Induced Catecholamine Release Research Articles

O-43 - Stimulatory effects of histamine and ouabain on Ca2+-induced catecholamine release from cultured bovine adrenal chromaffin cells.

O-43 - Stimulatory effects of histamine and ouabain on Ca2+-induced catecholamine release from cultured bovine adrenal chromaffin cells.

Involvement of Ca2+ entry and inositol trisphosphate-induced internal Ca2+ mobilization in muscarinic receptor-mediated catecholamine release in dog adrenal chromaffin cells.

Catecholamine (CA) release from adrenal medulla evoked by muscarinic receptor stimulation has been studied using isolated perfused adrenal gland and cultured chromaffin cells from dogs. Muscarine and oxotremorine (1-100 microM), and bethanechol (0.1-1 mM) dose-dependently stimulated CA release. Muscarine-evoked CA release was antagonized with M1-antagonist, pirenzepine and, to a lesser extent, with atropine; and was reduced either by removal of extracellular Ca2+ or treatment with Ca2+ channel blockers. Muscarine caused an increase of 45Ca uptake and 22Na uptake. Tetrodotoxin (TTX) did not affect muscarine-evoked increase of 22Na uptake and CA release. Under the absence of extracellular Ca2+, muscarine stimulated a 45Ca efflux. Muscarine-induced CA release was attenuated by treating the cells with 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate-HCl (TMB-8) which blocks Ca2+ release from the intracellular store. A phospholipase C inhibitor, neomycin, markedly reduced muscarine-induced CA release but not nicotine- and high K(+)-evoked release. Cinnarizine, a Ca2+ channel blocker, attenuated muscarine-evoked but not caffeine-induced CA release and 45Ca efflux in the absence of extracellular Ca2+. Muscarine caused an increase in intracellular free Ca2+ concentration ([Ca2+]i) in the presence of extracellular Ca2+. It caused a similar increase, but to a lesser extent, in the absence of extracellular Ca2+. The increase of [Ca2+]i induced by muscarine without extracellular Ca2+ was reduced by neomycin and cinnarizine. Polymixin B and retinal, which reduced 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced CA release, had little effect on muscarine-induced CA release. Muscarine increased cellular Ins(1,4,5)P3 production, and atropine inhibited this increase.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the glycine prodrug milacemide (2-N-pentylaminoacetamide) on catecholamine secretion from isolated adrenal medulla chromaffin cells

1. Milacemide (2-n-pentylaminoacetamide) is a glycine prodrug which readily crosses the blood brain barrier and increases brain glycine and glycineamide. In vitro and in vivo studies, with numerous tissues, including adrenal chromaffin cells, have clearly shown that the formation of the latter metabolites is exclusively mediated by monoamine oxidase B for which milacemide is a substrate. 2. Milacemide, glycineamide and glycine caused a time- and dose-dependent release of catecholamines from bovine isolated chromaffin cells. 3. Milacemide (10(-4) M) induced catecholamine release was roughly 30% of that initiated by acetylcholine (10(-4) M), the natural secretagogue. 4. The combined effects of milacemide (10(-4) M) and acetycholine (10(-4) M) on catecholamine secretion from chromaffin cells is additive, suggesting that milacemide does not act through the normal nicotinic receptor release mechanism. 5. The release of catecholamines from chromaffin cells in response to milacemide (10(-4) M) was partially inhibited by the selective MAO-B inhibitors (-)-deprenyl (10(-7) M) and AGN 1135 (10(-6) M). This indicates that the MAO-B derived metabolites, glycineamide and glycine, contribute to the secretion of catecholamines as does milacemide itself. 6. It is apparent that release of catecholamines by glycine is mediated by its uptake into the cells since [3H]-glycine uptake and catecholamine release showed a highly significant correlation (r = 0.96).

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.

Palytoxin: A potent stimulator of catecholamine release from cultured bovine adrenal chromaffin cells

The effect of palytoxin (PTX), a potent marine toxin, on catecholamine release from cultured bovine adrenal chromaffin cells was examined. PTX at concentrations of over 10 −9 M induced catecholamine release dose-dependently. About 40–50% of the total cellular catecholamine was released during 20-min incubation with 3 × 10 −8 M PTX. PTX-induced catecholamine release was dependent on both extracellular Na + and Ca 2+, and was inhibited by organic and inorganic Ca 2+ channel blockers, but not by tetrodotoxin. PTX-induced increase in 45Ca 2+ influx into the cells, which was associated with catecholamine release, was also inhibited by these Ca 2+ channel blockers. These results indicated that PTX-induced catecholamine release was mediated by activation of Na +-dependent, tetrodotoxin (TTX) insensitive voltage-dependent Ca 2+ channels.

Effects of Hypoxia on the Catecholamine Release, Ca2+ Uptake, and Cytosolic Free Ca2+ Concentration in Cultured Bovine Adrenal Chromaffin Cells

The purpose of the present study is to clarify the effects of hypoxia on catecholamine release and its mechanism of action. For this purpose, using cultured bovine adrenal chromaffin cells, we examined the effects of hypoxia on high (55 mM) K(+)-induced increases in catecholamine release, in cytosolic free Ca2+ concentration ([Ca2+]i), and in 45Ca2+ uptake. Experiments were carried out in media preequilibrated with a gas mixture of either 21% O2/79% N2 (control) or 100% N2 (hypoxia). High K(+)-induced catecholamine release was inhibited by hypoxia to approximately 40% of the control value, but on reoxygenation the release returned to control levels. Hypoxia had little effect on ATP concentrations in the cells. In the hypoxic medium, [Ca2+]i (measured using fura-2) gradually increased and reached a plateau of approximately 1.0 microM at 30 min, whereas the level was constant in the control medium (approximately 200 nM). High K(+)-induced increases in [Ca2+]i were inhibited by hypoxia to approximately 30% of the control value. In the cells permeabilized by digitonin, catecholamine release induced by Ca2+ was unaffected by hypoxia. Hypoxia had little effect on basal 45Ca2+ uptake into the cells, but high K(+)-induced 45Ca2+ uptake was inhibited by hypoxia. These results suggest that hypoxia inhibits high K(+)-induced catecholamine release and that this inhibition is mainly the result of the inhibition of high K(+)-induced increases in [Ca2+]i subsequent to the inhibition of Ca2+ influx through voltage-dependent Ca2+ channels.

119 RELEASE OF CATECHOLAMINES DURTNG THE DIFFERENT PHASES OF BIRTH SIMULATION IN THE FETAL LAMB

The precise mechanisms for catecholamine release at birth are not well established. Labor and delivery, cold, cord clamping, hypoxia and acidosis are stimuli of catecholamine release. Lung liquid resorption is in relationship with catecholamine release. To study the effect of the different stress applied on lung tissue, we investigated the different phases of birth simulation in utero focused only on lung distension, ventilation with Nitrogen(N2) without modifications of fetal blood gases, then with air and oxygen. We studied 16 ewes at 134,75±4 days of gestational age. Mean fetal weight was 3,63±0,63 kg. Pressure of db tension was 25±10 cm of water. For rhythmic ventilation with N2, pressure of inflation was 27±6 mm Hg, PEEP 4±5 and I/E 36±13%. Catecholamine samples were withdrawn after 15 min following modifications. Catecholamine concentrations were measured by high-performance liquid chromatography with electrochemical detection. Statistical analysis compared the consecutive steps analyzed by the non-parametric Wilcoxon test and considered significant for a p<0,01 Ventilation increased 4,6 fold epinephrine concentrations. However distension did not increase catecholamine release. Ventilation with air and O2 decreased concentrations and placental blood flow was abolished. The precise mechanism which induces catecholamine release during rhythmic ventilation with N2 is not elucidated.

Mechanism of histamine secretion

After the discovery in the fifties that the largest store of histamine in the body is located in the mast cells [1], the most important observation concerning histamine release has been that an enzymatic mechanism is involved and the release requires calcium and metabolic energy [2 5]. But very little was known about the metabolic parameters of the mast cells at lhal time. When it became possible to isolate the rat peritoneal mast cells as a pure population [6], the metabolic functions could be correlated with the secretory response. These studies not only confirnled energy dependence, but also showed that the secretion is associated with an increased energy requirement (see [7]). The biological role of calcium has been characterized in various cellular responses. Calcium is mobilized into the cytosol by influx across the plasma membrane or by release from intracellular slores. The importance of calcium in histamine release from mast cells was demonstrated by using the calcium ionophore A23187, which in tlle presence ol 'calcimn in the medium increases its influx [8]. In adrenal medullary cells, permeabilized by intense electric fields, which caused localized breakdown of the plasma membrane thus giving access of extracellular calcium to the cytosol, very low (btM) concentrations of calcium could induce catecholamine release [9]. Similar results have been obtained by permeabilizing mast cells with ATP [10] and Sendai virus [11]. In these preparations the extracellular calcium concentrations reflect those needed in the cytosol for exocytosis. The measurement of changes in the cytosol calcium concentration alone does not tell us the source of the calcium, whether by influx of extracetlular calcium or by release of intracellular stores. Optimal histamine release from sensitized mast cells challenged wilh the antigen is dependent on extracellular calcium. The polyamine, compound 48/80, on the other hand can release histamine in the absence of calcium in the medium.

Affinity purified tetanus toxin binds to isolated chromaffin granules and inhibits catecholamine release in digitonin-permeabilized chromaffin cells

Tetanus toxin, a potent neurotoxin which blocks neurotransmitter release in the CNS, also inhibits Ca 2+-induced catecholamine release from digitonin-permeabilized, but not from intact bovine chromaffin cells. In searching for intracellular targets for the toxin we studied the binding of affinity-purified tetanus toxin to bovine adrenal chromaffin granules. Tetanus toxin bound in a neuraminidase-sensitive fashion to intact granules and to isolated granule membranes, as assayed biochemically and visualized by electron microscopic techniques. The binding characteristics of the toxin to chromaffin granule membranes are very similar to the binding of tetanus toxin to brain synaptosomal membranes. We suggest that the toxin-binding site is a glycoconjugate of the G1b type (a polysialoganglioside or a glycoprotein-proteoglycan) which is localized on the cytoplasmic face of the granule membrane and might directly be involved in exocytotic membrane fusion.

Evidence that prostaglandins activate calcium channels to enhance basal and stimulation-evoked catecholamine release from bovine adrenal chromaffin cells in culture

The effects of prostaglandins (PGs) on catecholamine (CA) secretion and Ca 2+ fluxes were studied in a primary culture of bovine chromaffin cells. PGD 2, PGF 2α and PGE 2 induced CA release from cultured bovine chromaffin cells in a concentration dependent manner (0.03–3 μM). PGD 2, PGF 2α and PGE 2 at 3 μM elicited maximum CA release of 0.043 ± 0.001, 0.059 ± 0.008, 0.062 ± 0.002 μg/10 6 cells, respectively. Three micromolar of PGD 2, PGF 2α and PGE 2 enhanced CA release induced by acetylcholine (ACh) in a degree of 186 ± 10, 206 ± 6, 150 ± 4% of control respectively. PGs also enhanced CA release induced by 20 mM K +, veratridine and A23187. In Ca 2+-free medium, PGs failed to affect basal and caffeine (50 mM)-induced CA release. PGF 2α increased 45Ca uptake and showed additive effect with ACh on 45Ca uptake. Nicardipine (0.1–10 μM) suppressed CA release and 45Ca uptake induced by PGF 2α, while diltiazem and verapamil failed to affect these responses to PGF 2α. BAY K 8644 (1 μM) potentiated CA release and 45Ca uptake evoked by PGF 2α. These results suggest that PGs enhance basal and stimulation-evoked CA release from chromaffin cells possibly through facilitation of Ca 2+ influx. The mechanisms of action of PGs in adrenal medulla are discussed.

Prostanoid responses of bovine adrenal medullary cells: lack of effect of opioids

The effects of opioid compounds on catecholamine (CA) secretion and phosphatidylinositol turnover induced by prostaglandins E 1 (PGE 1) and E 2 (PGE 2) in cultured bovine adrenal medullary cells have been studied. PGE 1 induced CA secretion at 100 nM and above. PGE 2 was more potent, inducing CA secretion at 1–10 nM. Both prostaglandins required extracellular calcium to induce CA release. Neither etorphine nor diprenorphine (1 nM-10 μM) affected CA secretion induced by 1 μM PGE 1 or 0.1 μM PGE 2. PGEP 1 produced a small increase in phosphatidylinositol turnover at 10 μM, but had no effect at lower concentrations. PGE 2 was effective at 1 and 10 μM. Etorphine and diprenorphine had no effect on phosphatidylinositol turnover induced by PGE 1 or PGE 2. The results indicate prostaglandins can facilitate CA secretion independently of their effects on phosphatidylinositol metabolism. They also indicate that endogenous adrenal opioid peptides do not act on the opioid binding sites found on adrenal medullary cells to modify their responses to prostaglandins.

Anti-alpha-fodrin inhibits secretion from permeabilized chromaffin cells.

Chromaffin cells release catecholamine- and peptide-containing granules by exocytosis, by a mechanism involving movement of secretory granules towards the cell membrane, their apposition to it and the fusion of the granule membrane with the plasma membrane. One of the two subunits of membrane-associated brain spectrin, alpha-fodrin is an actin-binding protein which is found at the periphery of chromaffin cells and may be involved in secretion. Because cultured chromaffin cells can be permeabilized with detergents, giving pores large enough to permit the entry of immunoglobulin molecules, we used permeabilized cells to test the effect of specific antibodies on secretory mechanisms. Incubation of permeabilized cells with polyclonal immunoaffinity-purified monospecific anti-alpha-fodrin antibody or its Fab fragments did not modify basal release but did specifically inhibit Ca2+-induced catecholamine release by exocytosis. Our observations indicate that fodrin and the cytoskeleton participate in the release mechanism.

Effect of chronic and acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration on the function and structure of cultured chromaffin cells

Cultured bovine adrenal chromaffin cells were treated chronically with various concentrations of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in the culture medium for 2-8 days or acutely for 10-15 min. Culture of cells with MPTP for periods of 2-8 days resulted in a marked loss of total cellular catecholamines and a parallel reduction in secretory response, but not the ratio of stimulated to unstimulated secretion. By the eighth day in culture, at the highest MPTP concentration (1000 ?M), cell catecholamine content and secretion were only about 10% that of untreated cells. The proportion of total cellular catecholamines secreted was not altered by MPTP, suggesting that the secretory process was unaffected by the drug. The loss of secretory output was not prevented by inhibitors of monoamine oxidase or catecholamine uptake, drugs known to prevent MPTP-induced damage to central dopaminergic neurons. The subcellular organelles of MPTP-treated cells appeared relatively normal except for extensive depletion of the vesicle contents, in agreement with the biochemical data. The severity of the depletion appeared to be lessened in cells treated with monoamine oxidase inhibitors. Short term exposure to MPTP at concentrations less than 100 ?M had little effect on secretion induced by carbachol. Higher concentrations of MPTP increased unstimulated release and reduced stimulated release. Pretreatment of the cells with MPTP resulted in a lasting reduction in their subsequent secretory responsiveness. MPTP alone, at concentrations greater than 100 ?M induced catecholamine release that was unaffected by pretreatment of the cells with monoamine oxidase inhibitors or the catecholamine uptake inhibitor desipramine. MPTP-induced secretion by intact cells was calcium-dependent, while the small increase by permeabilized cells was not.

Contrasting monoamine oxidase activity and tyramine induced catecholamine release in PC12 and chromaffin cells

PC12 (phaeochromocytoma derived) cells possess the catecholamine synthesizing enzymes as well as the ability to store and release the catecholamines in response to K +. However, their monoamine oxidase activity and catecholamine release in response to tyramine has not been examined previously. PC12 cells have monoamine oxidase activity which oxidizes type A (noradrenaline and serolonin) and type A-B (dopamine, tyramine and kynuramine) substrates, and is selectively inhibited by clorgyline ( ic 50 ~ 10 −8 M). In contrast, PC12 cell monoamine oxidase hardly oxidizes phenylethylamine, a type B substrate, and is relatively insensitive to inhibition by the selective monoamine oxidase type B inhibitor. I-deprenyl ( ic 50 ~ 10 −6 M). By the above criteria it is apparent that the monoamine oxidase in PC12 is solely type A. The kinetics of the oxidase are similar to those of monoamine oxidase type A reported in other tissues including the adrenergic neuron, having apparent K m values of 400, 280, 170 and 227 μM for noradrenaline, dopamine, serotonin and tyramine. The apparent K m value for phenylethylamine is 235 μM. On the other hand, isolated chromaffin cells have the B form of monoamine oxidase with high affinity ( K m~25μM) for phenylethylamine and low affinities for noradrenaline ( K m~ 1100μ M) and adrenaline ( K m~ 1700/gmM). This enzyme form is selectively inactivated by the monoamine oxidase type B inhibitor, I-deprenyl. In similar fashion to peripheral adrenergic neurons, PC12 cells share the capacity to express a tyramine releasable pool of catecholamines, a property entirely lacking in mature cultured chromaffin cells, even though the latter cells are capable of taking up tyramine by a cocaine sensitive process. However, both PC12 cells and chromaffin cells have a voltage and calcium dependent, K + induced, catecholamine release mechanism. Although previous studies have demonstrated that PC12 cells share some functional and biochemical properties with adrenergic neurons as well as chromaffin cells, the present study indicates that they have a closer resemblance to adrenergic neurons with regard to monoamine oxidase activity and tyramine induced catecholamine release than to chromaffin cells.

Synergistic actions of Ca 2+ and the phorbol ester TPA on K +-induced catecholamine release from bovine adrenal chromaffin cells

Enhancement of Ca 2+-dependent high K +-evoked catecholamine secretion was observed after pretreatment of cultured bovine adrenal chromaffin cells with the phorbol ester 4B-phorbol 12-myristate 13-acetate (TPA) in the absence of added extracellular Ca 2+. This effect of TPA was not reproduced when the secretagogues acetylcholine, nicotine, or veratrine were substituted for high K +. The implications of these results are discussed in relation to the role of protein kinase C in stimulus-secretion coupling in the chromaffin cell.

Mechanism of the inhibitory effect of thiopentone on stimulus-secretion coupling in chromaffin cells

This study was undertaken to clarify the mechanism of action of thiopentone on stimulus-secretion coupling in cholinergic postsynaptic cells using cultured bovine adrenal chromaffin cells. Thiopentone (20-100 microns) inhibited carbachol-induced Ca2+ uptake into, and catecholamine release from the cells in a concentration-dependent manner. The inhibition of catecholamine release was almost parallel to the inhibition of Ca2+ uptake. The inhibitory effect of thiopentone was not overcome by an increase in the concentration of carbachol, whereas that of alcuronium was. Contrary to the inhibition by diltiazem, which was reversed by increasing the Ca2+ concentration, the inhibition by thiopentone was not overcome by a high Ca2+ concentration. Compared with the inhibition of carbachol-induced catecholamine release, a higher concentration of thiopentone was required to inhibit high K+ (56 mM KCl)-induced catecholamine release (200 microns thiopentone exhibited 32% inhibition). The results suggest that thiopentone blocks the stimulus-secretion coupling in the chromaffin cells as a result of inhibiting Ca2+ uptake through nicotinic receptor-linked channels. The linkage between receptor stimulation and Ca2+ channel activation seems to be the process most susceptible to inhibition by the barbiturate.

Enhancement of Ca 2+-induced catecholamine release by the phorbol ester TPA in digitonin-permeabilized cultured bovine adrenal chromaffin cells

The phorbol ester, 4β-phorbol 12-myristate acetate (TPA), increased the extent of catecholamine release induced by Ca 2+, without affecting the basal release response in digitonin-permeabilized chromaffin cells. This finding is consistent with the hypothesis that protein kinase C has a role to play in stimulus-secretion coupling in the bovine adrenal medullary chromaffin cell. Phorbol ester Cathecholamine Protein kinase C Chromaffin Secretion Ca 2+-induced release

Desensitization to nicotinic cholinergic agonists and K+, agents that stimulate catecholamine secretion, in isolated adrenal chromaffin cells.

Desensitization of catecholamine (CA) release from cultured bovine adrenal chromaffin cells was studied to characterize the phenomenon of desensitization and to attempt an elucidation of the mechanism(s) involved in this phenomenon at the level of the isolated chromaffin cell. Prior exposure of chromaffin cells to nicotinic cholinergic agonists [acetylcholine (ACh) or nicotine] caused a subsequent depression or desensitization of CA release during restimulation of the cells with the same agonists. Rates of development of and recovery from nicotinic desensitization were in the minute time range and the magnitude of nicotinic desensitization of CA release was greater at 37 degrees C than at 23 degrees C. ACh- (or nicotine)-induced desensitization was shown to be the result of two processes: (1) a Ca2+-dependent component of desensitization, possibly due to a depletion of intracellular CA stores and (2) a Ca2+-independent, depletion-independent component of desensitization. Prior exposure of cultured chromaffin cells to an elevated concentration of K+ also resulted in desensitization of K+-induced CA release in these cells. K+-induced desensitization was completely Ca2+-dependent and was shown to be the result, at least in part, of a mechanism that is independent of depletion of CA stores.

Block by phencyclidine of acetylcholine and barium induced adrenal catecholamine secretion

Activation of the sympathetic system by phencyclidine (PCP) should result in catecholamine release from the adrenals. However, adrenalectomy does not reduce PCP-induced hypertension. In an attempt to rectify this inconsistency, the direct effects of PCP on the bovine adrenal medulla were examined. At (3×10 −6M), PCP reduced the acetylcholine-(ACh)-induced catecholamine release by 50%. Surprisingly, barium-induced secretion of catecholamines was also reduced by PCP. ACh-induced catecholamine release was not altered by 10 −3M 4-aminopyridine (4 AP), the potassium channel blocker. Thus, calcium antagonist actions of PCP and consequent block of catecholamine secretion from adrenal medulla may explain the lack of effect of adrenalectomy on PCP-induced hypertension. Possible contributions of calcium and/or potassium channel blockade to other manifestations of PCP overdosage are discussed.

Facilitation of stimulation-evoked catecholamine release by somatostatin in dog perfused adrenal glands.

The effect of somatostatin on the catecholamine (CA) release from the chromaffin cells was determined on the isolated dog adrenals perfused with 1.8 mM Ca2+ containing fluid except indicated. The extremely low concentrations of somatostatin (0.18 nM-18 nM) were found to stimulate acetylcholine (Ach, 5 microM)-evoked CA release with the maximum response (by 114% above control) at 1.8 nM but the relatively high concentrations (61, 610 nM) caused an inhibition. Somatostatin (6.1 nM) also facilitated excess K+ (15 mM)-induced CA release but failed to enhance the release evoked by a Ca2+ ionophore, A23187 (50 microM) and the release by caffeine (50 mM) under the condition of Ca2+-free. Somatostatin by itself did not affect significantly on the basal release of CA. Elevation of Ca2+ concentrations from 1.8 mM to 5 mM in the perfusion fluid reduced the stimulatory and inhibitory effect of somatostatin. It is possible that somatostatin enhances CA release by facilitating the influx of Ca2+ via the potential-sensitive permeability channel when chromaffin cells are depolarized. The present results provide the first demonstration that somatostatin stimulate the release of CA from the adrenal gland suggesting that somatostatin may function as a facilitatory modulator of the response to ACh at chromaffin cells.