic Factor may be a Neurohormone Involved in Hypothalamic Pituitary Control

Abstract

Atrial natriuretic factor (ANF) is produced in atriomyocytes (1, 2), released as a 28 amino-acid peptide into the systemic circulation and probably plays an important role in fluid and electrolyte balance (3,4). The facts that immunoreactive ANF (ir-ANF) is also present in nerve terminals in the external layer of the median eminence (ME) (5), ir-ANF can be released from the hypothalamus in vitro by potassium depolarization (6) and specific ANF binding sites are present at a high concentration in rat pituitary tissue (7) suggest that ANF may be involved in hypothalamic-pituitary regulation. We report here that ir-ANF concentrations in hypophysial portal blood are about two to four times greater than in peripheral plasma from hypophysectomized as well as pituitary-intact adult female rats. These results show for the first time that ir-ANF is secreted from the hypothalamus into the hypophysial portal circulation at a concentration (-lo-' M) consistent with a role for ANF as a hypothalamic-pituitary regulator or modulator.Hypophysial portal blood was collected from female rats as part of a wider study on neurohormone release in relation to reproductive function. For this purpose, portal blood was collected during two consecutive periods of 30 min each (first and second sampling periods) at dioestrus, when the secretion of luteinizing hormonereleasing hormone (LHRH) and gonadotrophin is low, and during the afternoon of pro-oestrus when LHRH and gonadotrophin secretion is high (8). During the afternoon of pro-oestrus there is also a massive surge of prolactin (9). Since ir-ANF is present in pituitary gland, and may therefore contaminate portal blood samples, we collected hypophysial portal blood from longterm and acutely hypophysectomized as well as from 'pituitaryintact' animals. The measurement of ir-ANF in portal blood from long-term hypophysectomized rats served the additional purpose of determining whether pituitary factors could influence ir-ANF release into portal blood by way of a short-loop, negative feedback mechanism.Serial dilutions of extracts of both portal and peripheral plasma inhibited binding in parallel with that of synthetic rat ANF (rANF) (1-28) used as a standard (Fig. 1). On gel filtration, most of the ir-ANF in extracts of portal blood eluted as a single peak coincident with the elution peak of synthetic rANF (1-28) (Fig. 2) used in the present study. Anti-ANF antiserum was raised in rabbits by Dr K. Ogawa (10) against synthetic human ANF (hANF) (1-28) (Gift of Professor J. Funder, Melbourne, Victoria, Australia). The antiserum (R178) at final dilution of 1:52,000 yielded a specific binding of 38% with a sensitivity of 2 pg (6 x mol) per sample and ED,, of 62 pg (20 fmol).[lZ5I]rANF (1-28) was prepared by iodination of synthetic rANF (1-28) using the Iodogen method, followed by Sep-Pdk separation. The antigenic site resided between 5-27 amino-acid sequence with amino-acid 12 in the mid portion constituting part of the epitope. The antiserum cross-reacted on an equimolar basis with smaller conge...

Review: Central non-glucocorticoid inhibitors of the hypothalamo-pituitary-adrenal axis

“…However, concentrations of SP are very low in hypophysial portal blood of sheep (Clarke et al 1993), rat (Lim et al 1990) and monkey (Eckstein et al 1980), and SP in physiological doses had no effect on ACTH release from dispersed anterior pituitary cells in vitro (Chowdrey et al 1990). Therefore, in the absence of evidence that SP plays any significant physiological role at the level of the anterior pituitary, it is probable that it modulates HPA axis activity by inhibiting the expression of CRH within the PVN.…”

Atrial natriuretic peptide and feeding activity patterns in rats

“…However, the mice with the highest ANP levels were significantly hypotensive indicating that these pharmacologic renal effects of the peptide are less important than its hemodynamic actions (4). On the other hand, in rats or in humans, high ANP concentrations are hypotensive even when they are lower than natural concentrations in the pituitary portal system, and are able to inhibit ACTH secretion in vitro (25). Nevertheless, under appropriate conditions, the antagonism between the natriuretic peptide system and the renin-angiotensin system can promote natriuresis.…”

Alterations in Brain Atrial Natriuretic Polypeptide Levels in Hypophysectomized Rats

“…ANP can be released from the hypothala mus in vitro by potassium depolarization [10]. The presence of ANP immunoreactivity in the external layer of the median eminence [11][12][13] and its release into the hypophysial portal blood [6] suggest possible neuroendo crine effects of this peptide unrelated to its action on salt and fluid balance and blood pressure.…”