Caffeine-induced oscillations of cytosolic Ca2+ in GH3 pituitary cells are not due to Ca2+ release from intracellular stores but to enhanced Ca2+ influx through voltage-gated Ca2+ channels

Nuclear calcium signaling by inositol trisphosphate in GH3 pituitary cells

It has been proposed that nuclear and cytosolic Ca(2+) ([Ca(2+)](N) and [Ca(2+)](C)) may be regulated independently. We address here the issue of whether inositol trisphosphate (IP(3)) can, bypassing changes of [Ca(2+)](C), produce direct release of Ca(2+) into the nucleoplasm. We have used targeted aequorins to selectively measure and compare the changes in [Ca(2+)](C) and [Ca(2+)](N) induced by IP(3) in GH(3) pituitary cells. Heparin, an IP(3) inhibitor that does not permeate the nuclear pores, abolished the [Ca(2+)](C) peaks but inhibited only partly the [Ca(2+)](N) peaks. The permeant inhibitor 2-aminoethoxy-diphenyl-borate (2-APB) blocked both responses. Removal of ATP also inhibited more strongly the [Ca(2+)](C) than [Ca(2+)](N) peak. The [Ca(2+)](N) and [Ca(2+)](C) responses differed also in their sensitivity to IP(3), the nuclear response showing higher affinity. Among IP(3) receptors, type 2 (IP(3)R2) has a higher affinity for IP(3) and is not inactivated by ATP removal. We find that IP(3)R2 immunoreactivity is present inside the nucleus whereas the other IP(3)R subtypes are detected only in the cytoplasm. The nuclear envelope (NE) of GH(3) cells showed deep invaginations into the nucleoplasm, with cytosol and cytoplasmic organella inside. These results indicate that GH(3) pituitary cells possess mechanisms able to produce selective increases of [Ca(2+)](N).

Dampening of cytosolic Ca2+ oscillations on propagation to nucleus

Ca(2+) signals may regulate gene expression. The increase of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) promotes activation and/or nuclear import of some transcription factors, but others require the increase of the nuclear Ca(2+) concentration ([Ca(2+)](N)) for activation. Whether the nuclear envelope may act as a diffusion barrier for propagation of [Ca(2+)](c) signals remains controversial. We have studied the spreading of Ca(2+) from the cytosol to the nucleus by comparing the cytosolic and the nuclear Ca(2+) signals reported by targeted aequorins in adrenal chromaffin, PC12, and GH(3) pituitary cells. Strong stimulation of either Ca(2+) entry (by depolarization with high K(+) or acethylcholine) or Ca(2+) release from the intracellular Ca(2+) stores (by stimulation with caffeine, UTP, bradykinin, or thyrotropin-releasing hormone (TRH)) produced similar Ca(2+) signals in cytosol and nucleus. In contrast, both spontaneous and TRH-stimulated oscillations of cytosolic Ca(2+) in single GH(3) cells were considerably dampened during propagation to the nucleus. These results are consistent with the existence of a kinetic barrier that filters high frequency physiological [Ca(2+)](c) oscillations without disturbing sustained [Ca(2+)](c) increases. Thus, encoding of the Ca(2+) signal may allow differential control of Ca(2+)-dependent mechanisms located at either the cytosol or the nucleus.

Modulation of secretion by the endoplasmic reticulum in mouse chromaffin cells

The endoplasmic reticulum (ER) has been suggested to modulate secretion either behaving as a Ca2+ sink or as a Ca2+ source in neuronal cells. Working as a Ca2+ sink, through ER-Ca2+ pumping, it may reduce secretion induced by different stimuli. Instead, working as a Ca2+ source through the Ca2+ induced Ca2+ release (CICR) phenomenon, it may potentiate secretion triggered by activation of plasma membrane Ca2+ channels. We have previously demonstrated the presence of CICR in bovine chromaffin cells, but we now find that mouse chromaffin cells almost lack functional caffeine-sensitive ryanodine receptors in the ER and, consistently, no CICR from the ER could be observed. In addition, inhibition of ER Ca2+ pumping with ciclopiazonic acid or thapsigargin strongly stimulated high-K+-evoked catecholamine secretion and cytosolic [Ca2+] ([Ca2+]c) transients. Surprisingly, 5 mm caffeine reduced high-K+-induced [Ca2+]c peaks but considerably potentiated secretion induced by high-K+ stimulation. However, this potentiation was insensitive to ryanodine and additive to that induced by emptying the ER of Ca2+ with thapsigargin, suggesting that it is unrelated to the activation of ryanodine receptors. We conclude that, in mouse chromaffin cells, CICR is not functional and the ER strongly inhibits secretion by acting as a damper of the [Ca2+]c signal.

Correlation between electrical activity and intracellular Ca2+ oscillations in GH3 rat anterior pituitary cells

Simultaneous measurements of electrical activity and intracellular Ca(2+) levels were performed in perforated-patch current-clamped individual GH3 cells. Both in cells showing brief (<100 ms) and long action potentials (APs), we found a good correlation between the averaged intracellular Ca2+ concentration ([Ca2+]i) and AP frequency, but not between the mean [Ca2+]i and AP duration. Nevertheless, the magnitude of spontaneous Ca2+ oscillations was highly dependent on the size and duration of the APs. The decay of the Ca2+ transients was not slowed when the size of the oscillations was varied either spontaneously or after elongation of the AP with the K+ channel blocker tetraethyl ammonium. Furthermore, the recovery from Ca2+ loads similar to those induced by the APs was slightly retarded after treatment of the cells with intracellular store Ca2+-ATPase inhibitors. Among previous results showing that caffeine-induced [Ca2+]i increases are secondary to electrical activity enhancements in GH3 cells, these data indicate that the Ca2+ entry triggered via APs is the primary determinant of the [Ca2+]i variations, and that Ca2+-induced Ca2+ release has a minor contribution to Ca2+ oscillations recorded during spontaneous activity. They also point to modulation of electrical activity patterns as a crucial factor regulating spontaneous [Ca2+]i signalling, and hence pituitary cell functions in response to physiological secretagogues.

Mitochondrial [Ca(2+)] oscillations driven by local high [Ca(2+)] domains generated by spontaneous electric activity

Mitochondria take up calcium during cell activation thus shaping Ca(2+) signaling and exocytosis. In turn, Ca(2+) uptake by mitochondria increases respiration and ATP synthesis. Targeted aequorins are excellent Ca(2+) probes for subcellular analysis, but single-cell imaging has proven difficult. Here we combine virus-based expression of targeted aequorins with photon-counting imaging to resolve dynamics of the cytosolic, mitochondrial, and nuclear Ca(2+) signals at the single-cell level in anterior pituitary cells. These cells exhibit spontaneous electric activity and cytosolic Ca(2+) oscillations that are responsible for basal secretion of pituitary hormones and are modulated by hypophysiotrophic factors. Aequorin reported spontaneous [Ca(2+)] oscillations in all the three compartments, bulk cytosol, nucleus, and mitochondria. Interestingly, a fraction of mitochondria underwent much larger [Ca(2+)] oscillations, which were driven by local high [Ca(2+)] domains generated by the spontaneous electric activity. These oscillations were large enough to stimulate respiration, providing the basis for local tune-up of mitochondrial function by the Ca(2+) signal.

Ca2+-induced Ca2+ release in chromaffin cells seen from inside the ER with targeted aequorin

The presence and physiological role of Ca2+-induced Ca2+ release (CICR) in nonmuscle excitable cells has been investigated only indirectly through measurements of cytosolic [Ca2+] ([Ca2+]c). Using targeted aequorin, we have directly monitored [Ca2+] changes inside the ER ([Ca2+]ER) in bovine adrenal chromaffin cells. Ca2+ entry induced by cell depolarization triggered a transient Ca2+ release from the ER that was highly dependent on [Ca2+]ER and sensitized by low concentrations of caffeine. Caffeine-induced Ca2+ release was quantal in nature due to modulation by [Ca2+]ER. Whereas caffeine released essentially all the Ca2+ from the ER, inositol 1,4, 5-trisphosphate (InsP3)- producing agonists released only 60-80%. Both InsP3 and caffeine emptied completely the ER in digitonin-permeabilized cells whereas cyclic ADP-ribose had no effect. Ryanodine induced permanent emptying of the Ca2+ stores in a use-dependent manner after activation by caffeine. Fast confocal [Ca2+]c measurements showed that the wave of [Ca2+]c induced by 100-ms depolarizing pulses in voltage-clamped cells was delayed and reduced in intensity in ryanodine-treated cells. Our results indicate that the ER of chromaffin cells behaves mostly as a single homogeneous thapsigargin-sensitive Ca2+ pool that can release Ca2+ both via InsP3 receptors or CICR.

Effects of cellular interactions on calcium dynamics in prolactin-secreting cells

Signals derived from other pituitary cells can have a dramatic effect on PRL gene expression and secretion by mammotropes. However, the intracellular mechanisms by which these effects are manifested on the target cell remain unexplored. Inasmuch as calcium is a key modulator of both gene expression and hormone export in mammotropes, we evaluated the effects of cell to cell contact vs. specific cellular interactions on calcium dynamics within these cells. This was accomplished by digital-imaging fluorescence microscopy of fura-2 in pituitary cells that were isolated in culture (singles) or adjoining one other cell (doublets). After calcium imaging, we then subjected cells to immunocytochemistry for PRL. Doublets were further categorized into mammotropes attached to another mammotrope (M-M) or to a nonmammotrope (M-nonM). We then calculated and compared Mean [Ca2+]i values as well as Oscillation Indices (which reflect the oscillatory behavior of cells) in singles and doublets and found that they were not different (P > 0.05). However, the phenotype of the adjoining cell had a profound influence on both of these calcium parameters, such that the presence of one mammotrope could consistently decrease (P < 0.05) the Mean [Ca2+]i value (39.17 +/- 3.83 vs. 56.24 +/- 5.56 in M-nonM) and Oscillation Index (10.19 +/- 1.76 vs. 21.21 +/- 3.73 in M-nonM) of its neighboring counterpart. A more detailed analysis of oscillatory patterns in these cells revealed that nonoscillators were more abundant in M-M (23%) than in M-nonM (12%) doublets. Taken together, our results indicate that PRL-secreting cells convey a signal that dampens the oscillatory behavior of neighboring mammotropes. Thus, it appears that it is the phenotype rather than the physical presence of a neighbor that controls intercellular regulation of calcium dynamics among mammotropes.

alpha-MSH potentiates the responsiveness of mammotropes by increasing Ca2+ entry

It is well known that the suckling stimulus renders mammotropes considerably more responsive to prolactin (PRL)-releasing stimuli, and the neurointermediate lobe peptide alpha-melanocyte-stimulating hormone (alpha-MSH) has been proposed to play a pivotal role in this priming. The objectives of the present study were to determine whether alpha-MSH could act directly on pituitary cells to potentiate PRL release in response to two physiologically relevant PRL secretagogues, thyrotropin-releasing hormone (TRH) and ATP, and, if so, to identify the mechanism by which this priming phenomenon is manifested. To this end, we cultured anterior pituitary cells from lactating rats overnight and then subjected them to a reverse hemolytic plaque assay for PRL to evaluate their responses to various test agents. We found that alpha-MSH, which had no effect on PRL export when tested alone, augmented by more than threefold the secretory responses to TRH and ATP. Next, we utilized digital-imaging fluorescence microscopy of fura 2 to evaluate the role of intracellular Ca2+ in this process. We found that PRL export induced by pharmacological activation of L-type voltage-operated calcium channels was also potentiated by alpha-MSH, as was Ca2+ entry induced by TRH. Our results indicate that alpha-MSH acts as a mammotrope-priming agent on a subset of mammotropes by increasing Ca2+ entry induced by PRL secretagogues.

Functional ATP receptors in rat anterior pituitary cells

The effects of ATP and other nucleotides on the cytosolic Ca2+ concentration ([Ca2+]i) of single immunocytochemically typed anterior pituitary (AP) cells have been studied. ATP increased [Ca2+]i in a large percentage (60-88%) of all five AP cell types: lactotropes, somatotropes, corticotropes, gonadotropes, and thyrotropes. Additivity experiments suggest the presence of at least two different receptors, one accepting both ATP and UTP (U receptor), producing Ca2+ release from the intracellular stores, and the other preferring ATP (A receptor), producing Ca2+ (and Mn2+) entry. The characteristics of the U and A receptors were consistent with those of P2Y2 and P2X2, respectively, and their distribution in the different AP cell types was not homogeneous. The presence of other ATP receptors such P2Y1 or P2X2/P2X3 heteropolymers in a small fraction of the cells cannot be excluded. Thus functional ionophoric P2X receptors, which are typical of neural tissue, are also present in the pituitary gland and could contribute to regulation of the gland's function.

Mechanisms involved in ATP-evoked Ca2+ oscillations in isolated human granulosa-luteal cells

Using single-cell microfluorimetry, we have shown that ATP evoked repetitive Ca2+ oscillations in intact Fura-2 loaded human granulosa-luteal cells (hG/LCs) in the absence of extracellular Ca2+. Sustained increases in [Ca2+]i required extracellular Ca2+ and ATP depleted stores were refreshed by brief (2 min) incubation with external Ca2+. Basal [Ca2+]i was unaffected by caffeine (1 mM), but 20 mM caffeine inhibited ATP-evoked Ca2+ release in the absence of external Ca2+. Thimerosal (10 microM) evoked repetitive Ca2+ spikes, under Ca(2+)-free conditions, which fused to form an elevated plateau when external Ca2+ was replaced. Thimerosal-induced changes in [Ca2+]i were reversibly inhibited by the thiol reducing agent dithiothreitol (1 mM). The periodicity and amplitude of the [Ca2+]i oscillations produced by thimerosal and ATP differ. ATP- or thimerosal-evoked changes in [Ca2+]i were unaffected by dantrolene sodium (10 microM). The Ca(2+)-ATPase inhibitor thapsigargin (1 microM) increased [Ca2+]i and attenuated subsequent ATP-evoked changes in [Ca2+]i. We conclude that ATP stimulates an oscillatory release of Ca2+ from InsP3-sensitive stores in hG/LCs.

Effects of caffeine on the influx of extracellular calcium in GH4C1 pituitary cells

Caffeine increases intracellular Ca2+ concentrations ([Ca2+]i) in a variety of cell types by triggering the mobilization of Ca2+ from intracellular Ca2+ stores. Caffeine also can change [Ca2+]i by affecting Ca2+ influx through voltage-operated Ca2+ channels (VOCCs). In the present study, we investigated the effects of caffeine on Ca2+ entry in GH4C1 pituitary cells. Pretreatment of the cells with caffeine attenuated the high K+-evoked influx of 45Ca2+ in a dose-dependent manner. This inhibition was not secondary to the caffeine-evoked elevation of [Ca2+]i because caffeine was able to inhibit VOCCs also in the presence of the intracellular Ca2+ chelator BAPTA. However, the inhibitory effect of caffeine on 45Ca2+ entry appeared to be dependent on the degree of depolarization of the plasma membrane. Only in cells depolarized with relatively high concentrations of K+ (20, 35, and 50 mM) was the caffeine-induced inhibition observed. A similar inhibitory effect of caffeine on the high K+-evoked calcium and barium entry was observed in experiments using Fura 2. Neither IBMX, forskolin nor dibutyryl cAMP reduced the enhanced [Ca2+]i induced by 50 mM K+, suggesting that the effect of caffeine was not due to increased intracellular cAMP. Furthermore, high doses of caffeine inhibited the plateau level of the TRH-induced increase in [Ca2+]i, which is caused partly by influx of Ca2+ through VOCCs. The inhibitory effect of caffeine was, in part, due to an hyperpolarization of the plasma membrane observed at high doses of caffeine. On the other hand, low doses of caffeine enhanced depolarization-evoked Ba2+ entry as well as the TRH-evoked plateau level of [Ca2+]i. We conclude that caffeine has a dual effect on Ca2+ entry through activated VOCCs in GH4C1 cells: at low concentrations caffeine enhances Ca2+ entry, whereas high concentrations of caffeine block Ca2+ entry.

Phospholipase A2-dependent and -independent pathways of arachidonate release from vascular smooth muscle cells

[Arg8]vasopressin (AVP), through its V1 receptor coupled to GTP-binding proteins, and aluminum fluoride (AlF4-), which directly activates GTP-binding proteins, induced the release of [3H]arachidonate from prelabeled A7r5 vascular smooth muscle-like cells. Using fura-2-loaded cells, we observed that the release induced by AVP occurred concurrently with calcium (Ca2+) mobilization from internal stores and entry of external Ca2+, whereas AlF4(-)-dependent arachidonate release was much slower and was not accompanied by intracellular Ca2+ mobilization. Arachidonate transfer from phosphatidylcholine to phosphatidylethanolamine was an early event for both agonists, but phosphatidylinositol hydrolysis was an early event for AVP-stimulated cells and a late event for cells triggered with AlF4-. In addition, phospholipase inhibitors had no effect on arachidonate release induced by AlF4-. We investigated the enzymatic pathways involved in the releases of arachidonate, which occur in such different ways. Phospholipase A2 activities were assayed in a cell-free system with various substrates, which made it possible to differentiate between cytosolic, secretory and Ca2(+)-independent phospholipases A2. The specific activities were in the order alkenyl-AA-GPE > acyl-AA-GPE > acyl-AA-GPC in the presence of Ca2+. No significant activity was observed in the presence of Ca2+ chelators and when dipalmitoyl-glycerophosphocholine was used as a substrate. Phospholipase A2 activities did not change in homogenates from stimulated cells related to control cells. However, phospholipase A2 activity increased in membrane fractions from AVP-stimulated cells. Imunodetected phosphorylated and unphosphorylated forms of cytosolic phospholipase A2 (cPLA2) also clearly increased in the membrane fractions of AVP-stimulated cells, and only the unphosphorylated form of cPLA2 was present in AlF4(-)-triggered cells. We conclude that phospholipase C and translocation of cPLA2 can account for arachidonate release with AVP stimulation, whereas neither phospholipase C nor any phospholipase A2 activity appears to be implicated in AlF4(-)-dependent arachidonate release.

Caffeine enhancement of electrical activity through direct blockade of inward rectifying K+ currents in GH3 rat anterior pituitary cells

Treatment of rat anterior pituitary GH3 cells with caffeine causes a reversible enhancement of electrical activity superimposed over a depolarization of the plasma membrane potential. Similar results are obtained with theophylline, but not with isobutylmethylxanthine or forskolin. The effects of caffeine are not related to Ca2+ liberation from intracellular stores since they are not affected by incubation of the cells with ryanodine or thapsigargin. Furthermore, caffeine-induced hyperpolarization of the membrane is not detectable even in cells in which Ca2+ liberation from inositol 1,4,5-trisphosphate-sensitive compartments produces a prominent transient hyperpolarization in response to thyrotropin-releasing hormone. Reductions of Ca2+-dependent K+ currents caused by partial block of L-type Ca2+ channels by caffeine are not sufficient to explain the effects of the xanthine, since the results obtained with caffeine are not mimicked by direct blockade of Ca2+ channels with nisoldipine. GH3 cell inwardly rectifying K+ currents are inhibited by caffeine. Studies on the voltage dependence of the caffeine-induced effects indicate a close correlation between alterations of electrical parameters and reported values of steady-state voltage dependence of inactivation of these currents. We conclude that, as previously shown for thyrotropin-releasing hormone, modulation of inwardly rectifying K+ currents plays a major role determining the firing rate of GH3 cells and its enhancement by caffeine.