The effects of calcium ions, ionophore A23187 and inhibition of energy metabolism on protein degradation in the rat diaphragm and epitrochlearis muscles in vitro.

1. The effects of external Ca2+, EGTA, ionophore A23187, CN-, dinitrophenol and iodoacetamide on the rate of protein degradation in the rat diaphragm and epitrochlearis muscles in vitro were investigated. 2. External Ca2+ increased protein degradation when compared with external EGTA. Protein degradation was further increased by Ca2+ + ionophore A23187. 3. EGTA and ionophore A23187 decreased ATP and phosphocreatine concentrations and the ATP/ADP ratio. 4. CN-, dinitrophenol and iodoacetamide decreased protein degradation, presumably by interfering with energy metabolism. 5. The effects of EGTA may be caused by disturbances in energy metabolism. The effects of ionophore A23187 cannot be readily explained by disturbances in energy metabolism. 6. Incubation of diaphragms with Ca2+ causes a rapid increase in whole-tissue Ca content. This is further stimulated by ionophore A23187. The uptake of Ca2+ may be, at least in part, into the cytoplasm because an increase in the glycogen phosphorylase activity ratio is observed. 7. A Ca2+-activated proteinase is present in rat heart and diaphragm. This enzyme may mediate in part the effects of Ca2+ described above. The apparent KA of this enzyme for Ca2+ is about 0.25 mM. 8. Because effects of ionophore A23187 cause a large increase in whole-tissue Ca content and because the Ca2+-activated proteinase has a relatively low affinity for Ca2+, it is felt that the effects of Ca2+ upon muscle proteolysis are unlikely to be of importance in steady-state protein turnover in vivo. The mechanism may, however, be important in breakdown of necrotic tissue in the living animal.     

The stoichiometry of A23187- and X537A-mediated calcium ion transport across lipid bilayers

Initial rates of ionophore-mediated Ca2+ transport across egg phosphatidylcholine bilayers of large unilamellar vesicles were measured using the absorbance change of arsenazo III at 650 nm as an indicator of Ca2+ translocation. A23187 induced the movement of Ca2+ in a 2:1 ionophore: Ca2+ complex, whereas its methyl ester (CH3A23187) and X537A mediated Ca2+ movement in a 1:1 ionophore: Ca2+ complex. The relative potencies of these ionophores in transporting Ca2+ across lipid membranes were A23187 much greater than X537A greater than CH3A23187.     

Effects of Ca2+ and ionophore A23187 on protein synthesis in intact rabbit reticulocytes

1. Amino acid incorporation in intact rabbit reticulocytes was unaffected by depletion of Ca2+ with EGTA. 2. The Ca2+ ionophore A23187 strongly inhibited incorporation in reticulocytes incubated in 1 mM Ca2+ but not in EGTA. Polysomal profiles and average ribosomal transit times of cells treated with Ca2+ ionophore at 1 mM Ca2+ were characteristic of translational elongation block. 3. The behavior of reticulocytes with respect to Ca2+ and A23187 contrasts with that of nucleated cells possessing endoplasmic reticulum in which protein synthesis is inhibited at translational initiation by either Ca2+ depletion or by exposure to Ca2+ ionophore.     

The effect of intracellular calcium ions on adrenaline-stimulated adenosine 3'':5''-cyclic monophosphate concentrations in pigeon erythrocytes, studied by using the ionophore A23187.

1. The bivalent cation ionophore A23187 was used to increase the intracellular concentration of Ca2+ in pigeon erythrocytes to investigate whether the increase in cyclic AMP content caused by adrenaline might be influenced by a change in intracellular Ca2+ in intact cells. 2. Incubation of cells with adrenaline, in the concentration range 0.55--55 muM, resulted in an increase in the concentration of cyclic AMP over a period of 60 min. The effect of adrenaline was inhibited by more than 90% with ionophore A23187 (1.9 muM) in the presence of 1 mM-Ca2+. This inhibition could be decreased by decreasing either the concentration of the ionophore or the concentration of extracellular Ca2+, and was independent of the concentration of adrenaline. 3. The effect of ionophore A23187 depended on the time of incubation. Time-course studies showed that maximum inhibition by ionophore A23187 was only observed when the cells were incubated with the ionophore for at least 15 min before the addition of adrenaline. 4. The inhibition by ionophore A23187 depended on the concentration of extracellular Ca2+. In the absence of Mg2+, ionophore A23187 (1.9 muM) inhibited the effect of adrenaline by approx. 30% without added Ca2+, by approx. 66% with 10 muM-Ca2+ and by more than 90% with concentrations of added Ca2+ greater than 30 muM. However, even in the presence of EGTA [ethanedioxybis(ethylamine)tetra-acetate](0.1--10 mM), ionophore A23187 caused an inhibition of the cyclic AMP response of at least 30%, which may have been due to a decrease in cell Mg2+ concentration. 5. The addition of EGTA after incubation of cells with ionophore A23187 resulted in a partial reversal of the inhibition of the effect of adrenaline. 6. Inclusion of Mg2+ (2 mM) in the incubation medium antagonized the inhibitory action of ionophore A23187. This effect was most marked when the ionophore A23187 was added to medium containing Mg2+ before the addition of the cells. 7. The cellular content of Mg2+ was decreased by approx. 50% after 20 min incubation with ionophore A23187 (1.9 muM) in the presence of Ca2+ (1 mM) but no Mg2+. When Mg2+ (2 mM) was also present in the medium, ionophore A23187 caused an increase of approx. 80% in cell Mg2+ content. Ionophore A23187 had no significant effect on cell K+ content. 8. Ionophore A23187 caused a decrease in cell ATP content under some conditions. Since effects on cyclic AMP content could also be shown when ATP was not significanlty lowered, it appeared that a decrease in ATP in the cells could not explain the effect of ionophore A23187 on cyclic AMP. 9. Ionophore A23187 (1.9 muM), with 1 mM-Ca2+, did not enhance cyclic AMP degradation in intact cells, suggesting that the effect of ionophore A23187 on cyclic AMP content was mediated through an inhibition of adenylate cyclase rather than a stimulation of cyclic AMP phosphodiesterase. 10. It was concluded that in intact pigeon erythrocytes adenylate cyclase may be inhibited by intracellular concentrations of Ca2+ in the range 1-10 muM.     

Intracellular uptake and alpha-amylase and lactate dehydrogenase releasing actions of the divalent cation ionophore A23187 in dissociated pancreatic acinar cells.

Intracellular uptake of A23187 and the increased release of amylase and lactate dehydrogenase (LDH) accompanying ionophore uptake was studied using dissociated acinar cells prepared from mouse pancreas. Easily detected changes in the fluorescence excitation spectrum of A23187 upon transfer of the ionophore from a Tris-buffered Ringer's to cell membranes were used to monitor A23187 uptake. Uptake was rapid in the absence of extracellular Ca2+ and Mg2+ (t1/2=1 min) and much slower in the presence of Ca2+ or Mg2+ (t1/2=20 min). Cell-associated ionophore was largely intracellular as indicated by fluorescence microscopy, lack of spectral sensitivity to changes in extracellular Ca2+ and Mg2+, and by equivalent interaction of ionophore with membranes of whole and sonicated cells. A23187 (10 micronm) increased amylase release 200% in the presence of extracellular Ca2+ and Mg2+. In the absence of Ca2+ (but in the presence of Mg2+) A23187 did not increase amylase release. A23187 (10 micronm) also produced Ca2+ -dependent cell damage, as judged by increased LDH release, increased permeability to trypan blue, and by disruption of cell morphology. The cell damaging and amylase releasing properties of A23187 were distinguished by their time course and dose-response relationship. A23187 (1 micronm) increased amylase release 140% without increasing LDH release or permeability to trypan blue.     

The effects of Ca2+ and Sr2+ on Ca2+-sensitive biochemical changes in human erythrocytes and their membranes.

1. The Ca2+-dependency of K+ efflux, microvesiculation and breakdown of polyphosphoinositides and of ankyrin have been measured in intact human erythrocytes exposed to ionophore A23187 and HEDTA [N'-(2-hydroxyethyl)ethylenediamine NNN'-triacetate]-Ca2+ buffers. Half-maximal responses were observed at pCa values of 6.4, 4.1, 5.0 and 4.8 respectively. 2. The Ca2+ dependencies of K+ efflux and breakdown of polyphosphoinositides and ankyrin measured in erythrocyte ghosts without addition of ionophore showed almost identical values with those seen in whole cells treated with ionophore. 3. We conclude that ionophore A23187 is able to cause rapid equilibration of extracellular and intracellular [Ca2+] in intact cells and that in the presence of a suitable Ca2+ buffer, ionophore A23187 can be used to precisely fix the intracellular concentration of Ca2+ in erythrocytes. 4. The relatively high concentration of Ca2+ required to produce microvesiculation in intact cells may indicate that microvesiculation could be at least partly dependent on a direct interaction of Ca2+ with phospholipid. 5. Results obtained with Sr2+ paralleled those with Ca2+, although higher Sr2+ concentrations were required to achieve the same effects as Ca2+. Mg2+ produced none of the changes seen with Ca2+ or Sr2+.     

The effect of ETH 1001 on ion fluxes across red blood cell membranes

The calcium selective ionophore, ETH 1001, and the divalent cation ionophore, A23187, promoted Ca2+ flux across RBC membranes under various experimental conditions. ETH 1001 did not promote the passive movement of Mg2+ whereas A23187 did. The results confirm the potential application of ETH 1001 as a Ca2+ selective ionophore for biological membranes.     

Participation of the microtubular-microfilamentous system on intracellular Ca2+ transport and acid secretion in dispersed parietal cells

Biphasic responses of amino[14C]pyrine accumulation and oxygen consumption were registered by gastrin stimulation in dispersed parietal cells from guinea pig gastric mucosa, and this was mimicked with the calcium ionophore A23187. The characteristics of these phases (first phase and second phase) were distinguished by the differences in the requirements of extracellular Ca2+. The first phase evoked by gastrin or ionophore A23187 was independent of extracellular Ca2+, whereas the second phase was not. In the first phase, fluorescence of a cytosolic Ca2+ indicator (quin2-AM) increased with the stimulation of ionophore A23187 and carbamylcholine chloride in the presence of extracellular Ca2+. In addition, an increase in cytosolic Ca2+ induced by ionophore A23187, but not by carbamylcholine chloride was also observed in the absence of extracellular Ca2+, suggesting that Ca2+ pool(s) in parietal cells might be present in the intracellular organelle. Cytochalasin B and colchicine, but not oligomycin, could eliminate this cytosolic Ca2+ increase induced by A23187 in a Ca2+-free medium. On the other hand, in a Ca2+-free medium, addition of ATP after pretreatment with digitonin could diminish the cytosolic Ca2+ increase brought about by A23187. This was also observed with oligomycin-treated cells, but not with cytochalasin B-treated cells. Similarly, subcellular fractionation of a parietal cell which had been pretreated with cytochalasin B or colchicine in an intact cell system reduced the rate of ATP-dependent Ca2+ uptake. These observations indicate that intracellular Ca2+ transport in dispersed parietal cells may be regulated by the microtubular-microfilamentous system. In conclusion, this study demonstrates the possibility of the existence of intracellular Ca2+ transport mediated by gastrin or ionophore A23187 and regulated by the microtubular-microfilamentous system in parietal cells.     

The effects of the ionophore A23187 on pattern formation in the alga Micrasterias

We have used the divalent cation ionophore A23187 to investigate the hypothesis that cytoplasmic localization of Ca2+ is responsible for localized growth in the alga Micrasterias. In a preliminary study we found that, of the major salts contained in the cell's medium, only CaCl2 was needed for normal development. In cells developing in the presence of A23187 and extracellular Ca2+, we postulated that the ionophore would induce a spatially uniform influx of Ca2+ that would overwhelm endogenous Ca2+ gradients. When developing cells were treated with A23187 and 2 mM CaCl2, we observed a delocalization of the cell's normal pattern of wall deposition. This effect was less pronounced when cells were exposed to A23187 and 2 mM MgCl2. These results support the hypothesis that localized regions of high Ca2+ concentration normally mediate localized expansion of tip-growing lobes in Micrasterias.     

Calcium dependence of the stimulatory action of hypertonicity on renal medullary prostaglandin synthesis

Previous studies have shown that hypertonic mannitol or NaCl increases the release of [3H]arachidonate and immunoreactive prostaglandin E in inner medullary slices incubated in Ca2+-free media containing EGTA. By contrast, the stimulation of these parameters by ionophore A23187 and by arginine-vasopressin are abolished in Ca2+-free media plus EGTA. In the present study, the effects of Ca2+ deprivation and the intracellular Ca2+ antagonist TMB-8 [8-N,N-diethylamino)octyl-3,4,5 -trimethoxybenzoate-HCl) were further examined to assess the Ca2+ dependence of the actions of different stimuli of prostaglandin E synthesis in rat renal inner medulla. Ca2+-free media without EGTA abolished increases in [3H]arachidonate and immunoreactive prostaglandin E release induced by ionophore A23187, but not those induced by arginine-vasopressin, suggesting that different pools of Ca2+ subserve expression of the actions of these two stimuli. At low concentrations, TMB-8 (10-25 microM) inhibited increases in [3H]arachidonate and immunoreactive prostaglandin E release induced by arginine-vasopressin, but did not influence effects of Ca2+ plus ionophore A23187 or hypertonicity on these parameters. At higher concentrations (100-500 microM), TMB-8 suppressed effects of ionophore A23187, hyperosmolar NaCl and mannitol on immunoreactive prostaglandin E and [3H]arachidonate release from slices. The effects of a sub-optimal inhibitory concentration of TMB-8 on ionophore A23187 actions were overcome by increasing Ca2+ in the media from 1.5 to 5 mM. Ca2+ deprivation, or concentrations of EGTA or TMB-8, that were effective in suppressing increases in immunoreactive prostaglandin E induced by ionophore A23187, arginine-vasopressin or hypertonicity, did not modify increases in immunoreactive prostaglandin E induced by exogenous arachidonate. Moreover, in microsomal fractions of inner medulla, TMB-8 suppressed Ca2+-dependent increases in phospholipase A2 and C activities, an effect which was competitive with Ca2+. Thus, Ca2+ deprivation and TMB-8 act at a step in the immunoreactive prostaglandin E synthetic pathway proximal to cyclooxygenase activity, and probably at the level of Ca2+-dependent acyl hydrolase activity. The results with TMB-8 indicate that an intracellular pool of Ca2+ is involved in expression of the actions of hypertonicity to increase [3H]arachidonate release and immunoreactive prostaglandin E in inner medulla.     

Prevention of Ca(2+)-induced or thromboxane B2-induced hepatocyte plasma membrane bleb formation by thromboxane receptor antagonists.

Isolated hepatocytes incubated in the presence of either Ca2+ ionophore A23187 or thromboxane B2 develop many plasma membrane blebs which are a characteristic feature of toxic or ischaemic cell injury. When hepatocytes are incubated in the presence of both Ca2+ ionophore A23187 and any one of three thromboxane receptor antagonists (SK and F 88046, B.M. 13505, B.M. 13177), bleb formation is strongly inhibited. Hepatocytes incubated in the presence of both thromboxane B2 and any one of the three thromboxane receptor antagonists are also well protected from the formation of blebs. Treatment of isolated hepatocytes with Ca2+ ionophore A23187 is known to stimulate the production of thromboxanes. The data presented are consistent with thromboxane B2 acting as an intermediary in a proposed mechanism of cell injury and death in which elevated cytosolic free Ca2+ levels activate phospholipase A2 and the arachidonate cascade.     

influence of Ca channel blockers on the positive inotropic effect of ionophore A23187 in the myocardium

Calcium ionophore A23187 (10(-5) M) increases the force of contraction of the frog atrium up to 27 + 4.8%. The calcium antagonists d-600 (5 X 10- M), Zn2+ (2 X 10(-5) M), Mn2+ (2 X 10(-4) M) decrease the force of contraction 50, 10 and 20%, respectively, and inhibit the positive inotropic effect of ionophore A23187. Inhibition of the ionophore effect by the blockers is determined by the ability of D-600, Zn2+ and Mn2+ to form complexes with the ionophore. Besides, the affinity of these blockers to the ionophore is higher than that of Ca2+. It is assumed that Zn2+, Mn2+ and D-600 possess higher affinity to ionophore A23187 as compared with myocardial Ca-channels. Fenigidin interacts with Ca-channels to a larger degree than with ionophore A23187.     

Potentiation of PGE1-induced increase in cyclic AMP by chemotactic peptide and Ca2+ ionophore through calmodulin-dependent processes

The interaction between prostaglandin E1 (PGE1) and chemotactic peptide formylmethionyl-leucyl-phenylalanine (fMLP) in cAMP production in guinea pig neutrophils was investigated. Both PGE1 and fMLP increased the cAMP content in neutrophils. At low concentrations of PGE1 (less than 10 nM), the effects of fMLP and PGE1 in stimulating cAMP accumulation were additive, but at high concentrations of PGE1, their effects were synergistic. The effects of PGE1 and Ca2+ ionophore A23187 instead of fMLP on cAMP accumulation were also synergistic. The synergy did not appear to be related to change in cyclic nucleotide phosphodiesterase activity, because it was still marked in the presence of isobutyl-3-methyl-1-xanthine, a phosphodiesterase inhibitor. Studies on the time course of PGE1-induced cAMP accumulation showed that cAMP production ceased within 5 min after the addition of high concentrations of PGE1. The period of cAMP production could not be prolonged by combined treatment with PGE1 and fMLP or Ca2+ ionophore A23187. The synergy was found to be caused through Ca2+-dependent processes, because depletion of the medium of Ca2+ and addition of the Ca2+ antagonist TMB-8 inhibited the synergistic increase in cAMP. Moreover, the calmodulin antagonist W-7 also effectively inhibited the synergistic increase in cAMP. These results suggest that the potentiation of PGE1-induced cAMP production by fMLP or Ca2+ ionophore A23187 is catalyzed by calmodulin-dependent processes. However, the synergistic increase in cAMP production was not inhibited by arachidonic acid cascade inhibitors such as indomethacin, BW755C, or nordihydroguiaretic acid, and a combination of PGE1 and a protein kinase C activator, tetradecanoyl phorbol acetate (TPA), did not cause synergistic increase in cAMP. Marked increase in cAMP was also induced by a combination of cholera toxin and fMLP or Ca2+ ionophore A23187, but not by a combination of forskolin and fMLP or Ca2+ ionophore A23187. The synergistic increase in cAMP was not sustained in isolated membranes. On the contrary, PGE1-induced cAMP production in isolated membranes was suppressed by their pretreatment with fMLP or Ca2+ ionophore A23187. These data suggest that the synergistic effects of PGE1 and fMLP or Ca2+ ionophore in increasing the cAMP level are due to potentiation of PGE1-induced cAMP production by Ca2+ and calmodulin-dependent processes.     

Effects of ionophore A23187 and lanthanum on pepsinogen secretion from frog esophageal mucosa in vitro

The role of Ca2+ in the mediation of pepsinogen secretion from frog esophagus was investigated by means of ionophore A23187 and LaCl3. The esophageal mucosa from Asian bullfrog Rana tigerina was mounted in a double-chamber system to preserve its polarity and was incubated in a medium containing 1.5 mM CaCl2. Pepsinogen secreted was measured and expressed as % of total. The basal secretion averaged 3.5%/h. Bethanechol (25 microM), dibutyryl-cAMP (10 mM), ionophore A23187 (30 microM) and 3-isobutyl-1-methylxanthine (0.1 mM) increased the secretion to 8.7, 7.4, 7.1 and 6.8%, respectively. The stimulatory effect of bethanechol and of dibutyryl-cAMP were not affected by removing the exogenous Ca2+ with EGTA. The basal secretion was, however, reduced by 50% when Ca2+ in the incubation medium was lowered to 20 microM. At this low Ca2+ concentration, ionophore A23187 not only lost its stimulatory effect but also diminished the stimulation caused by bethanechol and dibutyryl-cAMP. While LaCl3 at 1 mM had no effect on basal and bethanechol-stimulated secretion, at 10 mM it abolished the stimulation evoked by bethanechol or dibutyryl-cAMP. The conclusions are: (1) both Ca2+ and cAMP are involved in the mediation of pepsinogen secretion from frog esophagus, (2) basal secretion is dependent on extracellular Ca2+, whereas bethanechol-stimulated secretion is not, (3) in the plasma membranes of peptic cells may exist a distinct Ca2+ pool (La3+-and ionophore A23187-sensitive) which is involved in the stimulated pepsinogen secretion.     

Calcium ion-flux across phosphatidylcholine membranes mediated by ionophore A23187.

The antibiotic A23187 carries Ca2+ across Müller-Rudin membranes made from 1,2-dierucoyl-sn-glycero-3-phosphocholine and n-decane. The conductance of the membranes is not increased by the Ca2+-transport. The flux depends linearly on Ca2+ concentration and ionophore concentration (above pH 6). It increases with increasing pH, approximately by a factor of 4-5 between pH 6 and pH 8. Maximal Ca2+-fluxes of about 10(-10) mol-cm-2-s-1 were found. A counter transport of H+ could not be detected. The complex formation between A23187 and Ca2+ in egg phosphotidylcholine vesicles was studied spectroscopically. The results are consistent with the formation of a 2:1 complex. Optical absorption measurements on single phophatidylcholine membranes were used to calculate the concentration of membrane-bound ionophore A23187.     

The role of calcium ions in the regulation of rat thymocyte pyruvate oxidation by mitogens.

1. Calcium concentrations in the nanomolar range cause a specific stimulation of the oxidation of pyruvate by isolated mitochondria from rat thymus that is sufficient to account precisely for the stimulation of pyruvate oxidation observed when rat thymocytes are incubated with the mitogens concanavalin A or ionophore A23187. 2. Higher concentrations of Ca2+ (more than 50 nM) inhibit the oxidation of NAD+-linked substrates by rat thymus mitochondria without affecting the oxidation of succinate or ascorbate+ NNN'N'-tetramethyl-p-phenylendiamine. 3. The addition of Ni2+ or Co2+ (2mM) to rat thymocytes prevents the response to concanavalin A at the level of pyruvate oxidation without affecting the stimulation of glycolysis induced by this mitogen. In contrast, the complete metabolic response to the ionophore A23187 is abolished by these ions. Ni2+ and Co2+ interfere with the ability of the ionophore to transport Ca2+ across the plasma membrane. 4. Concanavalin A, but not ionophore A23187, increases the respiratory inhibition induced by Ni2+ and Co2+. 5. These results support the view that mitogens stimulate lymphocyte pyruvate oxidation through an increase in cellular Ca2+ uptake.     

The dissociation of exocytosis and respiratory stimulation in leucocytes by ionophores.

By exploiting the unique characteristics of three ionophores, experimental conditions were found which permit the dissociation of respiratory stimulation from secretion in polymorphonuclear leucocytes. A marked stimulation of respiration was produced by ionophore X537A, which binds and transports both alkali-earth and alkali cations. The stimulatory activity of this ionophore was the same at either high or low Na+/K+ ratios in the medium and was virtually unaffected by extracellular Ca2+. A slight stimulation of oxygen consumption was also caused by the K+-selective ionophore valinomycin and by ionophore A23187, which complexes and transfers bivalent cations. Ionophore X537A and valinomycin were unable to stimulate selective release of granuleassociated beta-glucuronidase and gradually increased cell fragility, as monitored by increased leakage of lactate dehydrogenase. Ionophore A23187 slightly increased exocytosis of beta-glucuronidase. In a Mg2+-free medium, Ca2+, added simultaneously with ionophore A23187, greatly enhanced respiration and secretion of the granule enzyme. If Ca2+ was added a few minutes after the ionophore, exocytosis occurred, but no respiratory burst was observed. If the latter experiment was repeated in the presence of extracellular Mg2+, both secretion and respiration were stimulated. This effect was not produced by Mn2+ or Ba2+. It is proposed that Ca2+ is required for triggering selective secretion of granule enzymes from leucocytes is caused by an intracellular redistribution of cations, which may invovle Mg2+-dependent mechanisms.     

Rapid clearance of Plasmodium yoelii-infected erythrocytes after exposure to the ionophore A23187

1. The effects of Ca2+ and the calcium ionophore A23187 on the intraerythrocytic development of the asexual forms of Plasmodium yoelii were examined. 2. Erythrocyte-free parasites obtained by saponin lysis of infected cells remained viable after exposure to 1 mM Ca2+. 3. A23187 inhibited the growth of P. yoelii and the inhibition was augmented by Ca2+ in cells infected with parasites at young stage of development. 4. A23187-treated infected cells disappeared from the circulation shortly after intravenous injection and this disappearance was profound in infected cells treated with the ionophore in the presence of Ca2+.     

Calcium-dependent irreversible effect of ionophore A23187 on melanophores

The calcium ionophore, A231187, induces a Ca2+ -dependent movement (dispersion) of melanosomes within skin melanophores of the lizard, Anolis carolinensis, in vitro. The effects of A23187 are irreversible, since after repeated rinsing of the skins in the absence of the ionophore they will always darken in Ringer containing Ca2+ but will immediately lighten when transferred to Ca2+ -free Ringer. These results suggest that the ionophore is irreversibly localized to the melanophore membrane and that its melanosome-dispersing effect is continuously dependent upon extracellular calcium.     

Stimulation of an exocytotic event, the hamster sperm acrosome reaction, by cis-unsaturated fatty acids

The influence of extracellular Mg2+ on Ca2+ ionophore (A23187 and ionomycin) induced secretion and changes in the cytosol pH of rabbit neutrophils suspended in Ca2+-free buffer has been investigated. While extracellular Ca2+ is obligatory for ionomycin induced secretion, we have defined conditions under which A23187 can induce secretion in Ca2+-free media. The different behaviour of these two Ca2+ ionophores is discussed on the basis of their different counter cation specificities.