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.     

Mechanism of Inhibition of N-Methyl-d-Aspartate-Stimulated Increases in Free Intracellular Ca2+ Concentration by Ethanol

Dissociated brain cells were isolated from newborn rat pups and loaded with fura-2. These cells were sensitive to low N-methyl-D-aspartate (NMDA) concentrations with EC50 values for NMDA-induced intracellular Ca2+ concentration ([Ca2+]i) increases of approximately 7-16 microM measured in the absence of Mg2+. NMDA-stimulated [Ca2+]i increases could be observed in buffer with Mg2+ when the cells were predepolarized with 15 mM KCl prior to NMDA addition. Under these predepolarized conditions, 100 mM ethanol inhibited 25 microM NMDA responses by approximately 50%, which was similar to the ethanol inhibition observed in buffer without added Mg2+. Ethanol did not alter [Ca2+]i prior to NMDA addition. In the absence of Mg2+, 50 and 100 mM ethanol did not significantly alter the EC50 value for NMDA, but did inhibit NMDA-induced increases in [Ca2+]i in a concentration-dependent manner at 4, 16, 64, and 256 microM NMDA. Whereas NMDA-induced increases in [Ca2+]i were dependent on extracellular Ca2+ and were inhibited by Mg2+, the ability of 100 mM ethanol to inhibit 25 microM NMDA responses was independent of the external Ca2+ or Mg2+ concentrations. Glycine (1, 10, and 100 microM) enhanced 25 microM NMDA-induced increases in [Ca2+]i by approximately 50%. Glycine (1-100 microM) prevented the 100 mM ethanol inhibition of NMDA-stimulated [Ca2+]i observed in the absence of exogenous glycine. MK-801 (25-400 nM) inhibited 25 microM NMDA-stimulated rises in [Ca2+]i in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition and activation of bovine heart NAD-specific isocitrate dehydrogenase by ATP

In the absence of added calcium, inhibition of NAD-specific isocitrate dehydrogenase by ATP occurred without ADP (I0.5 = 1.8 mM) and with 0.2 mM ADP3- (I0.5 = 1.0 mM) at subsaturating substrate concentrations at pH 7.4. Inhibition by ATP was competitive with NAD+ in the presence and absence of ADP and was not reversed by magnesium citrate. No reversal of ATP inhibition by free Ca2+ was observed in the presence of ADP (0.2 mM). However, when ADP was absent, increasing Ca2+ first caused progressive reversal of ATP inhibition followed by activation by ATP. Without ADP, the S0.5 for calcium activation was 80-140 microM at ATP concentrations between 0.6 and 3.0 mM. The S0.5 for ATP activation, in the absence of ADP, was 1.1 and 2.1 microM when free Ca2+ was held constant at 0.1 and 1.0 mM, respectively. As in activation by ADP, ATP decreased the S0.5 for magnesium isocitrate without affecting V. However, in contrast to ADP, the activation by ATP occurred without lowering the Hill coefficient for the substrate. GDP activated the enzyme at relatively high concentrations of Ca2+ but not without added Ca2+.     

Regulation of abalone sperm cyclic AMP concentrations and the acrosome reaction by calcium and methylxanthines

When Ca2+ is added to abalone sperm (Haliotis rufescens) in Ca2+-free artificial seawater (CaFASW) to a final concentration of 9.6 mM a 4-fold elevation in sperm cAMP occurs within 15-30 sec. The methylxanthines, theophylline and 1-methyl-3-isobutylxanthine (MIX), also elevate sperm cAMP concentrations. In CaFASW, either compound causes up to a 3-fold increase in cAMP within 15-30 sec. MIX (150 microM), added to sperm in the presence of 9.6 mM Ca2+, elevates sperm cAMP 100-fold within 15-30 sec and also triggers the acrosome reaction (AR) in the same period. Under identical conditions theophylline (1.67 mM) is much less potent at elevating cAMP and inducing the AR. The effects of methylxanthines on cAMP of sperm incubated in the presence of Ca2+ appear to represent a potentiation by these compounds of the action of Ca2+. Neither compound induces the AR in the absence of Ca2+. All of the observed effects on sperm cAMP and the AR are dependent on Ca2+ and methylxanthine concentrations. Added cyclic nucleotides or their derivatives do not induce the AR in either the absence or presence of Ca2+. Experiments with isolated sperm heads and flagella indicate that the dramatic stimulatory response of sperm cAMP to Ca2+ plus MIX is present in the head region (acrosome, nucleus, midpiece) of the cell. The data suggest that the dramatic elevation of cAMP by MIX in the presence of Ca2+ may occur directly by an inhibition of phosphodiesterase activity and indirectly by an increase in cellular Ca2+. A strong temporal correlation between the cAMP elevation and the abalone AR exists, although cAMP elevation by itself does not act as the primary mediator of this exocytotic event.     

Calcium requirement for the inhibition by theophylline of histamine release from mast cells

When added to Ca2+-free Hanks' solution, Ca2+ (0.1-2.5 mM) had no significant effect on antigen-induced histamine release from rat mast cells, but Sr2+ (1.0-3.0 mM) dose-dependently increased the release. Ba2+ (1.0 and 2.0 mM) also enhanced the release. Ca2+ and Ba2+ inhibited compound 40/80-induced histamine release, in a dose-dependent manner. In ordinary Hanks' medium, theophylline and 3-isobutyl-1-methylxanthine (IBMX) dose-dependently inhibited the antigen-induced histamine release but these drugs were ineffective in Ca2+-free medium. Theophylline (1.0 mM) also inhibited compound 48/80-induced histamine release in the presence but not absence of Ca2+. There was an optimal Ca2+ concentration for the theophylline effect. Sr2+ but not Ba2+ could substitute for Ca2+ in supporting the theophylline effect. Theophylline (1.0 mM) and IBMX (1.0 mM) increased mast cell cyclic AMP levels both in the presence and absence of Ca2+. These results suggest that Ca2+ is required in the interaction of theophylline and specific sites on mast cells or in the mast cell response to theophylline which probably does not involve the cyclic AMP increase and is linked to the inhibition of histamine release.     

Comparative enzymatic properties of avian liver and skeletal muscle D-fructose-1,6-bisphosphate 1-phosphohydrolase.

The enzymatic properties of purified preparations of chicken liver and chicken skeletal muscle fructose bisphosphatases (D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) were compared. Both enzymes have an absolute requirement for Mg2+ or Mn2+. The apparent Km for MgCl2 at pH 7.5 was 0.5 mM for the muscle enzyme and 5 mM for the liver enzyme. Fructose bisphosphate inhibited both enzymes. At pH 7.5, the inhibitor constants (Ki) were 0.18 and 1.3 mM for muscle and liver fructose bisphosphatases, respectively. The muscle enzyme was considerably more sensitive to AMP inhibition than the liver enzyme. At pH 7.5 and in the presence of 1 mM MgCl2, 50% inhibition of muscle and liver fructose bisphosphatases occurred at AMP concentrations of 7 X 10(-9) and 1 X 10(-6) M, respectively. EDTA activated both enzymes. The degree of activation was time and concentration dependent. The degree of EDTA activation of both enzymes decreased with increasing MgCl2 concentration. Ca2+ was a potent inhibitor of both liver (Ki, 1 X 10(-4) M) and muscle (Ki, 1 X 10(-5) M) fructose bisphosphatase. This inhibition was reversed by the presence of EDTA. Ca2+ appears to be a competitive inhibitor with regard to Mg2+. There is, however, a positive homeotropic interaction among Mg2+ sites of both enzymes in the presence of Ca2+.     

The activation by Ca2+ of platelet phospholipase A2. Effects of dibutyryl cyclic adenosine monophosphate and 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate.

Thrombin-induced release of arachidonic acid from human platelet phosphatidylcholine is found to be more than 90% impaired by incubation of platelets with 1 mM dibutyryl cyclic adenosine monophosphate (Bt2 cyclic AMP) or with 0.6 mM 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8), an intracellular calcium antagonist. Incorporation of arachidonic acid into platelet phospholipids is not enhanced by Bt2 cyclic AMP. The addition of external Ca2+ to thrombin-treated platelets incubated with Bt2 cyclic AMP or TMB-8 does not counteract the observed inhibition. However, when divalent cation ionophore A23187 is employed as an activating agent, much less inhibition is produced by Bt2 cyclic AMP or TMB-8. The inhibition which does result can be overcome by added Ca2+. Inhibition of arachidonic acid liberation by Bt2 cyclic AMP, but not by TMB-8, can be overcome by high concentrations of A23187. When Mg2+ is substituted for Ca2+, ionophore-induced release of arachidonic acid from phosphatidylcholine of inhibitor-free controls is depressed and inhibition by Bt2 cyclic AMP is slightly enhanced. The phospholipase A2 activity of platelet lysates is increased by the presence of added Ca2+, however, the addition of either A23187 or Bt2 cyclic AMP is without effect on this activity. We suggest that Bt2 cyclic AMP may promote a compartmentalization of Ca2+, thereby inhibiting phospholipase A activity. The compartmentalization may be overcome by ionophore. By contrast, TMB-8 may immobilize platelet Ca2+ stores in situ or restrict access of Ca2+ to phospholipase A in a manner not susceptible to reversal by high concentrations of ionophore.     

Activation of potassium channels in erythrocytes of marine teleost Scorpaena porcus

To assess the possibility of stimulating Ca2+-activated K+ channels, marine fish erythrocytes were incubated at 20-22 degrees C in saline containing a Ca2+-ATPase inhibitor (orthovanadate), a Ca2+ ionophore (A23187), propranolol or Pb2+. Incubation of the cells for up to 2 h under control conditions or in the presence of 5 mM NH4VO3 and 1 mM Ca2+ did not affect the intracellular K+ and Na+ concentrations. About 50% cellular K+ was lost from erythrocytes incubated in the presence of 0.01 mM A23187, 1 mM EGTA and 0.4-1.0 mM Ca2+. There was a significant loss of cellular K+ after the addition of 0.05-0.2 mM propranolol to the incubation medium. The stimulatory effect of propranolol on the K+ efflux was independent of external Ca2+. Blockers of Ca2+ transport, verapamil and Co2+, caused only a small decrease in the K+ loss induced by propranolol. The treatment of erythrocytes with 1-2 microM Pb2+ led to a minor K+ loss, but at a Pb2+ concentration of 20-50 microM, about 70% cellular K+ was lost. The K+ efflux induced by propranolol or Pb2+ was completely blocked by 1 mM quinine. The induced K+ loss from the erythrocytes was accompanied by a slight increase in the intracellular Na+ concentration. These data indicate the possibility of inducing Ca2+- and Pb2+-activated potassium channels in erythrocytes of S. porcus. A distinctive feature of the cells is a high sensitivity to propranolol, which activates K+ channels in the absence of external Ca2+.     

Calcium-sensitive modulation of the actomyosin ATPase by fodrin

Fodrin, a spectrin-like protein isolated from brain, is a long flexible molecule which binds calmodulin and cross-links F-actin. The effects of fodrin on the actin-activated ATPase of myosin have been examined. When added after ATP, fodrin inhibited the actomyosin ATPase. Two to three times as much fodrin was required for inhibition in the presence of Ca2+ as in its absence. Complete inhibition in the absence of Ca2+ occurred at about one fodrin to 200 actins. Inhibition does not appear to result from fodrin cross-linking F-actin, and, thereby, preventing the myosin filaments from reaching the actin filaments; but cross-linking may promote inhibition by trapping the myosin filaments within the cross-linked F-actin. When added before ATP, fodrin stimulated the actomyosin ATPase almost 3-fold in the presence of Ca2+ and by less than 50% in the absence of Ca2+. Stimulation is thought to result from fodrin cross-linking F-actin. After several minutes the stimulations in Ca2+ were greatly reduced, and in the absence of Ca2+ the actomyosin ATPases were substantially inhibited. Whether added before or after ATP, fodrin inhibited the actin-activated ATPase of myosin subfragment 1. This inhibition was also slightly Ca2+ sensitive.     

Accumulation of cadmium in pancreatic beta cells is similar to that of calcium in being stimulated by both glucose and high potassium

The transport of Cd2+ and the effects of this ion on secretory activity and metabolism were investigated in beta cell-rich pancreatic islets isolated from obese-hyperglycemic mice. The endogenous cadmium content was 2.5 mumol/kg dry wt. After 60 min of incubation in a Ca2+-deficient medium containing 2.5 microM Cd2+ the islet cadmium content increased to 0.18 mmol/kg dry wt. This uptake was reduced by approx. 50% in the presence of 1.28 mM Ca2+. The incorporation of Cd2+ was stimulated either by raising the concentration of glucose to 20 mM or K+ to 30.9 mM. Whereas D-600 suppressed the stimulatory effect of glucose by 75%, it completely abolished that obtained with high K+. Only about 40% of the incorporated cadmium was mobilized during 60 min of incubation in a Cd2+-free medium containing 0.5 mM EGTA. It was possible to demonstrate a glucose-induced suppression of Cd2+ efflux into a Ca2+-deficient medium. Concentrations of Cd2+ up to 2.5 microM did not affect glucose oxidation, whereas, there was a progressive inhibition when the Cd2+ concentration was above 10 microM. Basal insulin release was stimulated by 5 microM Cd2+. At a concentration of 160 microM, Cd2+ did not affect basal insulin release but significantly inhibited the secretory response to glucose. It is concluded that the beta cell uptake of Cd2+ is facilitated by the activation of voltage-dependent Ca2+ channels. Apparently, the accumulation of Cd2+ mimics that of Ca2+ also involving a component of intracellular sequestration promoted by glucose.     

Calcium uptake by placental plasma membrane vesicles.

The placental plasma membrane vesicles are capable of accumulating up to 190 mM Ca2+. This is 24-fold higher than the external Ca2+ concentration. This process is dependent on ATP hydrolysis by the placental Ca2+-ATPase. The Pi/Ca ratio is dependent on the external Ca2+ concentration, and reaches the value of 2 at 10 mM Ca2+. Phosphate (5 mM) can double Ca2+ uptake when measured in the presence of 5 mM Ca2+. Mg2+ increased Ca2+ uptake only at low Ca2+ concentrations, and had no significant effect at 5 mM Ca2+.     

Participation of calcium in flagellar shortening and regeneration in Chlamydomonas reinhardii

Cation chelators cause flagellar shortening in Chlamydomonas reinhardii. Most effective are EDTA and EGTA (1 mM) but pyrophosphate (10 mM) also is effective. Addition of 5 mM Ca2+ after shortening caused by 4 mM EGTA results in flagellar regeneration. Other divalent cations can replace Ca2+ with the following relative activities: Ca2+ greater than Sr2+ = Mn2+ much greater than Ba2+ = Mg2+. Although the specific ion requirement to reverse shortening is not clear, it is possible that all of the ions act by displacing one bound cation, presumably Ca2+. A specific requirement for Ca2+ in flagellar regeneration could be demonstrated, however, because as little as 50 microM EGTA in the presence of 500 microM Mg2+ delayed regeneration and prevented full regeneration. Ca2+ at 100 microM overcame this inhibition.     

Effects of zinc chloride on the hydrolysis of cyclic GMP and cyclic AMP by the activator-dependent cyclic nucleotide phosphodiesterase from bovine heart.

In the presence of 10 micrometer Ca2+ and 5 mM Mg2+ (or 0.25 mM Mg2+), the addition of 100 micrometer Zn2+, Ni2+, Co2+, Fe2+, Cu2+ or 1 mM Mn2+ resulted in varying degrees of stimulation or inhibition of 10(-6) M cyclic GMP and cyclic AMP hydrolysis by the activator-dependent cyclic nucleotide phosphodiesterase from bovine heart in the absence or presence of phosphodiesterase activator. The substrate specificity of the enzyme was altered under several conditions. The addition of Zn2+ in the presence of 5 mM Mg2+ and the absence of activator resulted in the stimulation of cyclic GMP hydrolysis over a narrow substrate range while reducing the V 65% due to a shift in the kinetics from non-linear with Mg2+ alone to linear in the presence of Zn2+ and Mg2+. Zn2+ inhibited the hydrolysis of cyclic GMP and cyclic AMP in the presence of activator with Ki values of 70 and 100 micrometer, respectively. Zn2+ inhibition was non-competitive with substrate, activator and Ca2+ but was competitive with Mg2+. In the presence of 10 micrometer Ca2+ and activator, a Ki of 15 micrometer for Zn2+ vs. Mg2+ was noted in the hydrolysis of 10(-6) M cyclic GMP. Several effects of Zn2+ are discussed which have been noted in other studies and might be due in part to changes in cyclic nucleotide levels following phosphodiesterase inhibition.     

Regulation of citric acid cycle by calcium

The relationship of extramitochondrial Ca2+ to intramitochondrial Ca2+ and the influence of intramitochondrial free Ca2+ concentrations on various steps of the citric acid cycle were evaluated. Ca2+ was measured using the Ca2+ sensitive fluorescent dye fura-2 trapped inside the rat heart mitochondria. The rate of utilization of specific substrates and the rate of accumulation of citric acid cycle intermediates were measured at matrix free Ca2+ ranging from 0 to 1.2 microM. A change in matrix free Ca2+ from 0 to 0.3 microM caused a 135% increase in ADP stimulated oxidation of 0.6 mM alpha-ketoglutarate (K0.5 = 0.15 microM). In the absence of ADP and the presence of 0.6 mM alpha-ketoglutarate, Ca2+ (0.3 microM) increased NAD(H) reduction from 0 to 40%. On the other hand, when pyruvate (10 microM to 5 mM) was substrate, pyruvate dehydrogenase flux was insensitive to Ca2+ and isocitrate dehydrogenase was sensitive to Ca2+ only in the presence of added ADP. In separate experiments pyruvate dehydrogenase activation (dephosphorylation) was measured. Under the conditions of the present study, pyruvate dehydrogenase was found to be almost 100% activated at all levels of Ca2+, thus explaining the Ca2+ insensitivity of the flux measurements. However, if the mitochondria were incubated in the absence of pyruvate, with excess alpha-ketoglutarate and excess ATP, the pyruvate dehydrogenase complex was only 20% active in the absence of added Ca2+ and activity increased to 100% at 2 microM Ca2+. Activation by Ca2+ required more Ca2+ (K0.5 = 1 microM) than for alpha-ketoglutarate dehydrogenase. The data suggest that in heart mitochondria alpha-ketoglutarate dehydrogenase may be a more physiologically relevant target of Ca2+ action than pyruvate dehydrogenase.     

Transient state kinetic studies of phosphorylation by ATP and Pi of the calcium-dependent ATPase from sarcoplasmic reticulum.

The ATPase of the sarcoplasmic reticulum is phosphorylated by ATP in the presence of Ca2+. A rapid phosphorylation was observed when the enzyme was preincubated with Ca2+ prior to the addition of 0.1 or 1 mM ATP. The rate of phosphorylation was decreased when Ca2+ was omitted from the preincubation medium and added with ATP when the reaction was started. The rate of phosphorylation by ATP was further decreased when Pi was included in the preincubation medium without Ca2+. In this case, the enzyme was phosphorylated by Pi during the preincubation. When Ca2+ and ATP were added, a burst of phosphorylation by ATP was observed in the initial 16 ms. In the subsequent incubation intervals, the phosphorylation by ATP was synchronous with the hydrolysis of the phosphoenzyme formed by Pi. The rate of hydrolysis of the phosphoenzyme formed by Pi was measured when either the Pi concentration was decreased 10 fold, or when Ca2+, ATP or ATP plus Ca2+ was added to the medium. Upon the single addition of Ca2+, the time for half-maximal decay was in the range 500--1000 ms. In all other conditions it was in the range 70--90 ms.     

Mechanism of stimulation of cyclic-GMP level in a neuronal cell line mediated by serotonin (5-HT3) receptors. Involvement of nitric oxide, arachidonic-acid metabolism and cytosolic Ca2+

The mechanism by which serotonin (5-HT3) receptors mediate a rise in cyclic-GMP level was investigated in a neuronal cell line. Inhibitors of phospholipase A2 (mepacrine) and of lipoxygenase (eicosatetraynoic acid or nordihydroguaiaretic acid) suppressed the action of serotonin. On the other hand, inhibition by hemoglobin indicates a role for nitric oxide which could be in part responsible for the cyclic-GMP effect as an intercellular stimulant. The suppression of the serotonin effect by the arginine analogues N omega-methyl-L-arginine and canavanine is consistent with the notion that nitric oxide could be released from arginine. The serotonin-induced rise of cyclic-GMP level depends on the presence of extracellular Ca2+ with half-maximal stimulation at 0.3 mM Ca2+. The serotonin-stimulated rise of cyclic GMP was inhibited by (a) addition of inorganic blockers of Ca2(+)-permeable channels (La3+, half-maximal inhibitory concentration (IC50) 0.04 mM; Mn2+, IC50, 0.4 mM; Co2+, IC50, 0.9 mM; Ni2+, IC50, 1.2 mM) and (b) of organic blockers (diltiazem: IC50, 6 microM, methoxyverapamil: IC50, 3 microM and (c) intracellular application of the Ca2+ chelator bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (IC50, 2 microM). Thus, two pathways for the activation of soluble guanylate cyclase by serotonin are possible: (a) via lipoxygenase products of arachidonic acid and/or (b) via nitric oxide or a related nitroso compound. Serotonin mediates a rise of cytosolic Ca2+ activity due to entry of extracellular Ca2+. It still has to be investigated which step depends on a rise of cytosolic Ca2+ activity that appears to be a prerequisite for activation of guanylate cyclase.     

Fendiline increases [Ca2+]i in Madin Darby canine kidney (MDCK) cells by releasing internal Ca2+ followed by capacitative Ca2+ entry

The effect of fendiline, a documented inhibitor of L-type Ca2+ channels and calmodulin, on Ca2+ signaling in Madin Darby canine kidney (MDCK) cells was investigated using fura-2 as a Ca2+ probe. Fendiline at 5-100 microM significantly increased [Ca2+]i concentration-dependently. The [Ca2+]i rise consisted of an initial rise and a slow decay. External Ca2+ removal partly inhibited the Ca2+ signals induced by 25-100 microM fendiline by reducing both the initial rise and the decay phase. This suggests that fendiline triggered external Ca2+ influx and internal Ca2+ release. In Ca(2+)-free medium, pretreatment with 50 microM fendiline nearly abolished the [Ca2+]i rise induced by 1 microM thapsigargin, an endoplasmic reticulum Ca2+ pump inhibitor, and vice versa, pretreatment with thapsigargin prevented fendiline from releasing internal Ca2+. This indicates that the internal Ca2+ source for fendiline overlaps with that for thapsigargin. At a concentration of 50 microM, fendiline caused Mn2+ quench of fura-2 fluorescence at the 360 nm excitation wavelenghth, which was inhibited by 0.1 mM La3+ by 50%, implying that fendiline-induced Ca2+ influx has two components separable by La3+. Consistently, 0.1 mM La3+ pretreatment suppressed fendiline-induced [Ca2+]i rise, and adding La3+ during the rising phase immediately inhibited the signal. Addition of 3 mM Ca2+ increased [Ca2+]i after preincubation with 50-100 microM fendiline in Ca(2+)-free medium. However, 50-100 microM fendiline inhibited 1 microM thapsigargin-induced capacitative Ca2+ entry. Pretreatment with 40 microM aristolochic acid to inhibit phospholipase A2 inhibited 50 microM fendiline-induced internal Ca2+ release by 48%, but inhibition of phospholipase C with 2 microM U73122 or inhibition of phospholipase D with 0.1 mM propranolol had no effect. Collectively, we have found that fendiline increased [Ca2+]i in MDCK cells by releasing internal Ca2+ in a manner independent of inositol-1,4,5-trisphosphate (IP3), followed by external Ca2+ influx.     

Regulation by calcium and calmodulin of adenylate cyclase from rabbit intestinal epithelium

The effect of calcium on adenylate cyclase from rabbit small intestine has been studied using a particulate preparation obtained from isolated epithelial cells. Both basal and vasoactive intestinal peptide-stimulated activities were inhibited by calcium concentrations in the micromolar range. In the presence of calmodulin, a biphasic response was obtained. At low calcium concentration (4 X 10(-9)-6 X 10(-8) M) the enzyme was activated up to 50%. As the Ca2+ concentration was increased, the enzyme was concomitantly inhibited. Half-maximal inhibition of calmodulin-dependent activity was obtained at 1 microM free Ca2+. The activation of the enzyme was also dependent on the concentration of Mg2+. At less than 1 microM Ca2+, the enzyme exhibited a biphasic response, being activated at below 3 mM Mg2+ and inhibited at higher concentrations. At Ca2+ concentrations that were inhibitory, the enzyme did not show the biphasic response to Mg2+. At concentrations above 3 mM, the maximal rate (Vmax) remained constant. Vmax was inversely proportional to the concentration of Ca2+ present. Calmodulin altered Vmax when acting on vasoactive intestinal peptide-stimulated enzyme. Calmodulin had no effect on the Km for hormone activation. The calmodulin-dependent activity was inhibited by incubation with trifluoperazine.     

The effect of phytochrome action on the activity of cytosolic cholinesterase in oat cells

Cholinesterases in the oat cell were found to be distributed in the cell wall (50%) and cytoplasm (42%). Activity of the cytosolic enzyme was inhibited about 80% by 1 mM Ca2+. The enzyme activity was also inhibited by Mn2+, but no inhibition by Mg2+ was observed. Effects of red light and calcium ion on the enzyme activity were investigated in vivo to confirm the involvement of phytochrome action in the regulation process of this enzyme via Ca2+. It was observed that inhibition by red light only occurs when external Ca2+ existed in the cell medium. Based on a previous report (8) that red light stimulates the influx of Ca2+ into the cytosol of oat cell, inhibition of the enzyme activity by irradiation of red light can be suggested to occur via the influx of Ca2+.     

Ca2+ translocation and catalytic activity of the sarcoplasmic reticulum ATPase. Modulation by ATP, Ca2+, and Pi

The ratio between Ca2+ uptake and Ca(2+)-dependent ATP hydrolysis measured in sarcoplasmic reticulum vesicles of rabbit skeletal muscle was found to vary greatly depending on the concentrations of oxalate or Pi used. In the presence of 5 mM oxalate, 20 mM Pi, and 1 mM Pi, the ratios found were in the range of 1.4-2.3, 0.6-0.8, and 0.01-0.10, respectively. The rates of Ca2+ exchange and ATP synthesis were measured at the steady state by adding trace amounts of 45Ca and 32Pi, after the vesicles had been loaded with Ca2+. In the presence of 1 mM Pi, 10 mM MgCl2, and 0.2 mM CaCl2, the ratio between Ca2+ exchange and ATP synthesis varied from 9 to 14. This ratio approached two when Ca2+ in the medium was reduced to a very low level, or when in the presence of Ca2+, dimethyl sulfoxide was added to the assay medium, or when the Pi concentration was raised from 1 to 20 mM. A ratio of two was also measured when the steady state was attained using ITP instead of ATP. In all the conditions that led to a ratio close to two, there was an increase in the fraction of enzyme phosphorylated by Pi. It is proposed that the coupling between Ca2+ translocation and ATP hydrolysis or synthesis is modulated by the phosphorylation of the ATPase by Pi.