Synergistic effect of prostaglandin E2 and ouabain on catecholamine release from cultured bovine adrenal chromaffin cells

[Possible role of a neuropeptide PACAP (pituitary adenylate cyclase-activating polypeptide) on stimulus-secretion coupling in catecholamine neuron]

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide first isolated from ovine hypothalamic tissue. This peptide stimulates adenylate cyclase activation. However, few details were known of the function of this peptide on stimulus-secretion coupling in neuronal cells. The authors have investigated the role of PACAP on catecholamine biosynthesis and secretion using cultured bovine adrenal chromaffin cells as a model for catecholamine-containing neurons. PACAP38, the 38-amino acid form of PACAP, increased cAMP formation in bovine adrenal chromaffin cells. In addition, PACAP38 increased [Ca2+]i associated with PI turnover and Ca2+ influx into the cells. The synthesis of catecholamine and the phosphorylation of tyrosine hydroxylase, a rate-limiting enzyme of catecholamine biosynthesis, stimulated by the maximal effective concentration of dibutyryl cAMP or a high concentration (56 mM) of K+ were further enhanced by PACAP38. Thus PACAP38 stimulated the pathway of catecholamine biosynthesis mainly by both activation of cAMP- and Ca2(+)-dependent protein kinases. On catecholamine secretion from the cells, the effect of PACAP38 was markedly potentiated by addition of ouabain, an inhibitor of Na+/K+ ATPase. This markedly potentiated secretion was greatly reduced with Na+ omitted-sucrose medium. PACAP38 increased 22Na+ influx into the cells treated with ouabain. Thus PACAP38 with ouabain stimulated catecholamine secretion by accumulation of intracellular Na+, resulting in an increase in Ca2+ influx. These results indicate that the neuropeptide PACAP has an important role in stimulus-secretion coupling in adrenal chromaffin cells.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Potentiation by indomethacin of receptor-mediated catecholamine secretion in rat adrenal medulla

Effects of indomethacin on catecholamine secretion evoked by receptor agonists, muscarine, bradykinin or histamine, in rat adrenal chromaffin cells were studied. Indomethacin at 200 microM increased a sustained component of secretion during stimulation with muscarine, bradykinin and histamine by a factor of 2.3, 2.1 and 2.9, respectively, whereas it did not significantly alter basal, high-K(+)- and nicotine-evoked secretions. Although indomethacin at above 400 microM dose-dependently increased basal secretion, the amount of secretion induced by indomethacin alone was much smaller than that in muscarine-evoked secretion as compared at the same concentration of indomethacin applied. Bradykinin-evoked secretion and its potentiation by indomethacin were not inhibited by 20 microM nifedipine but were suppressed by 0.5 mM Ni2+. The cyclooxygenase inhibitor, ibuprofen (200 microM) did not mimic the effect of indomethacin; prostaglandin E2 (20 microM) and arachidonic acid (100 microM) did not significantly alter either bradykinin-evoked secretion itself or its potentiation by indomethacin. Bradykinin increased the intracellular free Ca2+ concentration, [Ca2+]i, in cells loaded with indo-1, and this response was enhanced in the presence of indomethacin. These results suggest that indomethacin may promote Ca2+ entry to potentiate agonist-evoked catecholamine secretions through a novel action that is not directly related to the inhibition of cyclooxygenase activity with indomethacin.

Potentiation of histamine-induced catecholamine secretion by ouabain in cultured bovine adrenal chromaffin cells is dependent on calcium and sodium influx

The effects of histamine on catecholamine secretion from cultured bovine adrenal chromaffin cells were studied in the presence of ouabain, an inhibitor of Na+-K+ ATPase. The purpose of this study was to determine whether Na+, as well as Ca2+, was involved in histamine receptor-mediated catecholamine secretion. Histamine (10(-8)-10(-5) M)-induced catecholamine secretion was markedly potentiated by addition of ouabain (10(-5) M) and was inhibited by a histamine-H1 receptor antagonist or incubation in a Ca2+-free medium. Histamine-induced 45Ca2+ influx was also potentiated by addition of ouabain. Ouabain alone or in the presence of histamine increased 22Na+ influx into the cells. In an additional set of experiments, cells were preincubated in the presence or absence of Na+ for 30 min (+/- histamine and ouabain), washed and then catecholamine secretion was measured following exposure to 2.2 mM Ca2+ for 15 min. Preincubation with histamine alone with or without Na+ had no effect of Ca2+-induced secretion of catecholamine. Preincubation with ouabain alone or with ouabain plus histamine produced a slight stimulation of catecholamine secretion in Na+-free medium and a large stimulation in Na+-containing medium. These results suggested that stimulation of the histamine-H1 receptor and inhibition of the Na+ pump both increase intracellular Na+ levels, resulting in increases in Ca2+ influx and catecholamine secretion.

Stimulatory effect of ouabain on VCAM-1 and iNOS expression in murine endothelial cells: involvement of NF-kappa B

Endothelial cells play a pivotal role in the development of atherosclerosis. An 'activated' phenotype of these cells is manifested by signal transduction-dependent expression of genes encoding cytokines, pro- and anticoagulant factors, and cell adhesion molecules. In the current study we examined the effect of ouabain, an inhibitor of Na+/K(+)-ATPase, on the process of endothelial cell activation. We demonstrated that ouabain was able to stimulate VCAM-1 expression and potentiate the effect of IFN-gamma on this process. Moreover, ouabain provided a complementary signal for either TNF or IFN-gamma in inducing iNOS expression. Our data also show, for the first time, that inhibition of Na+/K(+)-ATPase led to activation of the transcription factor, NF-kappa B, which may provide an explanation for the effects of ouabain on endothelial cells.

Potentiation by ouabain of bradykinin-induced catecholamine secretion and calcium influx into cultured bovine adrenal chromaffin cells: evidence for involvement of Na+ influx associated with the bradykinin B2-receptor

The effect of bradykinin (BK), in the presence of ouabain, an inhibitor of Na(+)-K+ ATPase, on catecholamine (CA) secretion was studied in cultured bovine adrenal chromaffin cells, to determine whether Na+, as well as Ca2+, is involved in BK-receptor mediated CA secretion. BK (10(-8)-10(-5) M)-induced CA secretion was markedly potentiated by addition of ouabain (10(-5) M), was blocked by a BK-B2 receptor antagonist, and was decreased in Ca(2+)-free medium. BK-induced increase in 45Ca2+ influx was also potentiated by addition of ouabain. The cultured cells were first incubated with BK for 30 min in Ca(2+)-free medium in the presence or absence of ouabain and then stimulated for 15 min with Ca(2+)-medium without BK or ouabain. Prior stimulation of the cells, BK induced 22Na+ influx and increased Ca(2+)-induced CA secretion and these stimulatory effects of BK were potentiated by added ouabain. When the cells were stimulated with BK and ouabain in Na(+)-free sucrose medium, the Ca(2+)-induced CA secretion was greatly reduced. These results indicated that activation of the BK-B2 receptor and inhibition of the Na+ pump both increase the intracellular Na+ level, resulting in increase in Ca2+ influx and CA secretion.

Influence of arachidonic acid on catecholamine secretion in the perfused rat adrenal medulla

The present study was conducted to investigate the influence of arachidonic acid, which is known to be an important unsaturated fatty acid component of membrane phospholipids and to be liberated by phospholipase A2 action, on secretion of catecholamines (CA) from the isolated perfused rat adrenal glands and to clarify the mechanism of its action. Arachidonic acid (10 uM) perfused into an adrenal gland of the rat for 20 min caused a significant inhibition of CA secretion evoked by ACh (5.32 x 10(-3) M), DMPP (10(-4) M) and muscarine (10(-4) M) while it did not affect that induced by excess K+ (5.6 x 10(-2) M). Arachidonic acid, in the presence of ouabain (100 uM), an inhibitor of Na+, K(+) -ATPase, also produced a marked inhibitory effect of CA secretion evoked by ACh, DMPP and muscarine but did not modify the secretory effect of excess K+. The perfusion of arachidonic acid along with indomethacin (30 uM), which is an inhibitor of cyclooxygenase, for 20 min attenuated markedly CA secretory effect evoked by ACh, DMPP and muscarine while it did not influence that by excess K+. Prostaglandin F2 alpha perfused in a retrograde direction for 20 min inhibited greatly the CA secretion evoked by DMPP but did not affect the effect evoked by excess K+. All of arachidonic acid, ouabain, indomethacin and prostaglandin F2 alpha used in the present study did not affect the spontaneous basal release of CA in the perfused rat adrenal glands. Taken together, these experimental results suggest that arachidonic acid, as well as prostaglandin F2 alpha, cause the inhibitory action of CA secretion evoked by cholinergic receptor-mediated stimulation, but not by membrane depolarization, and also play a modulatory role in regulating CA secretion from the rat adrenal medulla.

Cyclic-AMP-dependent Ca2+ influx elicited by prostaglandin D2 in freshly isolated nonchromaffin cells from bovine adrenal medulla

We previously reported that prostaglandin D2 (PGD2) specifically elevates intracellular cyclic AMP in nonchromaffin cells isolated from bovine adrenal medulla (Biochim. Biophys. Acta (1989) 1011, 75-80). Here we again found that PGD2 increased intracellular Ca2+ concentration ([Ca2+]i) in freshly isolated nonchromaffin cells and investigated the cellular mechanisms of PGD2-induced [Ca2+]i increase using the Ca2+ indicator fura-2 and a fluorescence microscopic imaging system. Treatment of the cells with PGD2 receptor agonists BW245C and ZK110841 resulted in both marked stimulation of cyclic AMP formation and an increase in [Ca2+]i. The [Ca2+]i response was also induced by bypassing of the receptor with forskolin, a direct activator of adenylate cyclase, but not by PGE2 or PGF2 alpha both of which are devoid of the ability to generate cyclic AMP in the cells. These cyclic AMP and [Ca2+]i responses induced by PGD2 were completely blocked by the PGD2 receptor antagonist BWA868C. The time-course of cyclic AMP production stimulated by PGD2 coincided with that of the [Ca2+]i increase. While the Ca(2+)-mobilizing hormone bradykinin stimulated a rapid inositol phosphate accumulation in nonchromaffin cells, PGD2 did not stimulate it significantly. Removal of extracellular Ca2+ markedly reduced the Ca2+ response to PGD2 in magnitude and duration, but did not alter the peak [Ca2+]i response to bradykinin. These results demonstrate that PGD2 receptor activation induces the increase in [Ca2+]i via cyclic AMP mainly by increasing the Ca2+ influx from the outside, unlike inositol trisphosphate which causes release of Ca2+ from internal stores.

Prostaglandin E2-induced arachidonic acid release and catecholamine secretion from cultured bovine adrenal chromaffin cells

We recently reported that prostaglandin E2 (PGE2) and arachidonic acid (AA) each induced a gradual secretion of catecholamines from cultured bovine adrenal chromaffin cells in the presence of ouabain by stimulation of phosphoinositide metabolism. In the present study, we examined the relationship between phospholipase A2 and C activation and catecholamine secretion by PGE2 in chromaffin cells. The phospholipase A2 inhibitors p-bromophenacyl bromide and mepacrine did not affect the basal and ouabain-induced release, but dose-dependently blocked PGE2-evoked phosphoinositide metabolism and the consequent catecholamine release at an IC50 value of 3 microM. PGE2 induced rapid hydrolysis of [3H]AA from prelabeled phospholipid pools: the release of [3H]AA could be detected at as early as 15 sec and reached a plateau after 1 min. While the phospholipase C inhibitor neomycin did not inhibit PGE2-induced AA release, phospholipase A2 inhibitors dose-dependently inhibited it at IC50 values comparable to those for catecholamine release. Pretreatment of intact cells with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate, but not with pertussis toxin, prevented AA release by PGE2. These results demonstrate that PGE2 activates phospholipase A2 as well as phospholipase C in a pertussis toxin-insensitive manner and suggest that the released arachidonic acid may be involved in PGE2-induced catecholamine release from chromaffin cells.

Characterization and Ca2+ requirement of histamine-induced catecholamine secretion in cultured bovine chromaffin cells

The stimulation of cultured bovine chromaffin cells with histamine induced a continuous catecholamine secretion (EC50 = 3 x 10(-7) M) via the H1 receptor, in addition to an initial catecholamine burst due to a nonspecific stimulatory effect at higher doses (greater than or equal to 10(-4) M). The continuous secretion showed little desensitization and lasted for more than 1 h. In fura-2-loaded cells, the stimulation with histamine evoked a transient rise of intracellular free Ca2+ concentration ([Ca2+]i) which lasted only for a few minutes and was followed by a sustained [Ca2+]i rise which continued for more than 20 min. The addition of an activator for the L-type voltage-sensitive Ca2+ channel, i.e., Bay K 8644 (1 microM), facilitated the sustained [Ca2+]i rise, as well as the secretion, whereas the addition of relatively high concentrations of Ca(2+)-channel blockers (10 microM) suppressed the sustained [Ca2+]i rise and part of the secretion. Removal of extracellular Ca2+ completely abolished continuous secretion and sustained [Ca2+]i rise. When the external Ca2+ level was elevated, both sustained [Ca2+]i rise and continuous secretion were enhanced in a similar Ca(2+)-dependent manner, showing saturation with around 1-3 mM Ca2+. This Ca2+ dependence was clearly different from that observed with high K+ and nicotine, which is mediated by the L-type Ca2+ channel, in which the responses showed little or no saturation when the Ca2+ level was increased. The results indicate that stimulation with histamine induces a continuous secretion via the H1 receptor, in addition to a transient and nonspecific secretion at higher doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of palytoxin-induced Na+ influx into cultured bovine adrenal chromaffin cells: possible involvement of Na+/H+ exchange system

To elucidate the mechanism of palytoxin (PTX)-induced Na+ influx, we examined the effect of amiloride, an inhibitor of Na+/H(+)-antiporter, on PTX-induced Na+ influx into cultured bovine adrenal chromaffin cells in relation to its effects on Ca2+ influx and catecholamine secretion. Amiloride dose-dependently inhibited PTX-induced 22Na+ influx, whereas tetrodotoxin (TTX) had no effect. Amiloride also inhibited PTX-induced Na(+)-dependent 45Ca2+ influx and catecholamine secretion. PTX alone did not significantly affect the intracellular pH, but it decreased in the presence of PTX and amiloride. These results indicate that an amiloride-sensitive Na+/H+ exchange mechanism is probably involved in PTX-induced, TTX-insensitive Na+ influx that triggers Ca2+ influx and catecholamine secretion from the cells.

Characterization of prostaglandin E2-induced Ca2+ mobilization in single bovine adrenal chromaffin cells by digital image microscopy

We recently reported that prostaglandin E2 (PGE2) stimulates phosphoinositide metabolism accompanied by an increase in intracellular free Ca2+ concentration ([Ca2+]i) in cultured bovine adrenal chromaffin cells. In the present study, temporal and spatial changes in [Ca2+]i induced by PGE2 in fura-2-loaded individual cells were investigated by digital image microscopy and were compared with those induced by nicotine and histamine. Image analysis of single cells revealed that responses to PGE2 showed asynchrony with the onset of [Ca2+]i changes. After a lag time of 10-30 s, PGE2-induced [Ca2+]i changes took a similar prolonged time course in almost all cells: a rapid rise followed by a slower decline to the basal level over 5 min. Few cells exhibited oscillations in [Ca2+]i. In contrast, nicotine and histamine induced rapid and transient [Ca2+]i changes, and these [Ca2+]i changes were characteristic of each stimulant. Whereas pretreatment of the cells with pertussis toxin (100 ng/ml, 6 h) did not block the response to any of these stimulants, treatment with 12-O-tetradecanoylphorbol 13-acetate (100 nM, 10 min) completely abolished [Ca2+]i changes elicited by PGE2 and histamine. In a Ca2(+)-free medium containing 3 mM EGTA, or in medium to which La3+ was added, the [Ca2+]i response to nicotine disappeared, but that to histamine was not affected significantly. Under the same conditions, the percentage of the cells that responded to PGE2 was reduced to 37% and the prolonged [Ca2+]i changes induced by PGE2 became transient in responding cells, suggesting that the maintained [Ca2+]i increase seen in normal medium is the result of a PGE2-stimulated entry of extracellular Ca2+. Whereas the organic Ca2(+)-channel blocker nicardipine inhibited [Ca2+]i changes by all stimulants at 10 microM, these [Ca2+]i changes were not affected by any of the organic Ca2(+)-channel blockers, i.e., verapamil, diltiazem, nifedipine, and nicardipine, at 1 microM, a concentration high enough to inhibit voltage-sensitive Ca2+ channels. These results demonstrate that PGE2 may promote Ca2+ entry with concomitant release of Ca2+ from intracellular stores and that the mechanism(s) triggered by PGE2 is apparently different from that by histamine or nicotine.

Sodium fluoride mimics the effect of prostaglandin E2 on catecholamine release from bovine adrenal chromaffin cells

We have reported recently that prostaglandin E2 (PGE2) stimulated phosphoinositide metabolism in bovine adrenal chromaffin cells and that PGE2 and ouabain, an inhibitor of Na+, K(+)-ATPase, synergistically induced a gradual secretion of catecholamines from the cells. Here we examined the involvement of a GTP-binding protein(s) in PGE receptor-induced responses by using NaF. In the presence of Ca2+ in the medium, NaF stimulated the formation of all three inositol phosphates, i.e., inositol monophosphate, bisphosphate, and trisphosphate, linearly over 30 min in a dose-dependent manner (15-30 mM). This effect on phosphoinositide metabolism was accompanied by an increase in cytosolic free Ca2+. NaF also induced catecholamine release from chromaffin cells, and the dependency of stimulation of the release on NaF concentration was well correlated with those of NaF-enhanced inositol phosphate formation and increase in cytosolic free Ca2+. Although the effect of NaF on PGE2-induced catecholamine release in the presence of ouabain was additive at concentrations below 20 mM, there was no additive effect at 25 mM NaF. Furthermore, the time course of catecholamine release stimulated by 20 mM NaF in the presence of ouabain was quite similar to that by 1 microM PGE2, and both stimulations were markedly inhibited by amiloride, with half-maximal inhibition at 10 microM. Pretreatment of the cells with pertussis toxin did not prevent, but rather enhanced, PGE2-induced catecholamine release over the range of concentrations examined. These results demonstrate that NaF mimics the effect of PGE2 on catecholamine release from chromaffin cells and suggest that PGE2-evoked catecholamine release may be mediated by the stimulation of phosphoinositide metabolism through a putative GTP-binding protein insensitive to pertussis toxin.

Involvement of phosphoinositide metabolism in GABA-induced catecholamine release from cultured bovine adrenal chromaffin cells

The effects of GABA on catecholamine release and phosphoinositide metabolism were studied in cultured bovine adrenal chromaffin cells. GABA and muscimol, a specific agonist for the GABAA receptor, each evoked a gradual secretion of catecholamines from the cells in the presence of ouabain, an inhibitor of Na+, K(+)-ATPase. This release was inhibited by bicuculline, a specific antagonist for the GABAA receptor, or by picrotoxin, a blocker of GABA-gated Cl- channels, and was potentiated by diazepam or pentobarbital. GABA or muscimol induced a concentration-dependent formation of inositol phosphates. This accumulation of inositol phosphates was also inhibited by bicuculline, picrotoxin or removal of extracellular Ca2+, and also potentiated by diazepam and pentobarbital. Nicardipine suppressed GABA-induced catecholamine release in the presence of ouabain and accumulation of inositol phosphates, while verapamil, diltiazem, and omega-conotoxin failed to inhibit these responses to GABA. The phosphoinositide-specific phospholipase C inhibitor neomycin also inhibited both GABA-induced accumulation of inositol phosphates and stimulation of catecholamine release in the presence of ouabain. These results taken together indicate that GABA evoked catecholamine release from the chromaffin cells in the presence of ouabain by stimulation of phosphoinositide metabolism in a Ca2(+)-sensitive manner via activation of GABAA receptor-coupled Cl- channels.

Arachidonic acid stimulates phosphoinositide metabolism and catecholamine release from bovine adrenal chromaffin cells

Arachidonic acid (AA) evoked a dose-dependent increase in the accumulation of inositol phosphates in cultured bovine adrenal chromaffin cells, and this effect was specific for AA. AA also induced a rise in [Ca2+]i, but this rise was markedly reduced by removal of extracellular Ca2+. AA-induced accumulation of inositol phosphates was absolutely dependent on extracellular Ca2+, and nicardipine and nifedine partially reduced it but verapamil had no effect. Moreover, AA dose-dependently stimulated catecholamine release from chromaffin cells in the presence of ouabain, and this effect was specific for AA. AA-induced catecholamine release in the presence of ouabain was also inhibited by nicardipine and nifedipine but not by verapamil. Furthermore, the phospholipase C inhibitor neomycin inhibited the release. These results taken together suggest that AA stimulates catecholamine release in the presence of ouabain by stimulation of phosphoinositide metabolism in a Ca2(+)-dependent manner.