Biomedical subjects

M A Greer

Publications and source records attributed to M A Greer.

At least 55 records · Page 3Linked to original sources

PubMed1990

Hyposmolarity stimulates myeloperoxidase exocytosis from human polymorphonuclear leukocytes.

Medium hyposmolarity induced in human polymorphonuclear leukocytes treated with cytochalasin B a rapid exocytosis of the lysosomal enzyme, myeloperoxidase (MPO), which was linearly proportional to the degree of hyposmolarity between a 5 and 30% decrease (r = 0.92, p less than 0.001). Cell viability was unaffected by the hyposmolarity. The kinetics of MPO exocytosis induced by opsonized zymosan (OZ) and hyposmolarity were indistinguishable; the combination of hyposmolarity and OZ was additive. Since hyposmolarity similarly stimulates a burst of hormone secretion by perifused adenohypophyseal and pancreatic islet cells, the authors suggest that hyposmolarity-induced exocytosis is a general cellular phenomenon.

Cell Survival↗

PubMed1990

Medium hyposmolarity stimulates prolactin secretion in GH4C1 cells by inducing an increase in cytosolic free calcium.

Extracellular hyposmolarity is a potent direct stimulus for hormone secretion for which a mechanism has not been delineated. The importance of plasmalemma Ca2+ permeability in this phenomenon in pituitary tumor-derived GH4C1 cells was evaluated by comparing the dynamics of changes in cytosolic free Ca2+ concentration [( Ca2+]i) with those of PRL secretion. At a normal physiological concentration of extracellular Ca2+ (1.5 mM), hyposmolarity induced a striking rise in both [Ca2+]i and PRL secretion, which was proportional to the stimulus between 0.50% reduction in medium osmolarity. Thirty percent hyposmolarity induced a 3-fold rise in [Ca2+]i and a 5-fold rise in PRL secretion above the basal level. These effects did not occur in cells incubated in a medium with a Ca2+ concentration lower than 30 microM. In cells incubated in 1.5 mM Ca2+, the Ca2(+)-channel antagonists, nifedipine and verapamil, significantly inhibited hyposmolar-induced increases in [Ca2+]i and PRL secretion. These data suggest that in GH4C1 cells medium hyposmolarity causes a burst of PRL secretion that depends on a similar preceding rise in [Ca2+]i produced by extracellular Ca2+ influx, most of which passes through dihydropyridine-sensitive Ca2(+)-channels.

Animals↗

PubMed1990

Evidence that ethanol induces prolactin secretion in GH4C1 cells by producing cell swelling with resultant calcium influx.

Although acute exposure to ethanol has been reported to affect hormone secretion, the data are sometimes conflicting, and the mechanism of action of ethanol is unclear. We have examined in GH4C1 cells the effect of isotonic ethanol on cell volume measured with a Coulter counter, the cytosolic Ca2+ concentration ([Ca2+]i) monitored with fura-2, and PRL secretion analyzed in a perifusion system. Isotonic ethanol caused prompt cell swelling and an explosive rise in both [Ca2+]i and PRL secretion proportional to the concentration of ethanol between 5-120 mM. The increases in both [Ca2+]i and PRL secretion induced by 80 mM isotonic ethanol were essentially abolished by removal of medium Ca2+ or by nifedipine; the nifedipine IC50 was approximately 20 nM. Cell swelling induced by hyposmolarity or isotonic urea similarly increased both [Ca2+]i and PRL secretion. Hypertonic ethanol did not cause cell swelling and was ineffective in inducing an increase in either [Ca2+]i or PRL secretion. These data suggest that in GH4C1 cells a major mechanism by which ethanol stimulates PRL secretion is to induce cell swelling, thus producing enhanced Ca2+ influx through dihydropyridine-sensitive Ca2+ channels.

Animals↗

PubMed1990

Role of extracellular calcium and calmodulin in prolactin secretion induced by hyposmolarity, thyrotropin-releasing hormone, and high K+ in GH4C1 cells.

The mechanism by which 30% medium hyposmolarity induces PRL secretion by GH4C1 cells was compared with that induced by 100 nmol/l TRH or 30 mmol/l K+. Removing medium Ca2+, blocking Ca2+ channels with 50 mumol/l verapamil, or inhibiting calmodulin activation with 20 mumol/l trifluoperazine, 10 mumol/l chlorpromazine or 10 mumol/l pimozide almost completely blocked hyposmolarity-induced secretion. The smooth muscle relaxant, W-7, which is believed relatively specific in inhibiting the Ca2(+)-calmodulin interaction, depressed hyposmolarity-induced PRL secretion in a dose-dependent manner (r = -0.991, p less than 0.01). The above drugs also blocked or decreased high K(+)-induced secretion, but had much less effect on TRH-induced secretion. Secretion induced by TRH, hyposmolarity, or high K+ was optimal at pH 7.3-7.65 and was significantly depressed at pH 6.0 or 8.0, indicating that release of hormone induced by all 3 stimuli is due to an active cell process requiring a physiologic extracellular pH and is not produced by nonspecific cell toxicity. The data suggest hyposmolarity and high K+ may share some similarities in their mechanism of stimulating secretion, which is different from that of TRH.

Animals↗

PubMed1990

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.

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 microM cytochalasin B, 0.1 mM ouabain, or 3 microM tetrodotoxin. The secretory response induced by medium hyposmolarity or by thyrotropin-releasing hormone was not appreciably different at 23, 37, or 43 degrees C, but was markedly reduced or abolished when the experiments were performed at 1 degree 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.

Animals↗

PubMed1989

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.

Animals↗

PubMed1989

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.

Animals↗

PubMed1989

Comparison of the nocturnal temporal profiles of N-acetyl-transferase and thyroxine 5'-deiodinase in rat pineal.

The nocturnal temporal patterns of pineal N-acetyltransferase (NAT) and type II thyroxine 5'-deiodinase (5'-D) were compared in the same animals. Both NAT and 5'-D had a similar rise to a midnight zenith with the same lag period but 5'-D rose to its peak 1 h before NAT. The temporal profiles of decrease in enzyme activities were quite different, with 5'-D declining long before NAT. Acute light exposure caused a rapid dramatic decrease in NAT but not in 5'-D. These data suggest that different mechanisms are in involved in the decrease of these two enzyme activities. The role of the antecedent rise in 5'-D activity in the nocturnal rise of NAT activity is apparently minor at most, since pretreatment with iopanoic acid completely blocked the nocturnal rise in 5'-D but had no effect on the nocturnal rise in NAT nor on the acute fall in NAT from its midnight zenith after exposure to light.

Acetylserotonin O-Methyltransferase↗

PubMed1989

Ontogenesis of pineal thyroxine 5'-deiodinase activity and plasma melatonin concentration in the rat.

Rat pineal thyroxine 5'-deiodinase (5'-D) activity was detectable by 5 days after birth. Nyctohemeral differences became significant by 10 days of age and gradually reached adult magnitude at 1-2 months, primarily due to a progressive increase in nocturnal enzyme activity. A nyctohemeral difference in N-acetyltransferase (NAT) activity was observed by 5 days postnatally and the maturation of NAT rhythmicity was characterized by both a decrease in diurnal and an increase in nocturnal enzyme activity. The maturation of 5'-D rhythmicity was slower than that of NAT. Isoproterenol stimulated 5'-D activity in 5-day-old rats in which there was no spontaneous nyctohemeral 5'-D rhythm, suggesting that the appearance of rhythmicity in pineal 5'-D may depend on the development of pineal innervation. Melatonin was detectable in plasma by 5 days postnatally and reached adult levels at 21 days. A nyctohemeral difference in plasma melatonin concentration existed from day 5 with the highest value at midnight. The difference between noon and midnight increased approximately 10-fold to the adult amplitude by 15 days of age. There was a transient rise of both diurnal and nocturnal plasma melatonin concentration which peaked at 10 days before dropping severalfold to the adult level for both noon and midnight values by 21 days. Since the rhythms of pineal NAT activity and of plasma melatonin concentration are established before that of pineal 5'-D, it is unlikely that the rhythm of melatonin secretion is initiated by a prior maturation of the pineal 5'-D rhythm.

Acetyltransferases↗

PubMed1989

Lidocaine inhibits dispersed anterior pituitary cell thyrotropin and prolactin secretion induced by thyrotropin-releasing hormone or high medium potassium.

Lidocaine at a concentration greater than or equal to 0.1 mM inhibited thyrotropin (TSH) and prolactin (PRL) secretion by perifused acutely dispersed rat adenohypophyseal cells stimulated by 10-100 microM thyrotropin-releasing hormone (TRH) or 30 mM K+. The concentration of lidocaine required for half-maximal inhibition of TSH and PRL secretion was approximately 1 and 0.5 mM, respectively. Maximal lidocaine inhibition of TRH-induced secretion was induced within 15 min and a normal response to these secretagogues returned within 20 min after removal of lidocaine from the perifusion medium. The inhibition of secretion by lidocaine may be caused by blocking depolarization of the cell membrane and depressing intracellular calcium mobilization and calcium influx across the plasma membrane.

Animals↗

PubMed1989

Evidence that rat pineal thyroxine 5'-deiodinase is primarily stimulated by beta- and not alpha-adrenergic agonists and that its adrenergic-stimulated and spontaneous rhythmic nocturnal rise require RNA and protein synthesis.

Pineal thyroxine 5'-deiodinase (5'-D) activity rose greater than 10-fold above the basal level 2-3 hr after 1 mg/kg isoproterenol and returned to near the basal level by 6 hr. The same dose of norepinephrine or phenylephrine was without effect, but phenylephrine modestly potentiated isoproterenol-stimulated 5'-D activity. Either actinomycin D or cycloheximide treatment markedly decreased diurnal isoproterenol stimulation and the spontaneous rhythmic nocturnal rise of pineal 5'-D. The data indicate that pineal 5'-D activity is very similar to pineal serotonin N-acetyl transferase in being primarily stimulated by beta-adrenergic agonists and requiring new RNA and protein synthesis for its activation.

Adrenergic alpha-Agonists↗

PubMed1988

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.

Animals↗

PubMed1988

Effect of short-term constant light or constant darkness on the nyctohemeral rhythm of type-II iodothyronine 5'-deiodinase activity in rat anterior pituitary and pineal.

Type-II iodothyronine 5'-deiodinase activity (5'-D) in both anterior pituitary and pineal was significantly elevated at 2400 h, approximately 0.5- and 20-fold higher than the noon value, respectively. The nocturnal rise in both organs was abolished by 6 h additional light. Short-term constant darkness did not alter 5'-D rhythmicity in either organ. These data suggest that environmental lighting plays an important role in the control of the 5'-D nyctohemeral rhythm in both anterior pituitary and pineal.

Animals↗

PubMed1988

There is a nyctohemeral rhythm of type II iodothyronine 5'-deiodinase activity in rat anterior pituitary.

We found a nyctohemeral rhythm of type II iodothyronine 5'-deiodinase (5'-D-II) with a zenith at midnight in rat anterior pituitary, but not in brown adipose tissue. There was no nyctohemeral rhythm of 5'-D-I in anterior pituitary, liver, or kidney. Hypothyroidism abolished the nyctohemeral rhythmicity in anterior pituitary 5'-D-II. The rhythmicity of anterior pituitary 5'-D-II may play a role in setting the nyctohemeral rhythm of TSH secretion by regulating the degree of negative feedback by locally generated T3 in the thyrotroph.

Adipose Tissue, Brown↗

PubMed1988

Anterior pituitary type II thyroxine 5'-deiodinase activity is not affected by lesions of the hypothalamic paraventricular nucleus which profoundly depress pituitary thyrotropin secretion.

Bilateral destruction of the hypothalamic paraventricular nuclei (PVN) produced a profound depression of plasma TSH and the median eminence TRH concentration in hypothyroid rats. Anterior pituitary type II iodothyronine 5'-deiodinase (5'-D) activity was consistently lower but not significantly different in sham- and PVN-lesioned rats. Treatment with suboptimal replacement doses of 0.15 and 0.75 micrograms T4/100 g BW.day produced a graded depression of plasma TSH in the PVN (P less than 0.02), but not in the sham (P greater than 0.8) groups. Adenohypophyseal 5'-D was depressed in both sham and PVN groups by the highest T4 dose. Plasma T4 was much lower in PVN than in sham rats given comparable doses of T4 (P less than 0.001), but plasma T3 was not significantly different. This suggests that an increase in peripheral T4 metabolism was produced by PVN lesions. Our data indicate that changes in adenohypophyseal 5'-D activity are not responsible for the decrease in plasma TSH in PVN-lesioned rats and that neither the PVN nor endogenous TRH plays a significant role in the regulation of anterior pituitary 5'-D activity.

Animals↗

PubMed1988

Evidence that thyroxine inhibits either basal or TRH-induced TSH secretion only after conversion to triiodothyronine.

When TRH was administered every 15 min for 2 hr in euthyroid rats, equivalent modestly supraphysiologic doses of either T4 or T3 suppressed TRH-induced TSH secretion after 45 min. Pretreatment with iopanoic acid blocked the ability of T4 but not of T3 to suppress TRH-induced TSH secretion 2 hr after administration of the respective thyroid hormone. Pretreatment with iopanoic acid also blocked the ability of T4, but not of T3, to depress the elevated basal plasma TSH concentration of hypothyroid rats within 2 hr. Propylthiouracil did not significantly inhibit the ability of T4 to depress TRH-induced TSH secretion and only slightly depressed the ability of T4 to reduce the elevated plasma TSH of hypothyroid rats. Our data support the concept that although equivalent physiologic doses of T4 or T3 inhibit basal or TRH-induced TSH secretion equally rapidly, TSH inhibition produced by T4 is probably dependent on its rapid conversion to T3, either within the pituitary or peripherally. T3 thus seems to be exerting almost all the negative feedback effects on TSH secretion under the conditions of our experiments.

Animals↗

PubMed1987

Thyroxine 5'-deiodinase and thyroid hormone metabolism in intermediate and neural lobes of rat posterior pituitary.

We have found thyroxine 5'-deiodinase in rat posterior pituitary. The isozyme pattern was different in intermediate and neural lobes. The former contained only type I enzyme. In the neural lobe, in euthyroidism only type I was found while in hypothyroidism type II was a prominent form. Since type II 5'-deiodinase has a high affinity for thyroxine and is a more tissue-specific isozyme than type I, our results suggest there may be a previously unrecognized role of thyroid hormone in posterior pituitary physiology.

Animals↗

PubMed1987

Rhythmicity of triiodothyronine generation by type II thyroxine 5'-deiodinase in rat pineal is mediated by a beta-adrenergic mechanism.

Type II T4 5'-deiodinase plays an essential role in converting T4 to T3 in extrathyroidal tissues, thus allowing the full development of a cellular effect of thyroid hormone. This enzyme is one of the most important factors in regulating local action of thyroid hormone. Its activity in the rat pineal was 20-30 times higher at midnight than at noon; this nocturnal rise was abolished by the beta-adrenergic blocker, propranolol. The beta-agonist isoproterenol, caused a 3-fold increase in diurnal enzyme activity. There is thus a marked nyctohemeral variation in T4 activation in the pineal, probably mediated by a beta-adrenergic mechanism. Since the temporal peak of pineal T4 activation corresponds to that of pineal function, our data suggest a previously unrecognized role of thyroid hormone in pineal regulation.

Animals↗