Oxytocin and vasopressin in rat hypophysial portal blood: experimental studies in normal and Brattleboro rats

Abstract

Oxytocin (OT) and vasopressin (VP) were measured by radioimmunoassay in hypophysial portal and peripheral blood from male Wistar rats and heterozygous and homozygous Brattleboro rats anaesthetized with urethane. In Wistar rats the concentrations of OT and VP were about 50 times greater than the concentrations in peripheral blood, whether or not the pituitary gland was left in situ during collection, and also considerably greater than the reported concentrations of the peptides in the cerebrospinal fluid. The release of both peptides was increased significantly by a lesion of the supraoptico-hypophysial tract that led to diabetes insipidus, but which left intact the external layer of the median eminence (ME). Concentrations of VP were undetectable in plasma from homozygous Brattleboro rats, but the portal plasma concentrations of VP in heterozygous Brattleboro rats were not significantly lower than in Wistar rats. The concentrations of OT in portal plasma from both types of Brattleboro rat were significantly higher than in Wistar rats. The output of VP and OT into hypophysial portal blood of Wistar rats was not significantly affected by electrical stimulation of the suprachiasmatic, supraoptic or paraventricular nuclei or the ME using two types of stimuli, one of which produced an increase in peripheral plasma concentrations of VP and OT in intact rats and a significant increase in the release of LH-releasing hormone into hypophysial portal blood. The output of VP and OT into portal blood was also not significantly affected by either adrenalectomy with or without injection of dexamethasone or the injection of either the 5-hydroxytryptamine (5-HT) synthesis blocker, parachlorophenylalanine, or the 5-HT uptake blockers, alaproclate or zimelidine. These results show that large amounts of OT as well as VP are released into hypophysial portal blood from fibres of the hypothalamo-neurohypophysial system that terminate in the external layer of the ME. Although distinct from the fibres that terminate in the pars nervosa (PN), the findings in Brattleboro rats show that the VP fibres of the ME system originate in neurones with a genomic mechanism for VP synthesis similar to that of the VP neurones that project to the PN. The lack of effect of adrenalectomy and the administration of 5-HT synthesis and uptake blockers must be interpreted with caution since the results obtained with electrical stimulation suggest that when the pituitary stalk is cut the release of OT and VP into portal blood approaches a maximum and may therefore be difficult to alter by experimental manipulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Oxytocin at physiological concentrations evokes adrenocorticotropin (ACTH) release from corticotrophs by increasing intracellular free calcium mobilized mainly from intracellular stores. Oxytocin displays synergistic or additive effects on ACTH-releasing factor or arginine vasopressin-induced ACTH secretion, respectively

“…Basal resting intracellular calcium levels were stable and Addition of OT (100 nM) increased the cytosolic free calcium concentrations to values ranging from 500-800 nM in 30 (of 32) identitied corticotrophs. Increased intracellular calcium was evident within 3 sec, then declined in [10][11][12][13][14][15] sec toward a plateau slightly higher than the calcium concentration observed underresting conditions (Fig. 6A).…”

“…In addition, CRF-, AVP-and OT-containing neurons in the hypothalamic paraventricular and supraoptic nuclei project to the median eminence and release these peptides into hypophyseal portal blood (8)(9)(10). Here, the concentration ofthese peptides are several times greater compared to peripheral blood (11)(12)(13)(14)(15). Thus, OT as apart of this hypophysiotropic cocktail in the adenohypophyseal circulation could contribute to physiological regulation of ACTH secretion by the hypothalamus.…”

Pituitary Portal Plasma Levels of Oxytocin during the Estrous Cycle, Lactation, and Hyperprolactinemia

“…Using the transpharengeal or parapharengeal approach, several groups of investigators have also obtained 20-to 50-fold higher concentrations of OT in pituitary portal plasma than in peripheral plasma in male and female rats. 22,40,[43][44][45] It is unlikely that a significant amount of OT in pituitary portal plasma is due to backflow from the posterior pituitary gland, since the level of pituitary portal OT did not change following removal of the pituitary gland (Table 1). It has been shown that a significant amount of OT is present in the cerebrospinal fluid (CSF) and in tanycytes lining the floor of the third ventricle.…”

“…Horn et al have recently presented evidence that OT from the cut axons of the supraoptico-hypophysial tract does not significantly contribute to the high neuropeptide level in pituitary portal plasma following sectioning of the pituitary stalk. 44 Thus, the OT concentration in pituitary portal blood reflects the secretory activity of OT neurons in the median eminence.…”

Blood vascular architecture of the rat cerebral hypophysis and hypothalamus. A dissection/scanning electron microscopy of vascular casts.

“…However, recent immunoassay studies of the rats have detected hypophyseal hormones, including MSH and oxytocin, in the blood in long portal vessels (OLIvER et al, 1977 ;GIBBS, 1984;HORN et al, 1985). These studies suggest that the blood in the long, short, intra-adenohypopyseal portal vessels and the intercommunicating capillaries can return in part to the median eminence, infundibular stalk and also infundibular process.…”