Role of calcium in the regulation of adenohypophysial hormone release

Paradoxical effect of neuroleptic drugs on prolactin secretion by rat pituitary cell cultures

Several antipsychotic drugs reverse the dopamine-induced inhibition of prolactin release by rat pituitary cell cultures. Paradoxically, at high doses and without dopamine, antipsychotic drugs can also inhibit prolactin secretion. The mechanism underlying this phenomenon is unclear. Some evidence suggests that these drugs have an agonistic action. We sought to verify whether clozapine and fluphenazine, at doses higher than those reversing dopamine-induced inhibition of prolactin secretion in vitro, show this paradoxical effect and eventually a partial agonistic action. Both antipsychotics inhibited prolactin secretion, clozapine at doses starting from 10(-6) M and fluphenazine from 10(-7) M. Haloperidol reversed clozapine-induced prolactin inhibition but left fluphenazine-induced inhibition unchanged. These in vitro findings suggest that clozapine has a partial agonistic action on dopaminergic receptors but fluphenazine does not.

Effects of lead on the somatic growth of children

Studies on the effects of lead on the somatic growth of children are limited and contradictory. The authors investigated the adverse effects of blood lead concentration on the somatic growth of primary-school-age children. In this study, there was a total of 522 children, aged 6-9 y, who resided in three areas of Greece (i.e., Loutraki, Lavrion, and Elefsina). The medical evaluation included medical history; physical examination; and measurements of height, head circumference, and chest circumference. The authors also evaluated dietary information, socioeconomic status, and height of parents. The authors conducted laboratory tests for hematological parameters and blood lead levels. The mean blood lead level was 12.3 microg/dl (standard deviation = 8.9 microg/dl), and levels ranged from 1.3 microg/dl to 51.2 microg/dl. There were negative monotonic relationships between growth parameters and blood lead levels, even after the authors allowed for confounding effects. An increase in blood lead level of 10 microg/dl was associated with a decrease of (a) 0.33 cm in head circumference (95% confidence interval = 0.12, 0.55; p = .002); (b) 0.86 cm in height (95% confidence interval = 0.14, 1.16; p = .020); and (c) 0.40 cm in chest circumference (95% confidence interval = -0.22, 1.02; p = .207). These findings led the authors to conclude that a decrease in growth in children may be associated with blood lead concentrations.

Sexual and ontogenic differences in K(+)-induced gonadotropin and prolactin release in vitro

Ontogenic and sexual differences have been described in the regulation of anterior pituitary hormone release. In the present experiments we studied basal release and the effect of a depolarizing concentration of K+ on in vitro gonadotropin and prolactin release from anterior pituitaries of male and female rats at 12, 20 and 28 days of age. Basal release of LH and FSH increased with age, values obtained from female glands being significantly higher than those obtained from male glands. K(+)-induced release of LH did not present differences among ages, although the response in females was always greater than that in age-matched males. If K(+)-induced release of LH was considered in relation to basal release, infantile 12-day-old rats of both sexes, had a significantly greater sensitivity to the effect of K+ in comparison to older ages, as has been described for the LH-releasing effect of LHRH and of other stimuli. K(+)-induced FSH release was maximal in females at 20 days of age, and in males at 28 days of age. Percentage increase relative to basal values, induced by K+ was also greatest at 12 days in both sexes, although values from female glands were significantly higher than those from males. Basal and K(+)-induced prolactin release increased significantly with age in both sexes. Basal prolactin release was greater in females than in males at 28 days of age, and no other sexual difference was evidenced.(ABSTRACT TRUNCATED AT 250 WORDS)

Requirement of extracellular calcium in fish pituitary gonadotropin release by gonadotropin hormone-releasing hormone

Influence of extracellular calcium on gonadotropin hormone-releasing hormone (GnRH)-stimulated gonadotropin hormone (GtH) release from a teleostean fish (Channa punctatus) pituitary was examined in vitro by preparing enzymatically dispersed pituitary cell incubation. Effect of Ca2+ on GnRH-augmented GtH release was evaluated with partially purified C. punctatus GnRH (cGnRH) and synthetic mammalian GnRH (mGnRH). Cells were dispersed by 0.3% collagenase plus 0.05% trypsin in culture medium and a high yield of viable cells were obtained. Addition of cGnRH (10 micrograms/ml) to pituitary cells in Ca2+-free medium resulted in a significant increase in GtH release, but the addition of Ca2+ (2 mM) enhanced it to about four- and threefold over cGnRH and mGnRH, respectively. Increasing concentrations of Ca2+ (0.1-2.0 mM/well) with fixed concentrations of GnRH (10 micrograms/ml) or increasing doses of GnRH (2.5 to 20 micrograms/ml) with fixed amount of Ca2+ (2 mM/well) resulted in a dose dependent increase in GtH release. EDTA or EGTA (2 mM/well) completely suppressed the Ca2+-augmenting effect of GnRH-stimulated GtH release. Addition of lanthanum (La3+, 4 mM/well), a competitive inhibitor of Ca2+, reduced 60% of the Ca2+ (2 mM/well) stimulation. Verapamil, a specific Ca2+ channel blocker, when added in increasing concentrations (1-100 microM/well) to pituitary cell incubations containing GnRH-stimulated GtH release in Ca2+-free medium could be waived by EGTA (2 mM/well), indicating availability of extracellular calcium from tissue sources. The uptake of radioactive Ca2+ by pituitary cells was greatly enhanced by GnRH while the addition of verapamil (10 microM/well) not only inhibited the GnRH-stimulated uptake, but also reduced it below the control level.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural study of the distribution of calcium in the pineal gland of the rat subjected to manipulation of the photoperiod

Using the pyroantimoniate technique, a study was conducted at electron microscope level on the distribution of the calcium ion in the pineal glands of normal adult Sprague-Dawley rats with initial weights of 150-200 g subjected to a 12:12 light dark cycle and others under the same conditions were subjected to modifications in the noradrenergic signal, such as continuous illumination over 7 days, blinding by bilateral enucleation (7 or 90 days) before sacrifice and bilateral superior cervical gangliectomy at 21 days before sacrifice. All the animals were sacrificed by decapitation, half of them at midday and the other half at midnight. Abundant fine precipitations of calcium were found in the intercellular spaces of the pineal glands of the normal rats. By contrast, in the gangliectomized animals subjected to constant illumination and chronic binding these precipitations were few in number. Additionally, two types of pinealocytes were observed regarding the distribution and concentration of intracytoplasmic calcium in both the normal and experimentally manipulated animals. Type I correspond to the classic light pinealocytes, with an absence of intracytoplasmic precipitations, although in the normal and gangliectomized animals sacrificed at midnight it was possible to observe fine deposits inside the mitochondrial matrix. Type II correspond to the classic dark pinealocytes, with a dense cytoplasmic matrix and numerous deposits of intracytoplasmic and intranuclear calcium; these were never seen in the type I pinealocytes.

Effects of calcium availability on the release of ovine choriomammotropin from cotyledonary cells incubated in vitro

1. In this study, we examined the basal release of choriomammotropin (oCM) from monolayer cultures of cotyledonary cells obtained from ewes at different gestational ages. 2. oCM release increased with gestational age and displayed a similar profile to the concentration of oCM observed in maternal plasma. 3. Release of oCM was significantly (P less than 0.05; n = 9) increased in calcium-depleted medium, and by treatment with either phospholipase C (0.125 units/ml) or KCl (50 mM). 4. The calcium antagonist MgCl (12 mM) and the calcium channel-blocking agents verapamil (50 microM) and nefidipine (10 microM) all significantly stimulated oCM release. 5. These data are consistent with the suggestion that oCM release is inversely related to extracellular calcium concentration.

cAMP- and diacylglycerol-mediated pathways elevate cytosolic free calcium concentration via dihydropyridine-sensitive, omega-conotoxin-insensitive calcium channels in normal rat anterior pituitary cells

The effect of 8-bromocyclic AMP (8-Br-cAMP) and phorbol 12-myristate 13-acetate (PMA), a protein kinase C activator, on cytosolic free calcium concentration ([Ca2+]i) in normal rat anterior pituitary cells was examined. [Ca2+]i was monitored directly by the fluorescent indicator fura-2. 8-Br-cAMP as well as PMA elevated [Ca2+]i in a concentration-dependent manner. Forskolin (10 mumol/l), which activates adenylate cyclase, and 1-oleoyl-2-acetyl-glycerol (10 mumol/l), another activator of protein kinase C, also increased [Ca2+]i. Both the 8-Br-cAMP (2 mmol/l)- and the PMA (100 nmol/l)-induced increase in [Ca2+]i was dependent on the presence of extracellular calcium and could be inhibited by the calcium channel blockers Mg2+ and nifedipine, but not by omega-conotoxin (100 nmol/l). The half-maximally inhibitory concentrations of Mg2+ and nifedipine were about 12 mmol/l and 160 nmol/l, respectively, for the [Ca2+]i response to 8-Br-cAMP (2 mmol/l), and were about 6 mmol/l and 400 nmol/l, respectively, for the PMA (100 nmol/l)-induced increase in [Ca2+]i. The sodium channel blocker tetrodotoxin (5 mumol/l) or PMA (100 nmol/l) on [Ca2+]i. After pretreatment for 3 min with PMA (100 nmol/l), the subsequent K+ (100 mmol/l)- or arachidonic acid (3 mumol/l)-induced increase in [Ca2+]i was decreased by about 50%. By contrast, pretreatment (3 min) with 8-Br-cAMP (2-10 mmol/l) markedly enhanced the subsequent [Ca2+]i response to K+ (100 mmol/l), and left the effect of arachidonic acid (3 mumol/l) on [Ca2+]i unimpaired.(ABSTRACT TRUNCATED AT 250 WORDS)

Ionophore A23187 partially reverses LH secretory defect of pituitary cells from old rats

We measured in vitro release of luteinizing hormone (LH) in the presence of 1.5 mM extracellular calcium, with and without LH-releasing hormone (LHRH; 10(-10) to 10(-7) M) or the ionophore A23187 (10(-7) to 10(-4) M), in primary cultures of anterior pituitary cells from intact mature (6 mo) and old (24 mo) male and intact and ovariectomized mature and old female Wistar rats. Base-line as well as LHRH- and A23187-mediated LH secretion was decreased from cells of old rats. However, exposure to A23187 led to a nearly twofold greater augmentation of LH release from cells of old rats, thus decreasing the apparent age-related LH secretory deficit by approximately one-half. We then measured LHRH-mediated (10(-8) M) vs. A23187-mediated (10(-4) M) LH release with and without extracellular calcium (0.08-1.5 mM). For cells from both mature and old rats, there was a similar calcium dependency for A23187- and LHRH-mediated LH release, with optimal LH secretion at 1.0-1.5 mM extracellular calcium concentrations. Again, both LHRH- and A23187-stimulated LH release was significantly lower and exposure to A23187 led to a greater increase in LH release from cells of old rats. Taken together with similar findings in other systems, these data suggest that the in vitro LH secretory defect of pituitary cells from old rats results in part from one or more defects in calcium mobilization and that such alterations may be a widespread manifestation of aging.

Is calcium or cyclic AMP involved in the inhibitory effect on pituitary hormone secretion of the tripeptide aldehyde proteinase inhibitors?

The mechanism by which tripeptide aldehyde proteinase inhibitors decrease prolactin (PRL) and growth hormone (GH) secretion was studied. Agents known to modify the intracellular levels of cyclic adenosine monophosphate (cAMP) or cytosolic free calcium were used in monolayer cultures of the rat anterior pituitary gland. The phosphodiesterase inhibitor isobutyl-methylxanthine (IBMX), 8-bromo-cAMP and forskolin all stimulated PRL release. Boc-D-Phe-Pro-arginal (Boc-DPPA) at 1 mmol/l concentration was a potent inhibitor of basal PRL release and significantly decreased the effect of 8-Br-cAMP, forskolin or IBMX (0.5 mmol/l). Forskolin (1 mumol/l) stimulated ACTH, PRL and GH release and all these effects were decreased by 100 mumol/l of Boc-D-Phe-Phe-lysinal (Boc-DPPL). Neither tripeptide aldehyde affected the forskolin-induced rise in intracellular cAMP. Growth hormone releasing factor (hpGRF, 1 nmol/l) stimulated both GH release and intracellular cAMP generation; Boc-DPPL (100 mumol/l) significantly decreased stimulated GH release without affecting cAMP accumulation. Increasing medium K+ to 10 times normal level stimulated PRL release presumably by enhancing Ca2+ entry into the cells and 1 mmol/l Boc-DPPA decreased high potassium-stimulated PRL release. The ionophore A-23187 stimulated PRL release at 10 mumol/l but not at 1 mumol/l. At 1 mumol/l A-23187 prevented the Boc-DPPA-induced inhibition of PRL release. These findings suggest that the tripeptide aldehyde proteinase inhibitors inhibit PRL and GH release at a site beyond cAMP formation.

The effects of A23187 and phorbol ester on the phosphorylation of proteins in rat anterior pituitary cells

The effects of A23187 and 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on the phosphorylation of proteins in normal rat anterior pituitary cells were compared. A23187 rapidly activated the phosphorylation of at least 5 proteins (45, 47, 53, 54 and 58 kDa), which reached the maximal level in 2-10 min, and decreased gradually thereafter. In contrast, TPA activated the phosphorylation of at least 6 proteins (45, 62, 64, 72, 76 and 82 kDa), which were mostly distinguished from those activated by A23187. TPA-induced response was elicited more slowly, reached a plateau after about 10 min, and was sustained thereafter. These results suggest that the protein phosphorylation stimulated by A23187 and TPA is conducted by different mechanisms.

Stimulation of ovine choriomammotrophin release, in vitro, by phospholipase C

Phospholipid metabolites have previously been implicated in receptor-mediated stimulation of protein hormone secretion. As the factors which regulate the release of choriomammotrophin remain to be elucidated, we investigated the potential involvement of phospholipase C-induced phospholipid metabolism in the release of this placental hormone. Phospholipase C (PLC) caused a dose-dependent release of choriomammotrophin from ovine placenta, incubated in vitro. At a concentration of 0.2 units/ml (0.25 microgram protein/ml), PLC caused the release of choriomammotrophin from placental tissue to approximately double that observed in control incubations (7.08 +/- 0.4 micrograms/50 mg/h and 3.26 +/- 0.3 micrograms/50 mg/h, respectively). PLC treatment did not significantly alter plasma membrane permeability, as indicated by the release of lactate dehydrogenase and protein. PLC-stimulated release of oCM was completely abolished by incubation in calcium-free medium or by preincubation with the inorganic calcium-channel blocking agents cobalt chloride (4 mM) and lanthanum chloride (1 mM). The effects of PLC treatment on ovine choriomammotrophin (oCM) release were also inhibited by preincubation of placental tissue with inhibitors of arachidonic acid metabolism: ibuprofen (10(-5) M), naproxen (10(-4) M) or nordihydroguaiaretic acid (NDGA) 5 X 10(-6) M). These results suggest that the effects of PLC on the release of choriomammotrophin are mediated via metabolites of arachidonic acid.

Multiple classes of calcium channels in mouse pituitary tumor cells

Synthetic corticotropin-releasing factor (CRF) is a potent adrenocorticotropin (ACTH) secretagogue in the mouse pituitary tumor cell strain AtT20/D16v (D16). In the absence of added calcium in the incubation medium a dose of 5 nM CRF stimulates ACTH secretion 2-fold over control values while at medium calcium concentrations greater than 1 mM the same dose of CRF elicits a 3-fold stimulation. In the presence of EGTA or of the calcium antagonists verapamil, cobalt, or lanthanum the CRF effect is abolished. Depolarizing concentrations of extracellular K+ lead to a rapid increase in cell-associated calcium, a response which is inhibited by the dihydropyridine calcium antagonist nimodipine. Although treatment with CRF does not alter the concentration of cell-associated calcium in D16 cells, ACTH secretion stimulated by both CRF and elevated medium K+ are inhibited by nimodipine in a dose-related manner. The results indicate that D16 cells possess both voltage-sensitive and CRF-activated calcium channels.

Calcium control of growth hormone release from chicken pituitary glands in vitro

The influence of calcium on the basal and stimulated release of growth hormone (GH) from chicken pituitary glands has been determined in vitro. Basal GH release occurred in Ca2+ deficient media, although it was increased in proportion to the medium Ca2+ concentration. Growth hormone release was stimulated by 10(-7)-10(-9) M thyrotrophin-releasing hormone (TRH), maximal stimulation being observed in the presence of 10(-8) M TRH and 1.5 mM Ca2+. Decreases in the Ca2+ concentration (to 0.75, 0.375, or 0 mM) suppressed the GH response to 10(-8) M TRH, as did increases (to 3.0 and 6.0 mM) in the Ca2+ concentration. These results suggest that GH release in chickens is regulated by Ca2+-dependent mechanisms.

Inhibition of prolactin and growth hormone secretion by nickel

Nickel (Ni++) is a potent inhibitor of prolactin (PRL) secretion from isolated rat pituitary quarters in vitro, suppressing both basal PRL release and the stimulation of PRL secretion due to theophylline and dibutyryl cyclic AMP. Stimulation of growth hormone (GH) secretion by synthetic GHRH is also blunted by Ni++, although basal GH release and stimulated GH release due to theophylline or dibutyryl cyclic AMP are not suppressed. Ni++ antagonizes the stimulation of both PRL and GH secretion by barium (Ba++) ion, suggesting that the inhibitory effects of Ni++ on hormone release are due to an antagonism of calcium uptake or redistribution.

Maitotoxin stimulates hormonal release and calcium flux in rat anterior pituitary cells in vitro

The marine dinoflagellate toxin maitotoxin (MTX), an activator of calcium channels, stimulates the release of prolactin (PRL), growth hormone (GH), thyroid-stimulating hormone (TSH), and luteinizing hormone (LH) from monolayers of anterior pituitary cells in a dose-dependent manner. Maitotoxin significantly increased PRL, GH, and LH release within 1.5 min and TSH release within 3.5 min, and the stimulation continued for at least 1 h (P less than 0.01). MTX-stimulated hormonal release was blocked by the calcium channel blocker manganese (P less than 0.01). In freshly dispersed perifused pituitary cells in columns, exposure to MTX for 10 min markedly increased PRL, GH, TSH, and LH release for at least 1 h after withdrawal of the toxin. In other experiments, MTX significantly stimulated 45Ca2+ exchange by dispersed pituitary cells within 30 s, continuing for at least 30 min. We conclude that MTX increases anterior pituitary hormonal release, possibly by activating calcium channels, thereby increasing cellular calcium influx. Thus MTX may be a useful agent for investigating the involvement of Ca2+ in hormonal secretory processes.

The modulation of placental lactogen secretion by calcium: studies with cultured human term trophoblast

Previous reports have documented the negative regulatory effect of calcium ion on the secretion of human placental lactogen by the placenta. Human term trophoblasts were dispersed from term placenta and maintained in monolayer cell culture. Incubation of the cultured trophoblast with EGTA produced a dose-dependent stimulation of placental lactogen secretion. The calcium ionophore A23187 inhibited hormone secretion. This inhibitory effect was eliminated by the addition of the calcium-calmodulin complex inhibitor trifluoperazine. It is concluded that calcium exerts a negative regulatory effect on the secretion of placental lactogen by a mechanism which requires the action of the calcium-calmodulin complex.

Lack of inhibition of anterior pituitary hormone release during chronic treatment with calcium antagonists

Calcium antagonists are widely used for the treatment of cardiovascular disorders, especially ischaemic heart disease. It has been demonstrated that these drugs, either in vitro or acutely administered in humans, inhibit the basal and stimulated secretion of pituitary hormones by blocking calcium influx through slow calcium channels. To see if a similar effect could be detected after their chronic administration, we studied the basal, TRH- and LHRH-stimulated TSH, PRL, LH and FSH release in 18 male subjects with chronic stable angina before and on the 30th day of oral treatment with verapamil (n = 8;80 mg three times a day) or with nifedipine (n = 10; 10 mg three times a day). Neither drug had any effect on basal TSH, PRL, LH and FSH values or on their response to the specific hypothalamic-releasing hormones. These results suggest that the chronic administration of calcium antagonists, at the usual therapeutic doses, does not effect the process of stimulus-secretion coupling of anterior pituitary hormones, ruling out any impairment of the related target glands which have been expected on the basis of previous studies.

Arachidonic acid metabolism and thyrotropin secretion in vitro

We investigated the role of arachidonic acid and certain of its metabolic products in the control of thyrotropin (TSH) secretion in vitro. Phospholipase A2 and 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA), which increase the intracellular availability of arachidonic acid, potently stimulated TSH release from anterior pituitary cells continuously perifused in columns and from hemipituitary glands in vitro. The effect was dose-dependent and reversible. Conversely, quinacrine (50 microM), an inhibitor of phospholipase A2 activity, inhibited basal and stimulated TSH release from pituitary cells perifused in columns. Exogenous arachidonic acid (1-100 microM) did not produce any significant effect on TSH release from hemipituitary glands in vitro. Nordihydroguaiaretic acid (NDGA), a specific inhibitor of the lipoxygenase pathway, dose-dependently inhibited basal TSH release from anterior pituitary glands incubated in vitro. Moreover, 50 microM NDGA antagonized the stimulatory effect of thyrotropin releasing hormone (TRH), phospholipase A2 and PMA on TSH release. BW755c, another lipoxygenase inhibitor, also inhibited TRH-stimulated TSH secretion. In contrast, 10-100 microM indomethacin, a potent blocker of the cyclooxygenase pathway, did not significantly modify either basal or TRH-stimulated TSH secretion from hemipituitary glands in vitro. These data suggest that arachidonic acid metabolism is involved in TSH secretion in vitro, although incubation of pituitary glands with the fatty acid did not apparently modify in our conditions basal TSH secretion. The eventual effect of arachidonate appears to be at least partially due to the action of its lipoxygenase pathway products.

The possible role of calmodulin in the inhibition of prolactin secretion by dopaminergic antagonists

Several previous reports have indicated that a number of dopaminergic antagonists paradoxically inhibit prolactin secretion at micromolar concentrations. It is well known that some of these drugs, including pimozide and the phenothiazines, are inhibitors of calmodulin activity. Here we report that micromolar concentrations of several dopaminergic antagonists inhibit prolactin secretion from isolated rat anterior pituitary cells and calmodulin activity (calmodulin-activated cyclic GMP phosphodiesterase). Inhibition of calmodulin activity may thus, at least partially, explain the inhibitory effect of these drugs on prolactin secretion.

Early changes in the adaptation to a low calcium diet in the chick

Twelve hours after the diet of 3-week-old chicks was changed from a 1% to a 0.1% concentration of calcium (Ca), the growth rate and circulating levels of growth hormone had fallen while renal 25-hydroxecholecalciferol-24-hydroxylase had risen. The amount of 47Ca incorporated into bone from an injection given 18 h previously was lower than in the control birds. Over the following 2 1/2 days on the low Ca diet, the renal 1-hydroxylase activity rose and the plasma prolactin level fell, but the other parameters moved back toward the control level. It was concluded that early adjustments in hormonal and mineral metabolism counteract the acute effects of a dietary Ca shortage until longer-term adaptive changes begin to compensate for a continuing Ca deficiency. The renal hydroxylase activities were not directly influenced by the level of circulating growth hormone or prolactin.

Calcium influx is not required for thyrotropin-releasing hormone stimulation of prolactin release from GH3 cells

TRH stimulation of prolactin release from GH3 cells is dependent on Ca2+; however, whether TRH-induced influx of extracellular Ca2+ is required for stimulated secretion remains controversial. We studied prolactin release from cells incubated in medium containing 110 mM K+ and 2 mM EGTA which abolished the electrical and Ca2+ concentration gradients that usually promote Ca2+ influx. TRH caused prolactin release and 45Ca2+ efflux from cells incubated under these conditions. In static incubations, TRH stimulated prolactin secretion from 11.4 +/- 1.2 to 19 +/- 1.8 ng/ml in control incubations and from 3.2 +/- 0.6 to 6.2 +/- 0.8 ng/ml from cells incubated in medium with 120 mM K+ and 2 mM EGTA. We conclude that Ca2+ influx is not required for TRH stimulation of prolactin release from GH3 cells.

Secretion of calcium into milk: review

Milk calcium exists in bound and ionized forms. Bound calcium is associated both with casein micelles and complexed to citrate and phosphate. Ionized calcium in milk is 1 to 4 millimolar, at least 1000 times its postulated concentration in the mammary alveolar cell. For this reason active transport mechanisms are necessary for transfer of this nutrient to the lumen of the mammary alveolus. Evidence that the major active transport system is a calcium adenosine triphosphatase residing in the membrane of the Golgi secretory vesicle is summarized. This adenosine triphosphatase appears to be activated by calcium concentrations in the micromolar range, to require magnesium ions, and to operate by phosphorylation of a 100,000 dalton enzyme intermediate. Metabolic processes are required to maintain a low concentration of calcium within the cytosol of the mammary alveolar cell. Because no evidence for sodium/calcium exchange could be found in the mammary gland of the lactating mouse, we suggest that these processes operate through a calcium adenosine triphosphatase in the basolateral membrane of the cell. Decreased calcium in the alveolar lumina decreased the integrity of the barrier between blood and milk. It is postulated from observations in other secretory systems that an increase in cystolic activity calcium may play a role in lactogenesis.

Modifications of membrane cholesterol content affects electrical properties and prolactin release of cultured pituitary cells

Treatment of cloned pituitary cells (GH3/B6) with cholesterol-enriched liposomes, which presumably increases membrane cholesterol content, affects the passive and active electrophysiological properties and stimulates the release of prolactin (PRL).

Does calcitonin modulate anterior pituitary hormone secretion?

In a group of 5 healthy subjects salmon CT (sCT) infusion was unable to induce significant variations on basal secretory levels of LH, FSH, PRL and TSH. In a second group of 5 normal subjects, GnRH and TRH tests were performed both during sCT and saline infusion; a clear inhibition of TSH-stimulated levels and of PRL area was documented, while gonadotropin secretion was not significantly affected by sCT infusion. These results suggest that CT effect might be attributed to a change in intracellular calcium of pituitary cells; however the different behavior between TRH-and GnRH-stimulated hormones might be due to a different hormonal release mechanism. Furthermore the widespread recognition of CT-like immunoreactivity in adenohypophysis and in portions of the central nervous system suggests that CT may be a neurotransmitter or paracrine regulator.

Hormonal changes associated with hypertension in neoplasia-induced hypercalcemia

Neoplasia-induced hypercalcemia in the Fischer rat results in hypertension 1 wk after Leydig cell tumor transplantation. Systolic blood pressure, plasma catecholamine, prolactin, plasma renin activity (PRA), and aldosterone responses to immobilization stress were evaluated in Fischer rats 10 days after tumor transplantation and in age-matched nontransplanted controls. Basal systolic blood pressure, norepinephrine, and PRA levels at 10 days after tumor transplantation were higher in association with elevated calcium levels in tumor-transplanted rats than in controls. Systolic pressure, norepinephrine, and epinephrine responses to immobilization stress were greater in the hypercalcemia 10-day transplanted rats. Although basal levels of prolactin and aldosterone were similar in the two groups. These observations suggest that elevated levels of the vasoactive hormones norepinephrine and angiotensin may play a pivotal role in development of hypertension in association with neoplasia-induced hypercalcemia. Further, neoplasia-induced hypercalcemia in the Fischer rat is associated with a relative hyperreninemic hypoaldosteronism state.

Evidence that TRH stimulates secretion of TSH by two calcium-mediated mechanisms

Thyrotropin-released hormone (TRH) stimulation of thyrotropin (TSH) release from mouse thyrotropic tumor (TtT) cells is dependent on Ca2+. We demonstrate that TRH action in TtT cells does not require extracellular Ca2+ but that Ca2+ influx induced by TRH can augment TSH secretion. TRH caused a 46% increase in 45Ca2+ uptake by TtT cells in medium with 100 micro M Ca2+. The increment in 45Ca2+ uptake caused by TRH was dependent on the concentration of Ca2+ in the medium. In contrast to the effect of 50 mM K+, which also causes Ca2+ influx, TRH caused 45Ca2+ efflux and TSH release from TtT cells even when the concentration of Ca2+ in the medium was lowered below 100 micro M. TRH stimulated TSH release during perifusion in medium in which the free Ca2+ concentration was lowered to approximately 0.02 micro M, and reintroduction of Ca2+ into the medium simultaneously with TRH markedly increased TSH release. We suggest that TRH may affect Ca2+ metabolism in TtT cells by both extracellular Ca2+-independent and -dependent mechanisms and that this dual mechanism of action serves to augment further TSH secretion induced by TRH.

Role of calcium in acute stimulated release of prolactin from neoplastic GH3 cells

Using a calcium-sensitive electrode to monitor calcium movements, we found that neoplastic GH3 cells experienced a net accumulation of calcium when exposed to elevated (50 mM) K+. Acute prolactin (PRL) release was also stimulated under these conditions. Both calcium uptake and PRL release could be blocked by the calcium antagonist methoxyverapamil (D-600). Thyrotropin-releasing hormone (TRH) also stimulated PRL release but had no effect on cellular calcium accumulation. Likewise, D-600 had no effect on TRH-induced PRL release. Such results indicate that enhanced secretory activity does not require an increase in intracellular calcium content. The observation that secretagogues do not stimulate PRL release in the absence of extracellular calcium was investigated. When GH3 cells were placed in a Ca-free medium, they underwent a prompt and sustained loss of cellular calcium. The loss of such intracellular calcium could be blocked with D-600. We conclude that the inability of TRH to stimulate the release of PRL in Ca-free medium is due to the loss of intracellular calcium and not to the absence of external calcium per se.

Human placental lactogen release in vitro: paradoxical effects of calcium

To study the effects of calcium on the release of human placental lactogen (hPL), placental explants were exposed to media containing lower or higher concentrations of calcium than normally available to the placenta. Explants exposed for 2 h to calcium-poor medium or medium containing either 2 mM EDTA or 2 mM EGTA released 160, 248, and 253% more hPL, respectively, than control explants. In contrast, explants exposed to medium containing higher than normal calcium concentrations released the same amounts of hPL as the control explants. At lower than normal extracellular calcium concentrations, the increased hPL release was inversely proportional to the calcium concentration. The increased release in calcium-poor medium was inhibited by subsequent exposure of the explants to medium containing calcium and was prevented by either barium or magnesium. Changes in barium or magnesium concentrations, however, had no effects on hPL release in the presence of normal extracellular calcium concentrations. Methoxyverapamil (D 600), an inhibitor of calcium flux, stimulated hPL release. Because low extracellular calcium and methoxyverapamil both inhibit calcium influx, these experiments suggest that calcium influx inhibits hPL release. The role of calcium in the regulation of hPL release therefore appears to be different from that reported in other release systems.

Kinetic analysis of calcium distribution in rat anterior pituitary slices

A kinetic analysis of exchangeable calcium associated with rat anterior pituitary slices was carried out. Slices were permitted to accumulate 45Ca for 1 h, and then a desaturation experiment was initiated. The resulting data was represented as a polyexponential function and analyzed with a weighted nonlinear least-squares method. The resulting coefficients and exponents were used to determine, at the steady state, the size of each calcium compartment, the unidirectional fluxes into and out of each compartment, and the corresponding rat constants. Three kinetically distinguishable compartments were defined. The first compartment exchanges with a half-time slower than calcium free in the extracellular space and can be removed by a 1.5-min exposure to Ca-free EGTA media suggesting that this calcium is bound in the extracellular matrix perhaps to the cell surfaces. The second compartment is intracellular and does not contain calcium accumulating structures such as mitochondria or microsomes. The third compartment has many of the characteristics associated with these organelles. Experiments were performed with two pituitary secretagogues, elevated K+ and ouabain, to assess changes in the study-state calcium distribution associated with enhanced secretion.

The effect of neoplasia-induced hypercalcemia on the neuroendocrine axis of the Fischer rat

The effects of neoplasia-induced hypercalcemia on the hypothalamic-pituitary axis of the Fischer rat were determined by measuring the responses of TSH to TRH, PRL to TRH and haloperidol, and LH to LHRH in 3 groups of 8 rats prior to and at 4 and 8 days after transplantation of a Leydig cell tumor. The mean serum calcium was 8.2 +/- 0.2 mg/dl in 8 Fischer rats pretransplantation; had risen at 4 days posttransplantation to 9.8 +/- 0.2 mg/dl; and at 8 days was 11.2 +/- 0.1 mg/dl. TSH response to TRH was greater in the pretransplant rats than in the groups studied at 4 and 8 days posttransplant. Basal PRL levels and PRL responses to TRH and haloperidol were less in the groups studied at 4 and 8 days posttransplant. Basal LH values were similar in all 3 groups, but the LH response to LHRH was less for the posttransplant groups. The TSH response to TRH, PRL responses to TRH and haloperidol, and LH responses to LHRH were less in the 8 days posttransplants than in the groups studied at 4 days posttransplantation. There were significant negative correlations between TSH (r = -0.79) and PRL (r = -0.80) responses to TRH, PRL responses (r = -0.94) to haloperidol and LH responses (r = -0.91) to LHRH and the serum calcium. The results indicate that increasing levels of serum calcium following Leydig cell tumor transplantation are associated with suppression of adenohypophysial release of TSH, PRL, and LH; the degree of suppression is related to the magnitude of hypercalcemia achieved in this model.