Rat Hypophysial Portal Blood Research Articles

The development of the releasing factor concept.

After four decades of intense and competitive research, three hypothalamic releasing hormones (formerly factors) have recently been isolated and characterized. These are the decapeptide gonadotrophin releasing hormone (GnRH), tripeptide thyrotrophin releasing hormone (TRH), and the tetradecapeptide somatostatin. Some aspects of these hormones are discussed, and GnRH is considered in greater detail to demonstrate the difficulties involved in fulfilling completely the criteria which determine whether a substance can be accepted as a physiological releasing hormone. A substance immunologically similar to GnRH is present in rat hypophysial portal vessel blood, but, while the amount of this substance released into the portal circulation can be increased significantly by electrical stimulation of the preoptic area, no significant changes occur during the oestrous cycle or after long-term castration. This may be due to interference with the normal secretion of GnRH by the trauma and anaesthesia which necessarily accompany exposure of the pituitary stalk. However, the possibility exists that our preconceived notions regarding the changes in plasma levels of releasing hormones under physiological conditions may be incorrect. Thus it seems likely that changes in the rate of secretion of thyrotrophin is effected by throid hormones modulating the responsiveness of the thyrotrophs to a steady input of TRH. Evidence is presented for the existence of a similar mechanism for gonadotrophin secretion, and the role of steroid hormones and the priming effect of GnRH in modulating the responsiveness of the gonadotrophs is considered. The intrinsic connexions of the hypothalamus, the role of the hypothalamic aminergic systems and the autonomy of the hypothalamus with respect to anterior pituitary control present many problems which will prove difficult to solve.

Absence of measurable amounts of epinephrine, norepinephrine and dopamine in rat hypophysial portal blood during various phases of the oestrous cycle.

Fluorescence techniques were used for the estimation of epinephrine, dopamine and norepinephrine in hypophysial portal vein blood of rats. The techniques employed allow the detection of norepinephrine and dopamine in blood collected from one adrenal vein in a single rat over a 2-hour period, and they also allow the detection of norepinephrine in systemic blood pooled from several rats.

Luteinizing hormone releasing factor in rat hypophysial portal blood collected during electrical stimulation of the hypothalamus

1. Ovulation was induced in Nembutal-blocked pro-oestrous rats by electrical stimulation of the hypothalamus.2. The same type of electrical stimulation was applied during the collection of hypophysial portal blood.3. Pooled hypophysial portal plasma from donors in pro-oestrus, oestrus and met-oestrus was assayed for ovarian ascorbic acid depleting (OAAD) activity.4. Electrical stimulation of the hypothalamus increased the OAAD activity, believed to be due to luteinizing hormone releasing factor (LRF), in pro-oestrus and met-oestrus, but not in oestrus.5. It is concluded that the hypothalamic nerve fibres responsible for releasing LRF into the hypophysial portal vessels are depleted of their store of this releasing factor, or are refractory to electrical stimulation, during oestrus.