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R M MacLeod

Publications and source records attributed to R M MacLeod.

At least 55 records · Page 3Linked to original sources
PubMed1987

Cholinergic stimulation of inositol phosphate production in cultured anterior pituitary cells.

The effects of acetylcholine and of the muscarinic receptor agonist carbachol on inositol phosphate production were studied in cultured rat anterior pituitary cells. In the presence of the cholinesterase inhibitor physostigmine, acetylcholine significantly (p less than 0.05-p less than 0.01) stimulated inositol phosphate formation in a concentration-related fashion: carbachol, but not oxotremorine, produced similar effects. The increase in the amount of inositol phosphates (primarily inositol trisphosphate and inositol bisphosphate) was very rapid, an effect potently antagonized by the muscarinic receptor antagonist atropine. This agent significantly attenuated the stimulatory effect of carbachol on growth hormone (GH) release. These results indicate that the effects exerted by acetylcholine on anterior pituitary function (i.e. GH release) may be mediated, at least in part, by receptor-activated polyphosphoinositide hydrolysis. In addition, acetylcholine and carbachol's relation with other intracellular pathways and with hormone release is discussed.

Acetylcholine↗
PubMed1987

Thymosin fraction 5 stimulates prolactin and growth hormone release from anterior pituitary cells in vitro.

Thymosin fraction 5 (TF5) is a partially purified extract of bovine thymus containing 40-60 peptides. In addition to its well documented immunopotentiating effects, TF5 reportedly modulates the secretion of some hypothalamic peptides and pituitary hormones. In this study, TF5 (10-100 micrograms/ml) stimulated PRL release from normal, MtTW15, and 7315a cells and GH release from normal and MtTW15 cells, but had no apparent effect on LH release. No changes in intracellular cAMP or cGMP levels could be correlated with these responses. Stimulation of PRL release from perifused normal anterior pituitary cells was rapid, sustained, and concentration related. Although it had no apparent effect on normal prelabeled anterior pituitary cells with respect to 45Ca2+ efflux, the calcium channel blocker D-600 inhibited TF5-mediated hormone release from these cells. Additive increases in TRH-stimulated PRL release and GRF-stimulated GH release by TF5 suggested independent mechanisms of action. Dopamine (500 nM) blocked TF5-stimulated PRL release, but somatostatin (10-100 nM) had no effect on TF5-stimulated PRL or GH release. TF5 failed to affect either basal or TRH-induced polyphosphoinositide hydrolysis. Perifused normal anterior pituitary cells prelabeled with [3H]arachidonate responded to TF5 treatment with a liberation of radioactive arachidonate and/or its metabolites. BW755c, an inhibitor of all known catabolic pathways of arachidonic acid, blocked the ability of TF5 to stimulate PRL and GH release. Reversed phase HPLC separation of TF5 into five fractions resulted in two fractions that exhibited hormone-releasing activity. These data suggest that TF5 stimulates pituitary hormone release through a mechanism different from that ascribed to TRH or GRF. The stimulus-secretion coupling mechanism involves neither polyphosphoinositide hydrolysis nor cAMP generation, but appears to be dependent on the generation of arachidonate metabolites.

Animals↗
PubMed1986

Nafazatrom, an arachidonate metabolism inhibitor, decreases prolactin and GH release.

Nafazatrom, an inhibitor of arachidonate metabolism by the lipoxygenase enzymes, decreases basal prolactin and growth hormone release in a concentration-dependent manner without significantly affecting the synthesis of either hormone. It is required that nafazatrom be incubated with pituitary cells for approximately 2 h for this effect to become evident; the blockade increases in magnitude for up to 4 h. Nafazatrom blocks the increase in prolactin release caused by the releasing factors TRH and angiotensin II and the increase in prolactin and growth hormone release due to the calcium channel activator maitotoxin. These data suggest that the lipoxygenase products of arachidonate metabolism may be important mediators in basal and secretagogue-induced release of prolactin and growth hormone.

Animals↗
PubMed1986

Growth hormone releasing factor (GRF) increases free arachidonate levels in the pituitary: a role for lipoxygenase products.

GRF, a specific stimulator of GH release, increased in a concentration- and time-dependent manner pituitary [3H]-arachidonate levels in vitro. This effect was antagonized by 100 nM somatostatin. Exogenous arachidonate also stimulated GH release in vitro. Quinacrine, a phospholipase A2 inhibitor, reduced both basal and GRF-stimulated free arachidonate levels as well as GH release. The cyclooxygenase inhibitor indomethacin was ineffective, while BW755c, which also inhibits the lipoxygenase pathway, produced a further increase in the levels of the fatty acid stimulated by GRF and potently reduced GH release. These results provide additional evidence for the involvement of arachidonate metabolism in the hormone-releasing effect of GRF at the somatotroph.

4,5-Dihydro-1-(3-(trifluoromethyl)phenyl)-1H-pyraz↗
PubMed1986

A possible role of arachidonate metabolism in the mechanism of prolactin release.

The cleavage of arachidonate from pituitary phospholipids may contribute to the process that regulates the release of prolactin. To test this hypothesis, primary cultures of anterior pituitary cells from female rats were preincubated with [3H]arachidonate to label their phospholipid-containing components. The cells were then washed and incubated with vehicle or test agents and the release into the medium of prolactin and [3H]arachidonate cleaved from the phospholipids was measured. Thyrotropin-releasing hormone (TRH) and neurotensin significantly increased the release of both [3H]arachidonate and prolactin. Although basal [3H]arachidonate release was not affected by dopamine or somatostatin, both of these agents reduced [3H]arachidonate release induced by TRH. The relationship between calcium mobilization and arachidonate release was investigated by exposing the cells to agents that modify calcium balance. Maitotoxin, a calcium channel activator, stimulated prolactin and arachidonate release. In contrast cobalt, a calcium channel blocker, penfluridol, a calcium-binding protein inhibitor, and low-calcium medium decreased basal and TRH-induced prolactin release and diminished the TRH-induced release of arachidonate. RHC 80267, an inhibitor of diacylglycerol lipase, decreased TRH-induced prolactin and arachidonate release. BW755c, an inhibitor of the conversion of arachidonate to its metabolites, decreased TRH-induced prolactin release but predictably increased arachidonate release. These findings support the hypothesis that arachidonate metabolites may be involved in the process regulating prolactin release.

4,5-Dihydro-1-(3-(trifluoromethyl)phenyl)-1H-pyraz↗
PubMed1986

Dopamine inhibits maitotoxin-stimulated pituitary 45Ca2+ efflux and prolactin release.

We examined the hypothesis that dopaminergic inhibition of prolactin release is coupled to modulation of cellular calcium flux. Dispersed female rat pituitary cells were prelabeled in 45Ca2+ and perifused to determine simultaneously fractional calcium efflux and prolactin release, as stimulated by maitotoxin, a calcium channel activator. The integrated response of each parameter to 5 ng/ml maitotoxin was obtained in individual perifusion columns in the absence or presence of various concentrations of dopamine. Maitotoxin-stimulated calcium efflux was suppressed by dopamine concentrations of 0.01 microM and greater (P less than 0.01) and achieved a maximal effect at approximately 0.1 microM, at which calcium efflux was reduced by 50%. Maitotoxin-stimulated prolactin release was inhibited by 0.03 microM dopamine and greater concentrations (P less than 0.01), and at a concentration of approximately 10.0 microM dopamine the effect became maximal at approximately 85% suppression. Haloperidol (0.1 microM) blocked the effects of 0.1 microM dopamine on both parameters. Simultaneous suppression of maitotoxin-stimulated calcium efflux and prolactin release by concentrations of dopamine within the nanomolar range suggests that dopamine receptor activation is negatively coupled to modulation of calcium flux in the physiological regulation of prolactin secretion.

Animals↗
PubMed1986

Protein kinase C activators and calcium-mobilizing agents synergistically increase GH, LH, and TSH secretion from anterior pituitary cells.

A series of studies was designed to determine the effects of protein kinase C activators on TSH, LH, and GH release from anterior pituitary cells. A 15-min incubation of cultured pituitary cells with synthetic diacylglycerol or phorbol myristate acetate, stimulators of protein kinase C, increased GH, LH, and TSH release. Similarly phospholipase C, which liberates endogenous diacylglycerol, stimulated GH, LH, and TSH secretion. The potentiation of the effects of protein kinase C activators is achieved by calcium mobilization in various cell types. The results of the present studies show that calcium ionophore A23187 or calcium channel activator maitotoxin potentiate diacylglycerol-, phorbol ester-, or phospholipase C-induced GH, LH, or TSH release. These findings suggest that activation of protein kinase C by diacylglycerol and mobilization of calcium may be synergistically involved in the regulation of GH, LH, and TSH release.

Animals↗
PubMed1986

Maitotoxin, a calcium channel activator, increases prolactin release from rat pituitary tumor 7315a cells by a mechanism that may involve leukotriene production.

Arachidonate and its metabolites may play an important role in the release of prolactin. In the present study, the effect of maitotoxin, a calcium channel activator, was measured on the release of arachidonate and its metabolites from the prolactin-secreting 7315a tumor. Maitotoxin increased the release of prolactin, arachidonate, prostaglandins E2 and F2 alpha (PGE2, PGF2 alpha) and leukotriene C4 (LTC4) from 7315a cells prelabeled with [3H]arachidonate. The magnitude of the increase of prolactin and arachidonate release was decreased in low-calcium medium. The release of arachidonate from cellular phospholipids is necessary for the effect of maitotoxin on prolactin release because quinacrine, an inhibitor of arachidonate hydrolysis from phospholipids, blocked the maitotoxin-induced release of prolactin. The ability of maitotoxin to induce prolactin release appears to require metabolic transformation of arachidonate to its metabolites because BW755c, an inhibitor of the conversion of arachidonate, blocked the maitotoxin-induced prolactin release. In particular, LTC4 may be an important component of the prolactin release process because nordihydroguaiaretic acid and nafazatrom, which block the production of leukotrienes and other lipoxygenase-generated products, decreased LTC4 and prolactin release without affecting arachidonate, PGE2 or PGF2 alpha production. In contrast, indomethacin, a prostaglandin synthesis inhibitor, decreased PGE2 and PGF2 alpha production without affecting LTC4 or prolactin release. These data indicate that release of LTC4 and prolactin are closely linked events in 7315a tumor cells.

Animals↗
PubMed1986

Dopaminergic inhibition of anterior pituitary adenylate cyclase activity and prolactin release: the effects of perturbing calcium on catalytic adenylate cyclase activity.

The dopaminergic inhibition of anterior pituitary adenylate cyclase activity, cAMP accumulation, and prolactin release was studied in the presence of the Ca2+ channel activator, maitotoxin. In isobutylmethylxanthine (IBMX)-treated cells, maitotoxin stimulated prolactin secretion within 30 s and cAMP accumulation within 1 min. Although dopamine reduced cAMP accumulation and prolactin release, the effectiveness of the catecholamine was reduced in the presence of maitotoxin. When hemipituitary glands were exposed for 10 min to 100 ng maitotoxin/ml, their membranes showed increased adenylate cyclase activity. The hypothesis that maitotoxin stimulates adenylate cyclase activity by increasing Ca2+ availability was supported by the observation that, at concentrations up to 100 microM, Ca2+-stimulated anterior pituitary adenylate cyclase activity. Although dopamine decreased basal and maitotoxin-stimulated pituitary cAMP accumulation, via changes in adenylate cyclase activity, the decrement in cyclic nucleotide production, but not prolactin release, can be ascribed to the effect of the catecholamine on the basal activities of these parameters. These data provide additional evidence that an increased Ca2+ flux is stimulating to cAMP generation and prolactin release, whereas dopamine is inhibitory to these processes.

Adenylyl Cyclases↗
PubMed1986

Angiotensin peptides stimulate phosphoinositide breakdown and prolactin release in anterior pituitary cells in culture.

We investigated the effects of angiotensin peptides on the breakdown of specific membrane phospholipids, the inositol lipids, in anterior pituitary cells in culture, measuring the water-soluble products (inositol phosphates) produced during the cleavage of phosphoinositides by phospholipase C. Both angiotensin II and angiotensin I in the presence of 10 mM LiCl potently increased, in a concentration-dependent manner, total [3H]inositol phosphate and PRL release in cultured rat anterior pituitary cells. The release of LH, TSH, or GH was not significantly enhanced by the peptides. The effect on inositol phosphate accumulation was significant at 0.01 nM, and maximal stimulation (approximately 5-fold increase) occurred at 10 nM, with an ED50 of about 0.3 nM. The stimulatory effects of both angiotensin II and angiotensin I were antagonized by the receptor antagonists saralasin and Sar1,Ile8-angiotensin II. Moreover, 1 microM captopril, an inhibitor of angiotensin-converting enzyme, antagonized the effects of 0.1 and 1 nM angiotensin I, suggesting that the effect of angiotensin I on phosphoinositide breakdown and PRL release is dependent on prior conversion of angiotensin I to angiotensin II. The effect of angiotensin II was very rapid. Fractionation of the water-soluble inositol phosphates showed that angiotensin II significantly increased inositol bisphosphate and inositol triphosphate at 10 sec, whereas inositol monophosphate was increased only after 40 sec. These data indicate that in the pituitary, and presumably in the lactotroph, the binding of angiotensin II to specific membrane receptors provokes increased polyphosphoinositide hydrolysis, leading to increased production of intracellular messengers, i.e. inositol triphosphate and 1,2-diacylglycerol, responsible for the stimulation of PRL release.

1-Sarcosine-8-Isoleucine Angiotensin II↗
PubMed1986

Association of 45Ca2+ mobilization with stimulation of growth hormone (GH) release by GH-releasing factor in dispersed normal male rat pituitary cells.

Dispersed normal male rat anterior pituitary cells were prelabeled with 45Ca2+ and perifused to study the influence of GH-releasing factor (GRF) on fractional calcium efflux and GH release. The cells were exposed for 2 min to 0, 0.03, 0.1, 0.3, 1.0, or 10.0 nM GRF in separate perifusion columns, and the response to each concentration was determined by integration of the area under the curve. Concentrations of 0.1 nM GRF and higher produced a simultaneous and significant stimulation of calcium efflux and GH release. The increase in calcium efflux was proportional to GRF concentration and was maximally responsive at 1 nM GRF. The value for the entire integrated response of GH release increased continuously with GRF concentration, but GH released rapidly (0-4 min) in response to GRF achieved a maximal response at 1 nM GRF and was significantly correlated with calcium efflux. Somatostatin (100 nM) abolished the stimulation of GH release and calcium efflux due to 10 nM GRF. We conclude that GRF receptor activation is intimately associated with calcium mobilization, although the relative dependence upon intracellular or extracellular calcium sources has yet to be defined. This interaction occurs at a GRF concentration about 10 times lower than that observed to cause a measurable increase in intracellular biochemical messengers such as cAMP, phosphatidylinositol, or arachidonate. We postulate that GRF-stimulated calcium mobilization is a rapid and very sensitive event contributing to GRF-stimulated GH release.

Animals↗
PubMed1986

Dopamine does not attenuate phosphoinositide hydrolysis in rat anterior pituitary cells.

The hydrolysis of membrane phosphatidylinositol to yield [3H]labelled inositol phosphates by anterior pituitary cells was stimulated significantly by angiotensin II, TRH and neurotensin over a broad range of concentrations. These secretagogues also stimulated release of prolactin. Although the coincident incubation of dopamine with these agents resulted in a marked diminution of prolactin release, no concomitant reduction in inositol phosphate production was observed. In addition, bromocriptine, a potent agonist of dopamine, also proved ineffective in blunting stimulated phosphatidylinositol catabolism. Although it slightly inhibited basal rates of inositol tris-, bis- and monophosphate production, these results show that the secretagogue-mediated enhancement of phosphatidylinositol catabolism may be correlated with an increased release of prolactin and that the inhibition of hormone release produced by dopamine is not achieved by reducing basal or secretagogue-mediated inositol phosphate production.

Angiotensin II↗
PubMed1985

Phorbol esters affect pituitary growth hormone (GH) and prolactin release: the interaction with GH releasing factor, somatostatin and bromocriptine.

Phorbol esters are tumor promotors that directly stimulate protein kinase C activity. We asked whether these agents affect basal or receptor initiated alterations in growth hormone (GH) and prolactin release. In 4 h incubations of anterior pituitary cells, phorbol esters enhanced basal and GH releasing factor (GRF)-induced GH release. Somatostatin reduced by 38% the 4-fold stimulation of GH release induced by phorbol ester. These tumor promoters also reversed the ability of bromocriptine, a dopamine agonist, to inhibit prolactin release, with no apparent effect on basal prolactin secretion. When these agents were applied for 24 h, an increase in the basal release of both GH and prolactin was apparent. These data lead us to suggest that an intact protein kinase C system may be necessary for the full expression of GRF-stimulated GH release and dopaminergic inhibition of prolactin release.

Animals↗
PubMed1985

Agents that increase cellular cyclic AMP or calcium stimulate prolactin release from the 235-1 pituitary cell line.

The 235-1 pituitary tumor clone was utilized to study prolactin secretion after perturbing cyclic AMP and calcium metabolism. Cellular cyclic AMP levels were elevated after treatment with PGE1, cholera toxin, forskolin, isobutylmethylxanthine as well as dibutryl cyclic AMP; these cyclic AMP responses were associated with increased prolactin release. Ionophore A23187 and maitotoxin, which enhance calcium uptake into cells, also amplified prolactin secretion. In contrast, the calmodulin antagonists penfluridol and W7 reduced basal prolactin release. These data support the hypothesis that cyclic AMP, calcium and calmodulin can participate in prolactin release from 235-1 cells.

Animals↗
PubMed1985

Diacylglycerol lipase and pituitary prolactin release in vitro: studies employing RHC 80267.

We studied the possible involvement of diacylglycerol lipase in the regulatory mechanisms governing the release of prolactin by primary cultures of anterior pituitary cells. This was accomplished by studying the effect of a selective inhibitor of diacylglycerol lipase activity, RHC 80267, on basal prolactin release and that stimulated by TRH and elevated potassium concentrations. RHC 80267 produced a concentration-dependent reduction in basal prolactin release and abolished its increase produced by TRH and potassium. These results are consistent with the hypothesis that the production of arachidonate from lipids via the diacylglycerol lipase pathway is an important event in the governance of prolactin release.

Animals↗