Thyrotropin-releasing hormone activates Na+/H+ exchange in rat pituitary cells

Rapid turnover of calcium in the endoplasmic reticulum during signaling. Studies with cameleon calcium indicators

HEK293 cells expressing the thyrotropin-releasing hormone (TRH) receptor were transfected with cameleon Ca(2+) indicators designed to measure the free Ca(2+) concentration in the cytoplasm, [Ca(2+)](cyt), and the endoplasmic reticulum (ER), [Ca(2+)](er). Basal [Ca(2+)](cyt) was about 50 nm; thyrotropin-releasing hormone (TRH) or other agonists increased [Ca(2+)](cyt) to 1 micrometer or higher. Basal [Ca(2+)](er) averaged 500 micrometer and fell to 50-100 micrometer over 10 min in the presence of thapsigargin. TRH consistently decreased [Ca(2+)](er) to 100 micrometer, independent of extracellular Ca(2+), whereas agonists for endogenous receptors generally caused a smaller decline. When added with thapsigargin, all agonists rapidly decreased [Ca(2+)](er) to 5-10 micrometer, indicating that there is substantial store refilling during signaling. TRH increased [Ca(2+)](cyt) and decreased [Ca(2+)](er) if applied after other agonists, whereas other agonists did not alter [Ca(2+)](cyt) or [Ca(2+)](er) if added after TRH. When Ca(2+) was added back to cells that had been incubated with TRH in Ca(2+)-free medium, [Ca(2+)](cyt) and [Ca(2+)](er) increased rapidly. The increase in [Ca(2+)](er) was only partially blocked by thapsigargin but was completely blocked if cells were loaded with 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. In conclusion, these new Ca(2+) indicators showed that basal [Ca(2+)](er) is approximately 500 micrometer, that [Ca(2+)](er) has to be >100 micrometer to support an increase in [Ca(2+)](cyt) by agonists, and that during signaling, intracellular Ca(2+) stores are continuously refilled with cytoplasmic Ca(2+) by the sarcoendoplasmic reticulum Ca(2+)-ATPase pump.

Upregulation of pancreatic polypeptide-sensitive neuropeptide Y (NPY) receptors in estrogen-induced hypertrophy of the anterior pituitary gland in the Fischer-344 rat

Implants of diethylstilbestrol inducing anterior pituitary prolactinomas in female Fischer-344 rats produced a considerable elevation of high-affinity binding of either rat or human pancreatic polypeptide (hPP). No comparable upregulation of high-affinity binding sites was noted for the ligand [125I](Leu31,Pro34)hPYY (LP-PYY) (masked by 2 nM hPP to force selectivity for the Y1 sites), or of the Y2-selective ligand [125I]hPYY(3-36). The Y5-like sites displayed the rank order of potency of hPP = rPP, hNPY, LP-PYY > pPYY(1-36) > hNPY(2-36) > hPYY(3-36) >> aPP, similar to the previously described rabbit kidney or hypothalamic Y5-like receptors. The PP binding in the anterior pituitary was not sensitive to the Y1-selective non-peptidic antagonist BIBP-3226. The PP binding was highly sensitive to alkali metal cations, and to a N5-substituted amiloride antagonist of sodium transport, but not to a C2-guanido substituted amiloride antagonist of sodium channels. The binding was also sensitive to phospholipase C antagonist U-73122, and to alkylating alpha-adrenergic agonist chloroethylclonidine. Lactotrophs isolated in Percoll gradients after enzymic dispersion of the anterior pituitary gland retained the high-affinity PP binding. Following removal of estrogen implants, the hPP binding sites decreased to very low levels within 3 days, in parallel to the decrease of plasma prolactin.

Effect of probucol on intracellular pH and proliferation of human vascular endothelial cells

We investigated the effect of probucol on the intracellular pH ([pH]i) and proliferation of human umbilical vein endothelial cells (HUVEC), as well as their production of prostacyclin (PGI2). The addition of probucol produced a biphasic shift in [pH]i, with a brief initial acidification followed by a rapid alkaline shift. After pretreatment with EGTA, the initial decrease in [pH]i was abolished, and the subsequent increase was inhibited. After pretreatment with amiloride, only the increase of [pH]i was abolished. These results suggest that the probucol-induced increase of [pH]i was mainly dependent on Na+/H+ exchange and partly on extracellular Ca2+. In contrast, the addition of LDL produced a decrease of [pH]i. Under Ca2+-free condition, [pH]i was further decreased by LDL. In cells pretreated with amiloride, however, [pH]i was not further decreased by LDL. It was found that probucol promoted cell proliferation, and LDL inhibited cell proliferation. Addition of probucol also enhanced prostacyclin generation by HUVEC. This enhancement of PGI2 generation resulted from increased release of Ca2+ from the storage sites, due not only to increased production of inositol 1,4,5-triphosphate (IP3) but also to the increase of [pH]i. These findings may help to explain the antiatherosclerotic action of probucol.

Regulation of intracellular pH in rat lactotrophs: involvement of anionic exchangers

Regulation of the intracellular pH (pHi) of normal rat lactotrophs was studied. As this cell type, cultured with 10% FCS, can achieve a relatively alkaline pHi (7.3-7.5), we investigated the presence of a mechanism based on Cl-/HCO3- exchange. Using the pHi-sensitive probe SNARF-1 (seminaphtorodafluor) in its permeant form, SNARF-1/AM, we studied pHi recovery after acidic loading in individual cells with a microspectrofluorometric approach. We showed the involvement of anionic exchange in lactotroph cell pHi regulation. Acute CO2-bicarbonate cell acidic loading combined with external Cl- depletion induces the activation of a Cl-/HCO3- exchange. This exchange is 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid sensitive and corresponds to the type 3 anionic exchanger (AE3). However, after nigericin acidification, Na+/H+ exchange can also participate in recovery. In addition, incubation experiments strongly suggest that a 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-insensitive anionic exchanger (type 2 anionic exchanger or AE2) is present in rat lactotrophs. The presence and involvement of carbonic anhydrase in pHi regulation have been demonstrated. Finally, using Northern blot and reverse transcription-PCR techniques, messenger RNAs for both AE2 and AE3 were identified in anterior pituitary cell extracts. We concluded that in normal rat lactotrophs, pHi regulation is achieved by a complex system in which Cl-/HCO3- exchange has a pivotal role.

Sphingosylphosphorylcholine activates an amiloride-insensitive Na+-H+-exchange mechanism in GH4C1 cells

The effect of sphingosylphosphorylcholine (SphPCho) on the intracellular pH (pHi) in GH4C1 cells was investigated. SphPCho evoked a very slow increase in basal pHi. In cells acidified with nigericin, SphPCho induced a rapid alkalinization of the cells. The effect was inhibited in a Na+-free buffer solution, but was insensitive to ethylisopropyl amiloride, a potent inhibitor of Na+-H+ exchangers (NHE). Reverse transcription and PCR showed that the predominant isoform of the antiport expressed in GH4C1 cells is NHE-1. The rate of alkalinization after stimulation with propionate, and after addition of Na+ to cells acidified with NH4Cl, was enhanced in cells treated with SphPCho. The initial rate of alkalinization after addition of Na+ to acidified cells treated with SphPCho gave an apparent Km value of 15 +/- 2 mM for Na+. The Vmax value was 9 +/- 2 mM H+/min. The effect was insensitive to ouabain, staurosporine and bafilomycin A. However, the SphPCho-evoked alkalinization was abolished in cells treated with 2-deoxy-D-glucose. The effect was not due to the charge of the molecule, as stearylamine increased pHi in Na+-containing and Na+-free buffer. The results show that SphPCho may activate Na+-H+ exchange, and that this effect is mediated via an amiloride-insensitive exchange mechanism.

Effects of high-density lipoproteins on intracellular pH and proliferation of human vascular endothelial cells

We investigated the effects of high-density lipoprotein (HDL) on the intracellular pH ([pH]i), and on the proliferation of human vascular endothelial cells (HUVEC), as well as on their production of prostacyclin (PGI2). The [pH]i was slightly acidified when extracellular Ca2+ was chelated with EGTA. Pretreatment of HUVEC with amiloride, the Na+/H+ exchange inhibitor, caused the [pH]i to become strongly acidic. The addition of HDL produced a biphasic shift in [pH]i, with a brief initial acidification followed by a rapid alkaline shift. The initial decrease in [pH]i was abolished in the cells pretreated with EGTA, and subsequent alkalinization was inhibited. The alkalinization of [pH]i disappeared in the cells pretreated with amiloride. These results suggest that [pH]i depends mainly on Na+/H+ exchange and partially on the extracellular Ca2+ of the HUVEC either in the resting unstimulated state or during HDL stimulation. In contrast, the addition of LDL produced an acidification of [pH]i, which was increased by LDL in the Ca(2+)-free condition. In the cells pretreated with amiloride, [pH]i was not further acidified by LDL. As a result, HDL promoted the proliferation of cells, an action that was inhibited by pretreatment with EGTA. However LDL inhibited cell proliferation, an action unaffected by EGTA pretreatment. The addition of HDL also enhanced the generation of prostacyclin in endothelial cells, the enhancement of PGI2 generation resulted from an increase in the release of Ca2+ from storage sites, due not only to an increased production of inositol 1,4,5-trisphosphate (IP3), but also to the alkalinization of [pH]i. These effects may be involved in the mechanism of HDL's anti-atherosclerotic action.

Role of the Na(+)-Ca2+ and Na(+)-H+ antiporters in prolactin release from anterior pituitary cells in primary culture

2',4'-dimethylbenzamilamiloride (DMB), a somewhat selective inhibitor of the Na(+)-Ca2+ exchanger, in concentrations of 10, 30 and 100 microM did not produce any significant effect on baseline prolactin release from anterior pituitary cells in primary culture. When prolactin secretion was stimulated by the inhibitor of the Na(+)-K(+)-ATPase, ouabain, that activates the Na(+)-Ca2+ exchanger as a Ca(2+)-influx pathway, DMB was able to produce inhibition of prolactin secretion. 5-(N,N-hexamethylene) amiloride (HMA), another amiloride analog which specifically inhibits the Na(+)-H+ antiporter and has no inhibitory activity on the Na(+)-Ca2+ exchanger, at the concentrations of 0.1, 1 and 10 microM, did not affect basal prolactin release whereas it significantly reduced prolactin release stimulated by thyrotropin releasing hormone (TRH) (1 microM). These results suggest that the Na(+)-Ca2+ antiporter is involved in the process of prolactin release elicited by the inhibition of the Na(+)-K(+)-ATPase whereas the Na(+)-H+ antiporter is involved in the prolactin secretion elicited by TRH.

Influence of Hepes- and CO2/HCO(3-)-buffer on Ca2+ transients induced by TRH and elevated K+ in rat pituitary GH4C1 cells

The influence of two buffer systems (Hepes and CO2/HCO3-) on intracellular Ca2+ ([Ca2+]i) transients evoked by TRH and by elevated K+ were studied in single, and small clusters of, clonal rat pituitary GH4C1 cells using Fura 2. The steady-state level of [Ca2+]i was virtually identical in Hepes and CO2/HCO3-. In both buffers, addition of TRH induced a transient increase in [Ca2+]i which attained a significantly higher peak in Hepes (357 +/- 43 nM) when compared with values measured in the presence of CO2/HCO3- (184 +/- 21 nM). In Hepes, the basal IP3-level was higher than in CO2/HCO3-. The TRH-evoked increase in IP3 was higher in magnitude in Hepes than in CO2/HCO3-, although the stimulated/basal ratio was not different between the two buffers. The buffer composition had no effect on the specific binding of 3H-TRH to the cells. Furthermore, the amplitude of the increase in [Ca2+]i evoked by 50 mM K+ was identical in both buffers. TRH and K+ had no effect on pHi in either buffer. The present results indicate that HCO3- has an influence on TRH-induced Ca2+ transient, at least in part by modifying the TRH-evoked production of IP3.

Simultaneous measurements of cytosolic pH and calcium interactions in bovine lactotrophs using optical probes and four-wavelength quantitative video microscopy

The properties of pH and calcium homeostasis have been investigated in single bovine lactotrophs by the use of the fluorescent indicators 2',7'-bis(carboxyethyl)-carboxyfluorescein (BCECF) and fura-2 respectively. A method of simultaneous recording from both dyes loaded in the same cell was used. Despite slight crosstalk between the two dyes, physiologically relevant information about the interrelationship between pH and calcium homeostasis was obtained. Three types of interactions were recorded. First, an increase in calcium due to the discharge of intracellular stores by thyrotropin-releasing hormone resulted in no change in cytosolic pH. Secondly, alkalinization by the addition of a weak base, NH4Cl, induced a large transient (around 1000 nM) and a small (a few tens of nanomoles per liter) sustained increase in cytosolic calcium. The former is partly due to release from agonist-sensitive stores. Thirdly, upon the removal of NH4Cl the cytoplasm became acidic, which induced a release of calcium from intracellular stores in some cells. In addition we demonstrate that the recovery from acid load is sensitive to extracellular Na+, suggesting the presence of Na+/H+ exchange mechanisms in bovine lactotrophs. Interestingly we have also found that, at rest, removal of Na+ from the bathing medium results in a decrease in resting [Ca2+]i, paralleled by a reduction in pHi. This suggests a role for Na+/H+ exchange in determining resting [Ca2+]i.

Intracellular free sodium concentrations in GH4C1 cells

In the present investigation, intracellular sodium ([Na+]i) levels were determined in GH4C1 cells using the fluorescent probe SBFI. Fluorescence was determined by excitation at 340 nm and 385 nm, and emission was measured at 500 nm. Intracellular free sodium ([Na+]i) was determined by comparing the ratio 340/385 to a calibration curve. The ratio was linear between 10 and 60 mM Na+. Resting [Na+]i in GH4C1 cells was 26 +/- 6.2 mM (mean +/- SD). In cells incubated in Na(+)-free buffer [Na+]i decreased to 3 +/- 3.6 mM. If Na+/K+ ATPase was inhibited by incubating the cells with 1 mM ouabain, [Na+]i increased to 47 +/- 12.8 mM in 15 min. Stimulating the cells with TRH, phorbol myristyl acetate, or thapsigargin had no effect on [Na+]i. Incubating the cells in Ca(2+)-free buffer rapidly increased [Na+]i. The increase was not inhibited by tetrodotoxin. Addition of extracellular Ca2+, nimodipine, or Ni2+ to these cells immediately decreased [Na+]i, whereas Bay K 8644 enhanced the influx of Na+. In cells where [Na+]i was increased the TRH-induced increase in intracellular free calcium ([Ca2+]i) was decreased compared with control cells. Our results suggest that Na+ enters the cells via Ca2+ channels, and [Na+]i may attenuate TRH-induced changes in [Ca2+]i in GH4C1 cells.

Characterization of the TRH-induced activation of Na+/H(+)-exchange in pituitary GH4C1 cells

In the present study in GH4C1 cells, the dependence of TRH-induced activation of Na+/H(+)-exchange on extracellular Na+ and Ca2+ was examined. Furthermore, the effects of both extracellular and intracellular H+ on Na+/H(+)-exchange were investigated. The buffering capacity was 63 +/- 11.8 mM (pH unit)-1 at basal intracellular pH (pHi) of 7.02 +/- 0.02. The initial rate of alkalinization in cells acidified with nigericin increased with increasing concentrations of extracellular Na+ according to simple Michaelis-Menten kinetics. The apparent Km-value for Na+ was 53 +/- 17.5 mM and the Vmax value was 28 +/- 4.5 mM H+/min. Addition of Na+ together with TRH increased Vmax to 56 +/- 6.4 mM H+/min (P < 0.05), while no difference was observed in Km. Decreasing extracellular pH (pHo) decreased the rate of alkalinization of acid-loaded cells, despite a large inward Na+ gradient. Furthermore, a decrease in pHi was necessary to obtain activation of Na+/H+ exchange. At pHi-values close to basal pHi no activation of Na+/H(+)-exchange was obtained. In addition, the results showed that extracellular Ca2+ was necessary for TRH-induced activation of Na+/H+ exchange. Blocking influx of extracellular Ca2+ with Ni2+ abolished the effect of TRH, suggesting that the TRH-induced activation of Na+/H(+)-exchange in GH4C1 cells is dependent on influx of extracellular Ca2+.

Characterization of Na+/H+ exchange in FRTL-5 thyroid cells. Evidence for dependence on activation of protein kinase C

Na+/H+ exchange activity was investigated in cultured rat thyroid follicular FRTL-5 cells using the pH sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Basal intracellular pH (pHi) was 7.13 +/- 0.10 in cells incubated in Hepes-buffered saline solution. The intracellular buffering capacity beta i was determined using the NH4Cl-pulse method, yielding a beta i value of 85 +/- 12 mM/pH unit. The relationship between extracellular Na+ and the initial rate of alkalinization of acid-loaded cells showed simple saturation kinetics, with an apparent Km value of 44 +/- 26 mM, and an Vmax value of 0.3 +/- 0.01 pH unit/min. The agonist-induced activation of Na+/H+ exchange was investigated in cells acidified with nigericin. Addition of 12-O-tetradecanoylphorbol 13-acetate (TPA) or ATP induced rapid cytosolic alkalinization in acid-loaded cells. The action of both TPA and ATP was abolished by preincubating the cells with 100 microM amiloride, by substituting extracellular Na+ with equimolar concentrations of choline+, and by pretreating the cells with TPA for 24 h. Chelating extracellular Ca2+, or depleating intracellular Ca2+ pools did not affect the ATP-induced alkalinization. The results indicate, that FRTL-5 cells have a functional Na+/H+ exchange mechanism. Furthermore, stimulation of protein kinase C activity is of importance in activating the antiport.

A depolarization can trigger Ca2+ uptake and the acrosome reaction when preceded by a hyperpolarization in L. pictus sea urchin sperm

The acrosome reaction (AR) is an exocytotic event that allows sperm to recognize and fuse with the egg. In the sea urchin sperm this reaction is triggered by the outer investment of the egg, the jelly, which induces ionic movements leading to increases in intracellular Ca2+ ([Ca2+]i) and intracellular pH (pHi), a K(+)-dependent transient hyperpolarization which may involve K+ channels, and a depolarization which depends on external Ca2+. The present paper explores the role of the hyperpolarization in the triggering of the acrosome reaction. The artificial hyperpolarization of Lytechinus pictus sperm with valinomycin in K(+)-free seawater raised the pHi, caused a small increase in 45Ca2+ uptake, and triggered some AR. When the cells were depolarized with KCl (30 mM) 40-60 sec after the induced hyperpolarization, the pHi decreased and there was a significant increase in 45Ca2+ uptake, [Ca2+]i, and the AR. This waiting time was necessary in order to allow the pHi change required for the AR to occur. Thus, the jelly-induced hyperpolarization may lead to the intracellular alkalinization required to trigger the AR, and, on its own or via pHi, may regulate Ca2+ transport systems involved in this process. Because of the key role played by K+ in the triggering of the AR, the presence and characteristics of ion channels in L. pictus isolated sperm plasma membranes are being explored. Planar lipid bilayers into which these membranes were incorporated by fusion displayed 85 pS single channel transitions which were cation selective.

Intracellular pH in individual pituitary cells: measurement with a dual emission pH indicator

Intracellular pH (pHi) can now be measured at the single cell level using dual emission wavelength microspectrofluorimetry with the fluorescent pH indicator SNARF 1 and its membrane permeant acetoxymethyl ester (SNARF 1/AM). We measured pHi of individual pituitary cells under both basal and stimulated conditions. The emitted fluorescence of SNARF 1 probe was calibrated following experimental manipulations of pHi in two types of rat pituitary cells. The calibration curves obtained in the two cell types were identical. We observed a Gaussian distribution of individual pHi with a wide dispersion (6.95 to 8) in the two cell populations. TRH (10(-7) M) and ionomycin (5 microM) induced a transient acidification followed by a sustained alkalinization, whereas K+ (50 mM) depolarization only exerted a transient acidification. These results show that the dual emission pH indicator SNARF 1 can be used to reliably investigate changes in pHi in individual endocrine cells.

Priming effect of hyperosmotic stress on TRH-induced activation of Na+/H+ exchange in GH4C1 rat pituitary cells

The effect of mild hyperosmotic stress on cytosolic pH (pHi) alone, and in combination with thyrotropin-releasing hormone (TRH) or the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) was investigated in GH4C1 cells at resting pHi. Hyperosmotic stress induced by addition of 50 mM choline was without an effect on pHi. In cells stimulated with either TRH or TPA after choline, pHi increased 0.15 +/- 0.05 and 0.14 +/- 0.03 pH units, respectively (mean +/- SD). A similar response was obtained if TRH or TPA was added prior to choline. The effect was abolished by replacing extracellular Na+ with choline+, and by pretreatment of the cells with amiloride, indicating that the change in pHi probably was dependent on activation of Na+/H+ exchange. The results thus indicate that, in GH4C1 cells, hyperosmotic stress in combination with TRH or TPA can activate Na+/H+ exchange at resting pHi levels.

The regulation of the intracellular pH in cells from vertebrates

Eukaryotic cells control their intracellular pH using ion-transporting systems that are situated in the plasma membrane. This paper describes the different mechanisms that are involved and how their activity is regulated.