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Biomedical subjects

M A Greer

Publications and source records attributed to M A Greer.

At least 19 recordsLinked to original sources
PubMed1992

Cell swelling induced by medium hyposmolarity or isosmolar urea stimulates gonadotropin-releasing hormone secretion from perifused rat median eminence.

Medium hyposmolarity between 10 and 50% and isotonic urea between 22.5 and 90 mM induced a dose-dependent burst of gonadotropin-releasing hormone (GnRH) secretion from perifused median eminence tissue which was maximal at 2-3 min and returned to near baseline by 5 min in spite of continued exposure to the stimulus. If Ca(2+)-free medium was used, osmotic stimulation of secretion was increased or unchanged, but secretion induced by 30 mM K+ was markedly reduced. Our data indicate that cell swelling induced by medium hyposmolarity or permeant molecules stimulates GnRH secretion from median eminence cells or cell processes as it does secretion from normal endocrine cells containing hormone stored in intracellular vesicles. In both, Ca2+ influx is not required or has a negative modulating influence on cell swelling-induced secretion.

Animals↗
PubMed1992

[Mechanism of suppressed prolactin secretion due to medium hyperosmolarity--the importance of Ca2+ influx].

It is still unknown how extracellular hyperosmolarity suppresses exocytosis. To evaluate the possibility that extracellular hyperosmolarity affects one of the most important second messenger system, Ca2+ signal, we evaluated the effect of hyperosmolarity on the thyrotropin releasing hormone (TRH)-induced changes in both intracellular Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion in GH4C1 cells. TRH caused two phases of [Ca2+]i: an initial high-amplitude phase (first phase), which was not inhibited by Ca2+ free medium, and a sustained low-amplitude phase (second phase), which was abolished by Ca2+ free medium. Medium hyperosmolarity (isotonic = 300mOsm, hypertonic = 338, 375, 450, and 600mOsm) suppressed both TRH-induced phases of [Ca2+]i in a dose dependent manner, however, the suppressive effect was clearly stronger in the second phase of [Ca2+]i than in the first phase of [Ca2+]i. Low doses of medium hyperosmolarity (338 and 375mOsm) suppressed PRL secretion, which was dependent on Ca2+ influx. However, high doses of medium hyperosmolarity (450 and 600mOsm) also blocked PRL secretion, which was dependent on Ca2+ mobilized from cytosolic Ca2+ pools. These data indicate that in GH4C1 cells medium hyperosmolarity may inhibit PRL secretion by both blocking Ca2+ influx and a mechanism unrelated to Ca2+.

Calcium↗
PubMed1992

An acute release of Ca2+ from sequestered intracellular pools is not the primary transduction mechanism causing the initial burst of PRL and TSH secretion induced by TRH in normal rat pituitary cells.

With 1.5 mM [Ca2+]e, 10 nM TRH induced a prompt high-amplitude burst of hormone secretion and an initial high-amplitude [Ca2+]i burst (first phase) followed by a sustained low-amplitude [Ca2+]i increment (second phase) in both tumor-derived GH4C1 and normal adenohypophyseal (AP) cells. With less than 2 microM [Ca2+]e, in both cell types the TRH-induced first phase rise in [Ca2+]i was suppressed 30% while the second phase rise was completely abolished; however, hormone secretion was inhibited only 20-30% in GH4C1 but greater than 80% in AP cells. Thapsigargin induced a first-phase rise in [Ca2+]i in AP cells equal to that induced by 10 nM TRH but only 20% as much first-phase hormone secretion. Blocking Ca2+ channels with nifedipine inhibited TRH-induced secretion in AP cells significantly more than in GH4C1 cells. Our data indicate that the TRH-induced first-phase spike in [Ca2+]i from intracellular Ca2+ stores may play a major transduction role in hormone secretion in GH4C1 cells but not in normal AP cells. Transduction mechanisms coupled to Ca2+ influx through Ca2+ channels in the plasmalemma are apparently a much more important component of TRH-induced secretion in normal than in tumor-derived pituitary cells.

Animals↗
PubMed1992

Protein kinase C modulates cell swelling-induced Ca2+ influx and prolactin secretion in GH4C1 cells.

In GH4C1 rat pituitary cells, cell swelling stimulates prolactin (PRL) secretion by increasing Ca2+ influx through nifedipine-sensitive Ca2+ channels; however, the mechanism by which cell swelling opens Ca2+ channels is still unclear. To evaluate the role of protein kinase C (PKC) in this phenomenon, we studied the effect of down-regulating PKC by 12-h pretreatment with phorbol ester or by treatment with H-7, a protein kinase C inhibitor. Cell swelling induced by either 27% medium hyposmolarity or 80 mM isotonic urea caused a prompt rise in both [Ca2+]i and PRL secretion in otherwise untreated control GH4C1 cells. Removal of medium Ca2+ enhanced the osmotically induced cell swelling but prevented the increase in [Ca2+]i and PRL secretion. Both PKC down-regulation and H-7 suppressed the cell swelling-induced increases in [Ca2+]i concentration and PRL secretion, although they enhanced the induced cell volume expansion. Our data indicate that in GH4C1 cells PKC plays an important positive modulating role in the osmotic opening of plasmalemma Ca2+ channels, a critical component of the early transduction chain by which cell swelling causes PRL secretion in tumor-derived clonal pituitary cells.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1992

Medium hyperosmolarity inhibits prolactin secretion induced by depolarizing K+ in GH4C1 cells by blocking Ca2+ influx.

Medium hyperosmolarity between 300 (normal medium osmolarity) and 600 mOsm inhibited in a concentration-correlated fashion (r greater than 0.97, p less than 0.001) the rise in intracellular Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion induced in GH4C1 cells by depolarizing 30 mM K+. [Ca2+]i concentration and PRL secretion were tightly related between 300 and 600 mOsm (r = 0.976, p less than 0.001); 50% inhibition of both occurred at 450 mOsm. Medium hyperosmolarity slowed the rate of Ca2+ influx. At 600 mOsm the rise in both [Ca2+]i and PRL secretion was abolished but PRL secretion induced by 1 microM phorbol 12-myristate 13-acetate was not significantly reduced. Our data suggest that inhibition of Ca2+ influx may be the primary mechanism by which extracellular hyperosmolarity inhibits PRL secretion induced by high medium K+ in GH4C1 cells. Depression of the Ca2+ intracellular transduction system may play a pathophysiological role in vivo in conditions such as dehydration and hypertonic coma.

Animals↗
PubMed1992

Alpha-adrenergic inhibition of thyrotropin-releasing hormone-induced prolactin secretion in GH4C1 cells is associated with a depressed rise in intracellular Ca2+.

alpha-Adrenergic receptors are present on the plasma membrane of normal anterior pituitary cells and alpha-adrenergic agonists may play a role in the secretion of corticotropin (ACTH) and thyrotropin (TSH). However, alpha-adrenergic involvement in prolactin (PRL) secretion is uncertain. We have therefore examined this question in the PRL-secreting clonal rat pituitary tumor-derived GH4C1 cells. Norepinephrine (NE), an alpha-adrenergic agonist, had no effect on basal PRL secretion but abolished thyrotropin-releasing hormone (TRH)-induced PRL secretion in a dose-dependent manner (EC50 100 nM). NE also significantly suppressed the TRH-stimulated rise in [Ca2+]i. Phentolamine (PA), a non-selective alpha-adrenergic antagonist, reversed the inhibitory effect of NE on both the TRH-stimulated PRL secretion and [Ca2+]i rise. NE did not inhibit the rise in PRL secretion or [Ca2+]i induced by depolarizing 30 mM K+, 30% hyposmolarity or BAY K-8644, a specific L-type Ca2+ channel agonist. The inhibitory effect of NE on TRH-induced PRL and [Ca2+]i changes was also present when Ca2+ influx was prevented by removing medium Ca2+ or by blocking L-type Ca2+ channels with 2 microM nifedipine. The TRH-stimulated first-phase rise in [Ca2+]i in GH4C1 cells is believed to result primarily from release of sequestered Ca2+ from an intracellular pool through the activation of inositol 1,4,5-trisphosphate (IP3) and this [Ca2+]i spike stimulates PRL secretion. Our data thus suggest that GH4C1 cells have alpha-adrenergic receptors and that alpha-adrenergic agonists either suppress IP3 generation or block IP3 release of sequestered intracellular Ca2+.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1992

Lidocaine inhibits prolactin secretion in GH4C1 cells by blocking calcium influx.

The mechanism of the inhibitory effect of local anesthetics on hormone secretion was studied in the GH4C1 line of rat pituitary tumor-derived cells. Lidocaine between 0.1 and 5 mM exerted significant dose-dependent inhibition on the increment in cytosol Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion induced by 30 mM K+. For both effects the IC50 was 0.25 mM and maximal inhibition occurred at 5 mM. A normal response returned within 20 min after removal of lidocaine from the incubation medium. 1 microM tetrodotoxin had no effect on the 30 mM K+ induced [Ca2+]i transient or PRL secretion, indicating that Na+ channels are not involved in the inhibitory effect of lidocaine. Lidocaine similarly inhibited the [Ca2+]i increment and PRL secretion induced by 30% medium hyposmolarity and 1 microM Bay K 8644. Lidocaine was much less effective in inhibiting secretion induced by 1 microM phorbol 12-myristate 13-acetate (TPA) or 5 microM forskolin. 5 mM procaine produced effects similar to those of lidocaine. Our data suggest that in GH4C1 cells local anesthetics depress secretagogue-induced PRL secretion primarily by blocking Ca2+ influx, probably through L-type Ca2+ channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1991

Evidence that potassium channels regulate prolactin secretion in GH4C1 cells by causing extracellular calcium influx.

Tetraethylammonium (TEA), a K+ channel blocker, induced prolactin (PRL) secretion in GH4C1 cells in a dose-dependent manner when applied at a concentration from 1-20 mM. During continuous exposure to TEA, a significant increase in PRL secretion occurred by 20 min and the response was sustained until the end of a 60-min exposure. Blocking Ca2+ influx by employing a Ca(2+)-depleted medium or the Ca2+ channel blocker, nifedipine, prevented induction of PRL secretion by 20 mM TEA. Preincubation of the cells for 10 min with 20 mM TEA did not inhibit PRL secretion induced by thyrotropin-releasing hormone (TRH), phorbol 12-myristate 13-acetate (TPA) or by cell swelling produced by 30% medium hyposmolarity, but significantly depressed that induced by depolarizing 30 mM K+. BaCl2, another K+ channel blocker, had the same effect on PRL secretion as TEA. The data suggest that blocking K+ channels may cause membrane depolarization, thereby inducing Ca2+ influx which is a potent stimulus for PRL secretion in GH4C1 cells.

Calcium↗
PubMed1991

Dual effect of osmotic cell swelling on prolactin secretion by acutely dispersed adenohypophyseal cells.

Cell swelling induced by acute exposure to the permeant molecule urea or by medium hyposmolarity evoked a prompt PRL secretory burst from dispersed rat anterior pituitary cells. However, during continuous exposure greater than or equal to 10 min to these conditions inhibition of basal and TRH-induced PRL secretion occurred and there was an "off" burst of PRL secretion following return to basal conditions. Compared with continuous TRH stimulation which causes biphasic PRL secretion with a rapid high amplitude first phase secretory burst followed by a sustained low level second phase of secretion, cell swelling induced only "first phase" secretion. Removing Ca2+ from the medium or adding 50 microM verapamil markedly depressed the "off" secretory burst following return to basal conditions but had no effect on the initial high amplitude burst. Our data suggest that the effect of cell swelling on PRL secretion is complex and that there are at least two mechanisms for PRL secretion in normal anterior pituitary cells; these are differently affected by cell swelling and Ca2+ influx.

Animals↗
PubMed1991

Dopamine inhibits cell swelling-induced prolactin secretion in MMQ cells by blocking Ca2+ influx.

To evaluate the role of Ca2+ influx on hormone secretion induced by cell swelling, we have utilized a prolactin (PRL)-secreting rat tumor cell line, MMQ, which has plasmalemma dopamine receptors. Medium hyposmolarity or osmotically equivalent isotonic urea caused prompt cell swelling and a rise in both [Ca2+]i and PRL secretion in a dose-dependent manner. Dopamine inhibited the induced increase in both [Ca2+]i and PRL secretion in a dose-dependent manner but the maximum inhibition was only 50%. This effect of dopamine was prevented by haloperidol. Depletion of medium Ca2+ or blocking Ca2+ influx with nifedipine completely abolished the osmotically induced rise in both [Ca2+]i and PRL secretion. These data indicate that Ca2+ influx through nifedipine-sensitive Ca2+ channels is an essential component of PRL secretion induced by osmotic cell swelling in MMQ cells and that a dopaminergic receptor-linked mechanism influences the opening of these channels.

Animals↗
PubMed1991

Medium hyperosmolarity depresses thyrotropin-releasing hormone-induced Ca2+ influx and prolactin secretion in GH4C1 cells.

We studied the influence of graded degrees of hyperosmolarity on the dynamics of the thyrotropin-releasing hormone (TRH)-induced rise in cytosol Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion in GH4C1 cells. TRH caused two phases of increase in [Ca2+]i that were differentially altered by hyperosmolarity: 100% hyperosmolarity (600 mOsm) depressed only 20% of an initial high-amplitude [Ca2+]i burst (first phase) dependent on Ca2+ mobilized from intracellular pools, but it abolished a sustained low-amplitude second phase dependent on extracellular Ca2+ influx. Low degrees of hyperosmolarity suppressed PRL secretion due to Ca2+ influx while high degrees suppressed secretion due to mobilized Ca2+. These data suggest that in GH4C1 cells hypertonic inhibition of secretion may result from both blocking Ca2+ influx and mechanisms unrelated to [Ca2+]i.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1991

Hormone secretion stimulated by ethanol-induced cell swelling in normal rat adenohypophysial cells.

Ethanol has been reported to affect endocrine functions, but its mechanism of action is unclear. To evaluate the hypothesis that cell swelling induced by ethanol permeation through the plasmalemma triggers hormone secretion, we studied the effect of ethanol on both hormone secretion and cell volume in acutely dispersed rat adenohypophysial cells under isotonic and hypertonic conditions. Isotonic ethanol caused a prompt cell swelling and an explosive secretory burst of prolactin and thyrotropin, which were proportional to the concentration of ethanol between 10 and 120 mM. The lowest effective dose of isotonic ethanol was 10 mM, which is below the plasma levels of legal intoxication (16 mM). Removal of medium Ca2+ enhanced the isotonic ethanol-induced increases in both cell volume and secretion. Hypertonic ethanol was ineffective in these effects. These data indicate that, in normal rat adenohypophysial cells, cell swelling caused by the rapid passage of ethanol through the plasmalemma is a potent mechanism for stimulating hormone secretion and this induced secretion is negatively modulated by extracellular Ca2+.

Animals↗
PubMed1991

The contrasting role of calcium influx in secretion induced by cell swelling can differentiate normal and tumor-derived rat pituitary cells.

We have evaluated whether cell swelling may be a generally useful technique to differentiate normal and neoplastic pituitary cells, making the comparison between normal lactotrophs and thyrotrophs and tumor-derived GH4C1 and MMQ cells. With 1.5 mM medium Ca2+, cell swelling induced by osmotically equivalent stimuli, 27% medium hyposmolarity or 80 mM isotonic urea, caused a prompt increase in both intracellular Ca2+ and hormone secretion by all cell types. Depletion of medium Ca2+ abolished the cell swelling-induced increase in intracellular Ca2+ in all cell types and hormone secretion in the tumor-derived cells. However, it enhanced hormone secretion in normal cells. The critical role of Ca2+ influx in osmotically induced secretion in neoplastic, but not normal, pituitary cells may reflect some fundamental alteration in the intracellular transduction system in tumor cells.

Animals↗
PubMed1991

Significant qualitative differences exist between thyrotropin and prolactin secretory dynamics induced by pituitary cell swelling.

Cell swelling produced by a variety of techniques is a potent stimulus intensity-related inducer of an immediate secretory burst of thyroid-stimulating hormone (TSH) and prolaction (PRL) secretion from anterior pituitary cells. A 2-min "square wave" exposure to either hyposmolarity or isotonic urea induced stimulus intensity-correlated TSH and PRL secretory bursts peaking within 3 min, but the PRL zenith occurred 1 min later than that of TSH. With continuous exposure to these stimuli, TSH secretion rapidly decreased and remained only slightly above the unstimulated rate after 5 min. PRL secretion fell to and remained below the unstimulated level after 10 min. After stopping the stimulus, another secretory burst ("off" response) occurred with PRL, but not with TSH. A progressive "ramp" increase in stimulus intensity over 18 min induced a corresponding gradual increase in TSH secretion; there was a progressive depression, rather than increase, in PRL secretion during the stimulus ramp, with an off response secretory burst when the stimulus was discontinued. Removal of extracellular Ca2+ or addition of verapamil to the medium did not alter the dynamics of hyposmolarity-induced TSH secretion, but markedly altered those of PRL secretion; there was no off response PRL secretion and a hyposmolar ramp induced a corresponding gradual increase in PRL secretion, with a return to baseline after removing the stimulus. The dramatic qualitative differences in the response of the thyrotroph and lactotroph may reflect differences between the cell types in the size of secretory vesicles, membrane potential, the mechanism of exocytosis, and/or the role of Ca2+ influx across the plasmalemma.

Animals↗
PubMed1991

Quinidine inhibits prolactin secretion induced by thyrotropin-releasing hormone, high medium potassium or hyposmolarity in GH4C1 cells.

In cultured GH4C1 cells quinidine inhibited basal prolactin (PRL) secretion and that induced by 0.1 to 10 nM thyrotropin-releasing hormone (TRH), 30 mM medium K+ or 30% medium hyposmolarity but did not inhibit secretion induced by 100 nM 12-O-tetradecanoylphorbol 13-acetate. Inhibition of basal PRL secretion was highly correlated with the drug concentration between 30 microM to 1 mM quinidine; 50% inhibition of basal secretion occurred at 300 microM and at this concentration quinidine completely blocked PRL secretion stimulated by TRH, K+ and hyposmolarity. Significant inhibition of TRH-induced PRL secretion was produced by 15 microM quinidine, a concentration equivalent to that in plasma during standard antiarrhythmic therapy with quinidine in humans. In rats in vivo, a single injection of 2 mg i.p. of quinidine gluconate/100 g b.wt. 1 hr before TRH injection significantly inhibited induced TSH secretion by 15%. Quinidine inhibition of secretion may be caused by blocking depolarization of the cell membrane, thus depressing voltage-gated Ca++ channels and preventing a rise in intracellular Ca++ release.

Anti-Arrhythmia Agents↗
PubMed1990

Lidocaine and procaine inhibit the increase in cytosol Ca2+ induced by thyrotropin-releasing hormone or K+ depolarization in GH4C1 cells.

Lidocaine at greater than or equal to 1 mM and procaine at greater than or equal to 2.5 mM exerted dose-dependent inhibition of the increment in [Ca2+]i induced by 100 nM thyrotropin-releasing hormone (TRH) or 30 mM K+ in GH4C1 cells. The rise in [Ca2+]i induced by K+ was more sensitive to this inhibition than that induced by TRH. Lidocaine was more potent than procaine in inhibiting the [Ca2+]i increment induced by secretagogues. Maximal lidocaine inhibition of the TRH-induced [Ca2+]i increment occurred within 15-20 min and a normal response to secretagogues returned within 20 min after removal of lidocaine from the incubation medium. Our data suggest that in GH4C1 cells local anesthetics depress secretagogue-induced intracellular Ca2+ mobilization, depolarization of the cell membrane, and the opening of voltage-dependent Ca2+ channels. This may explain the depression of secretagogue-stimulated hormone secretion induced by these agents.

Animals↗