Susan E. Greer

Cell swelling induced by the permeant molecules urea or glycerol induces immediate high amplitude thyrotropin and prolactin secretion by perifused adenohypophyseal cells

The permeant molecules, urea and glycerol, evoked a prompt secretory burst of TSH and PRL when added to the extracellular medium of acutely dispersed anterior pituitary cells. Secretion of both hormones was proportional to the concentration of urea or glycerol between 26 and 104 mM (r greater than 0.89, P less than 0.001). Equivalent concentrations of the impermeant molecule, mannitol, did not induce secretion. The acute TSH and PRL secretory responses to TRH, hyposmolarity, and permeant molecules were qualitatively indistinguishable. These data support our hypothesis that cell swelling and resultant plasmalemma expansion is a potent inducer of hormone secretion. Since the secretory response to permeant molecules was not reduced in a Ca2+-free medium containing 0.1 mM EGTA, an increase in Ca2+ transport across the plasmalemma to raise cytosol Ca2+ concentration does not appear involved.

Evidence that the Major Physiological Role of TRH in the Hypothalamic Paraventricular Nuclei May Be to Regulate the Set-Point for Thyroid Hormone Negative Feedback on the Pituitary Thyrotroph

If a primary physiologic action of TRH is to regulate the set-point for negative feedback, a sudden drop in plasma thyroid hormone concentration should stimulate the same rate of in vivo increase in TSH secretion from normal and TRH-deprived thyrotrophs. To test this hypothesis, 3 experiments were performed in which young adult female rats were divided into 3 groups of 6-10 rats each: intact controls, hypothalamic paraventricular nuclei ablation (PVN) and sham-ablated (Sham). Sham and PVN rats were thyroparathyroidectomized 2-4 weeks after brain lesions and serial blood samples taken in all groups at frequent intervals from 0 to 58 days post-thyroidectomy. Plasma TSH was significantly higher than in intact controls by 3 days post-thyroidectomy in both the Sham and PVN groups (p < 0.05). At 14 days PVN plasma TSH was 4 x higher and at 30 days 8 x higher than in intact controls and remained consistently at 50% of that of the Sham group. There was no statistical difference between PVN and Sham in the rate of increase in TSH. Plasma T4 was 40% lower in PVN than in Sham at the time of thyroidectomy and became undetectable in both by day 3. The prompt parallel rate of rise of plasma TSH in Sham and PVN groups following thyroidectomy indicates that a primary physiologic action of TRH in the thyrotroph is to control the set-point for thyroid hormone negative feedback on TSH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

The permeant molecule urea stimulates prolactin secretion in GH4C1 cells by inducing Ca2+ influx through dihydropyridine-sensitive Ca2+ channels

Isotonic urea in medium with a normal 1.2 mM Ca2+ concentration induced a striking rise in both cytosolic Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion, each of whose peaks were proportional to the concentration of urea between 5 and 120 mM. There was a significant linear relationship between the peaks of induced [Ca2+]i and PRL secretion (r = 0.99, P less than 0.001). The increase in both [Ca2+]i and PRL secretion was completely abolished by removal of medium Ca2+ or by 2 microM nifedipine. Hypertonic urea was ineffective in inducing either an increase in [Ca2+]i or PRL secretion. These data support the hypothesis that plasma membrane expansion is a potent non-toxic inducer of hormone secretion and that in GH4C1 cells an increase in [Ca2+]i produced by enhanced influx of extracellular Ca2+ through dihydropyridine-sensitive Ca2+ channels plays an important role in this phenomenon.

Hyposmolar stimulation of secretion of thyrotropin, prolactin, and luteinizing hormone does not require extracellular calcium and is not inhibited by colchicine, cytochalasin B, ouabain, or tetrodotoxin.

Abstract Hyposmolar stimulation of thyroid-stimulating hormone, prolactin, and luteinizing hormone secretion by dispersed perifused rat pituitary cells was not depressed by removal of Ca2+ from the perifusion medium or by 0.1 mM colchicine, 20 μM cytochalasin B, 0.1 mM ouabain, or 3 μM tetrodotoxin. The secretory response induced by medium hyposmolarity or by thyrotropin-releasing hormone was not appreciably different at 23, 37, or 43°C, but was markedly reduced or abolished when the experiments were performed at 1°C. These data indicate that microtubules or microfilaments, transport of extracellular Ca2+ into the cytoplasm, and plasmalemma ion transport mechanisms sensitive to ouabain or tetrodotoxin are not essential components of the mechanism by which extracellular hyposmolarity induces secretion.

Evidence that Ca2+-activated K+ channels participate in the regulation of pituitary prolactin secretion

We evaluated the role of Ca(2+)-activated K+ channels in the regulation of prolactin (PRL) secretion with a perifusion system using acutely dispersed rat anterior pituitary cells. Apamin, which blocks Ca(2+)-activated K+ channels, induced PRL secretion in a dose-dependent fashion between 1 and 300 nM (r = 0.99, P < 0.01). Charybdotoxin, another Ca(2+)-activated K+ channel-blocker, also induced PRL secretion at 20 nM concentration. These were not non-specific toxic effects, since stimulation of PRL secretion by 10 nM thyrotropin-releasing hormone (TRH) was not different before and after applying the channel-blockers. Both 10 microM dopamine and 2 microM nifedipine significantly, but incompletely, depressed PRL secretion induced by 100 nM apamin; 10 microM dopamine completely blocked PRL secretion induced by 20 nM charybdotoxin. Our data indicate that Ca(2+)-activated K+ channels may play an important role in the regulation of PRL secretion.

Comparison of isoproterenol and dibutyryl adenosine cyclic 3',5'-monophosphate stimulation of thyroxine 5'-deiodinase activity in cultured pineal glands from euthyroid and hypothyroid rats

Thyroxine 5-deiodinase was increased by isoproterenol and dibutyryl adenosine cyclic 3,5-monophosphate in a dose- and time-related manner in cultured rat pineal gland. Basal and stimulated activity was higher in glands from hypothyroid than from euthyroid animals. Our data suggest direct beta-adrenergic stimulation of intracellular cyclic AMP may be involved in the regulation of pineal thyroxine 5-deiodinase activity.

Comparison of hyposmolar and hyperosmolar effects on in vitro luteinizing hormone secretion by anterior pituitary cells.

Abstract It has previously been described that perifusion of acutely dispersed adenohypophyseal cells with hypotonic medium causes an immediate high-amplitude “on” burst of luteinizing hormone (LH) secretion. In the present report the converse study with hyperosmolar solutions has been made. Perifusion with hypertonic medium depressed LH secretion; return to isotonicity caused an immediate high-amplitude “off” burst of LH secretion closely resembling that induced by hypotonic perifusion. The data give further support to the theory that exocytotic secretion may involve expansion of the outer cell membrane, thus drawing secretory granules to the cell surface where their contents are extruded.

The role of the superior cervical ganglia in the nocturnal rise of pineal type-II thyroxine 5′-deiodinase activity

Superior cervical ganglionectomy (SCGx) abolished the nocturnal rise in pineal type-II thyroxine 5-deiodinase (5-D) activity in both euthyroid and hypothyroid rats. Isoproterenol induced at least as great a rise in diurnal pineal 5-D in SCGx as in intact rats. These data suggest that beta-adrenergic stimulation through the superior cervical ganglia is essential for the nocturnal rise in pineal 5-D activity.

Cell volume induced hormone secretion: studies on signal transduction and specificity.

Cell swelling causes an immediate secretory response in various cell types. Induced secretion possesses some unique features suggesting the involvement of a specific signal transduction pathway. The effect of 10-20 µM GdCl3, 100 µM HgCl2, 1-100 µM indomethacin and 1-20 µM nordihydroguaiaretic acid (NDGA) on cell swelling-induced hormone secretion (isosmotic 80 mM ethanol or 15-30% hyposmotic medium) from incubated rat hypothalamic paraventricular nucleus (PVN) and posterior pituitary (oxytocin and TRH), isolated pancreatic islets (TRH) and perifused anterior pituitary cells (prolactin) were examined. To determine how general the effect of cell swelling is on exocytotic secretion, the release of two different neurohormones (thyrotropin releasing hormone -TRH and oxytocin) from the same tissue explant were studied. Both hyposmotic medium or isosmotic ethanol containing medium induced immediate TRH and prolactin release from the tested tissues.The effect of GdCl3, HgCl2, NDGA or indomethacin showed no inhibition of cell swelling induced secretion. In contrast to TRH, oxytocin release was not induced by isosmotic ethanol containing medium from the PVN or posterior pituitary. CONCLUSION: These data indicate that signal transduction leading to exocytosis after cell swelling does not involve GdCl3 sensitive stretch activated channels, mercury sensitive aquaporins, or indomethacin and NDGA sensitive mediators including prostaglandins and leukotriens. Cell swelling-induced exocytosis possesses limited selectivity; cells specifically involved in water and salt regulation retain their specific response to osmotic stimuli.

Isotonic but Not Hypertonic Ethanol Stimulates LHRH Secretion from Perifused Rat Median Eminence

We utilized luteinizing hormone-releasing hormone (LHRH) secretion by perifused minced rat hypothalamic median eminence (ME) tissue to evaluate whether isotonic ethanol would stimulate neurosecretion as it does secretion from pituitary cells. Isotonic ethanol induced a dose-dependent burst of LHRH secretion which was maximal at 2-3 min and returned to near baseline by 10 min. In Ca(2+)-depleted media (< 2 microM Ca2+), stimulation of secretion by isotonic ethanol was enhanced, but secretion induced by depolarizing 30 mM K+ was abolished. Hypertonic ethanol was ineffective in stimulating LHRH secretion in either normal or Ca(2+)-free media. The secretory response of hypothalamic LHRH-secreting cells to ethanol and its negative modulation by medium Ca2+ is thus identical to that of normal anterior pituitary cells and presumably is caused by cell swelling resulting from influx of permeant ethanol molecules across the plasmalemma.