Activation by divalent cations of a Ca2+-activated K+ channel from skeletal muscle membrane

Several divalent cations were studied as agonists of a Ca2+-activated K+ channel obtained from rat muscle membranes and incorporated into planar lipid bilayers. The effect of these agonists on single-channel currents was tested in the absence and in the presence of Ca2+. Among the divalent cations that activate the channel, Ca2+ is the most effective, followed by Cd2+, Sr2+, Mn2+, Fe2+, and Co2+. Mg2+, Ni2+, Ba2+, Cu2+, Zn2+, Hg2+, and Sn2+ are ineffective. The voltage dependence of channel activation is the same for all the divalent cations. The time-averaged probability of the open state is a sigmoidal function of the divalent cation concentration. The sigmoidal curves are described by a dissociation constant K and a Hill coefficient N. The values of these parameters, measured at 80 mV are: N = 2.1, K = 4 X 10(-7) mMN for Ca2+; N = 3.0, K = 0.02 mMN for Cd2+; N = 1.45, K = 0.63 mMN for Sr2+; N = 1.7, K = 0.94 mMN for Mn2+; N = 1.1, K = 3.0 mMN for Fe2+; and N = 1.1 K = 4.35 mMN for Co2+. In the presence of Ca2+, the divalent cations Cd2+, Co2+, Mn2+, Ni2+, and Mg2+ are able to increase the apparent affinity of the channel for Ca2+ and they increase the Hill coefficient in a concentration-dependent fashion. These divalent cations are only effective when added to the cytoplasmic side of the channel. We suggest that these divalent cations can bind to the channel, unmasking new Ca2+ sites.     

'Slow' K+-stimulated Ca2+ influx is mediated by Na+-Ca2+ exchange: a pharmacological study

K+-stimulated 45Ca2+ influx was measured in rat brain presynaptic nerve terminals that were predepolarized in a K+-rich solution for 15 s prior to addition of 45Ca2+. This 'slow' Ca2+ influx was compared to influx stimulated by Na+ removal, presumably mediated by Na+-Ca2+ exchange. The K+-stimulated Ca2+ influx in predepolarized synaptosomes, and the Na+-removal-dependent Ca2+ influx were both saturating functions of the external Ca2+ concentration; and both were half-saturated at 0.3 mM Ca2+. Both were reduced about 50% by 20 microM Hg2+, 20 microM Cu2+ or 0.45 mM Mn2+. Neither the K+-stimulated nor the Na+-removal-dependent Ca2+ influx was inhibited by 1 microM Cd2+, La3+ or Pb2+, treatments that almost completely inhibited K+-stimulated Ca2+ influx in synaptosomes that were not predepolarized. The relative permeabilities of K+-stimulated Ca2+, Sr2+ or Ba2+ influx in predepolarized synaptosomes (10:3:1) and the corresponding selectivity ratio for Na+-removal-dependent divalent cation uptake (10:2:1) were similar. These results strongly suggest that the K+-stimulated 'slow' Ca2+ influx in predepolarized synaptosomes and the Na+-removal-dependent Ca2+ influx are mediated by a common mechanism, the Na+-Ca2+ exchanger.     

A cation channel for K+ and Ca2+ from Tetrahymena cilia in planar lipid bilayers

The single-channel properties for monovalent and divalent cations of a voltage-independent cation channel from Tetrahymena cilia were studied in planar lipid bilayers. The single-channel conductance reached a maximum value as the K+ concentration was increased in symmetrical solutions of K+. The concentration dependence of the conductance was approximated to a simple saturation curve (a single-ion channel model) with an apparent Michaelis constant of 16.3 mM and a maximum conductance of 354 pS. Divalent cations (Ca2+, Ba2+, Sr2+, and Mg2+) also permeated this channel. The sequence of permeability determined by zero current potentials at high ionic concentrations was Ba2+ greater than or equal to K+ greater than or equal to Sr2+ greater than Mg2+ greater than Ca2+. Single-channel conductances for Ca2+ were nearly constant (13.9 pS-20.5 pS) in the concentrations between 0.5 mM and 50 mM Ca-gluconate. In the experiments with mixed solutions of K+ and Ca2+, a maximum conductance of Ca2+ (gamma Camax) and an apparent Michaelis constant of Ca2+ (K Cam) were obtained by assuming a simple competitive relation between the cations. Gamma Camax and K Cam were 14.0 pS and 0.160 mM, respectively. Single-channel conductances in mixed solutions were well-fitted to this competitive model supporting that this cation channel behaves as a single-ion channel. This channel had relatively high-affinity Ca2+-binding sites.     

Uptake and intracellular sequestration of divalent cations in resting and methacholine-stimulated mouse lacrimal acinar cells. Dissociation by Sr2+ and Ba2+ of agonist-stimulated divalent cation entry from the refilling of the agonist-sensitive intracellular pool

The abilities of various divalent cations to enter the cytoplasm of mouse lacrimal acinar cells was examined under resting and agonist-stimulated conditions, by monitoring their effects on the fluorescence of cytosolic fura-2. In vitro, Ni2+, Co2+, and Mn2+ quenched the fura-2 fluorescence, whereas Sr2+, Ba2+, and La3+ produced an excitation spectrum and maximum brightness similar to Ca2+. Stimulation of mouse lacrimal acinar cells with methacholine (MeCh) caused a biphasic elevation of intracellular Ca2+ concentration [( Ca2+]i) resulting from a release of Ca2+ from intracellular pools followed by a sustained entry of extracellular Ca2+. Neither La3+ nor Ni2+ entered the cells under resting or stimulated conditions, but both blocked Ca2+ entry. Although both Co2+ and Mn2+ entered unstimulated cells, this process was not increased by MeCh. Both Sr2+ and Ba2+ were capable of supporting a sustained increase in fura-2 fluorescence in response to MeCh, indicating that these cations can enter the cells through the agonist-regulated channels. However, Sr2+, but not Ba2+, was capable of refilling the agonist-sensitive intracellular stores. These findings demonstrate dissociation of agonist-induced Ca2+ entry from intracellular Ca2+ pool refilling and thereby provide strong support for the recently modified version of the capacitative Ca2+ entry model according to which influx into the cytoplasm occurs directly across the plasma membrane and does not require a specialized cation channel directly linking the extracellular space and the intracellular Ca2+ stores.     

Activation of three types of membrane currents by various divalent cations in identified molluscan pacemaker neurons

We investigated membrane currents activated by intracellular divalent cations in two types of molluscan pacemaker neurons. A fast and quantitative pressure injection technique was used to apply Ca2+ and other divalent cations. Ca2+ was most effective in activating a nonspecific cation current and two types of K+ currents found in these cells. One type of outward current was quickly activated following injections with increasing effectiveness for divalent cations of ionic radii that were closer to the radius of Ca2+ (Ca2+ greater than Cd2+ greater than Hg2+ greater than Mn2+ greater than Zn2+ greater than Co2+ greater than Ni2+ greater than Pb2+ greater than Sr2+ greater than Mg2+ greater than Ba2+). The other type of outward current was activated with a delay by Ca2+ greater than Sr2+ greater than Hg2+ greater than Pb2+. Mg2+, Ba2+, Zn2+, Cd2+, Mn2+, Co2+, and Ni2+ were ineffective in concentrations up to 5 mM. Comparison with properties of Ca2(+)-sensitive proteins related to the binding of divalent cations suggests that a Ca2(+)-binding protein of the calmodulin/troponin C type is involved in Ca2(+)-dependent activation of the fast-activated type of K+ current. Th sequence obtained for the slowly activated type is compatible with the effectiveness of different divalent cations in activating protein kinase C. The nonspecific cation current was activated by Ca2+ greater than Hg2+ greater than Ba2+ greater than Pb2+ greater than Sr2+, a sequence unlike sequences for known Ca2(+)-binding proteins.     

Ca2+,Mg2+-ATPase of microsomal membranes from bovine aortic smooth muscle: effects of Sr2+ and Cd2+ on Ca2+ uptake and formation of the phosphorylated intermediate of the Ca2+,Mg2+-ATPase

The effects of various divalent cations on the Ca2+ uptake by microsomes from bovine aortic smooth muscle were studied. High concentrations (1 mM) of Co2+, Zn2+, Mn2+, Fe2+, and Ni2+ inhibited neither the Ca2+ uptake by the microsomes nor the formation of the phosphorylated intermediate (E approximately P) of the Ca2+,Mg2+-ATPase of the microsomes. The cadmium ion, however, inhibited both the Ca2+ uptake and the E approximately P formation by the microsomes. Dixon plot analysis indicated Cd2+ inhibited (Ki = 135 microM) the Ca2+ dependent E approximately P formation in a non-competitive manner. The inhibitory effect of Cd2+ was lessened by cysteine or dithiothreitol. The strontium ion inhibited the Ca2+ uptake competitively, while the E approximately P formation increased on the addition of Sr2+ at low Ca2+ concentrations. At a low Ca2+ concentration (1 microM), Sr2+ was taken up by the aortic microsomes in the presence of 1 mM ATP. It is thus suggested that Sr2+ replaces Ca2+ at the Ca2+ binding site on the ATPase.     

The effect of calcium channel blockers on blood platelet function, especially calcium uptake

The effects of organic and inorganic calcium antagonists on washed platelets from rat and human have been studied. Platelet aggregation was assessed by turbidimetry. Endogenous serotonin release was measured on the same sample by means of electrochemically treated carbon fiber electrodes. The organic calcium antagonist, nitrendipine, and the inorganic calcium channel blockers (Co2+, Mn2+, Cd2+, La3+) drastically inhibited rat and human platelet aggregation induced by thrombin, ADP or adrenaline in the presence of 0.32 mM Ca2+. In our conditions, the thrombin-induced release of endogenous serotonin was found to be external Ca2+-dependent and completely inhibited by 20 microM nitrendipine or 1 mM Cd2+. In addition, Ba2+ or Sr2+ ions can be substituted for Ca2+ to bring about platelet aggregation as well as endogenous serotonin secretion. In Ba2+ or Sr2+-containing media, rat platelet aggregation and/or serotonin secretion can be inhibited by either nitrendipine or Cd2+. Finally, we have also studied the thrombin- and external Ca2+-dependence of radiolabeled calcium uptake by rat platelets. We found that the thrombin-induced 45Ca uptake was inhibited by either 18 microM nitrendipine or 1 mM Cd2+. These results provide strong evidence for the existence of an influx of divalent cations (Ca2+, Sr2+, Ba2+) triggering platelet function. They also suggest, although they do not prove, that the translocation of these cations occurs through an agonist-operated channel as proposed by Hallam and Rink (FEBS Lett. 186 (1986) 175-179).     

Na+/Ca2+ antiport in cultured arterial smooth muscle cells. Inhibition by magnesium and other divalent cations

Cultured smooth muscle cells from rat aorta were loaded with Na+, and Na+/Ca2+ antiport was assayed by measuring the initial rates of 45Ca2+ influx and 22Na+ efflux, which were inhibitable by 2',4'-dimethylbenzamil. The replacement of extracellular Na+ with other monovalent ions (K+, Li+, choline, or N-methyl-D-glucamine) was essential for obtaining significant antiport activity. Mg2+ competitively inhibited 45Ca2+ influx via the antiporter (Ki = 93 +/- 7 microM). External Ca2+ or Sr2+ stimulated 22Na+ efflux as would be expected for antiport activity. Mg2+ did not stimulate 22Na+ efflux, which indicates that Mg2+ is probably not transported by the antiporter under the conditions of these experiments. Mg2+ inhibited Ca2+-stimulated 22Na+ efflux as expected from the 45Ca2+ influx data. The replacement of external N-methyl-D-glucamine with K+, but not other monovalent ions (choline, Li+), decreased the potency of Mg2+ as an inhibitor of Na+/Ca2+ antiport 6.7-fold. Other divalent cations (Co2+, Mn2+, Cd2+, Ba2+) also inhibited Na+/Ca2+ antiport activity, and high external potassium decreased the potency of each by 4.3-8.6-fold. The order of effectiveness of the divalent cations as inhibitors of Na+/Ca2+ antiport (Cd2+ greater than Mn2+ greater than Co2+ greater than Ba2+ greater than Mg2+) correlated with the closeness of the crystal ionic radius to that of Ca2+.     

The effect of divalent cations on bovine spermatozoal adenylate cyclase activity.

The effect of divalent cations on bovine sperm adenylate cyclase activity was studied. Mn2+, Co2+, Cd2+, Zn2+, Mg2+ and Ca2+ were found to satisfy the divalent cation requirement for catalysis of the bovine sperm adenylate cyclase. These divalent cations in excess of the amount necessary for the formation of the metal-ATP substrate complex were found to stimulate the enzyme activity to various degrees. The magnitude of stimulation at saturating concentrations of the divalent cations was strikingly greater with M2+ than with either Ca2+, Mg2+, Zn2+, Cd2+ or Co2+. The apparent Km was lowest for Zm2+ (0.1 - 0.2 mM) than for any of the other divalent cations tested (1.2 - 2.3 mM). The enzyme stimulation by Mn2+ was decreased by the simultaneous addition of Co2+, Cd2+, Ni2+ and particularly Zn2+ and Cu2+. The antagonism between Mn2+ and Cu2+ or Zn2+ appeared to have both competitive and non-competitive features. The inhibitory effect of Cu2+ on Mn2+-stimulated adenylate cyclase activity was prevented by 2,3-dimercaptopropanol, but not by dithiothreitol, L-ergothioneine, EDTA, EGTA or D-penicillamine. Ca2+ at concentrations of 1-5 mM was found to act synergistically with Mg2+, Zn2+, Co2+ and Mn2+ in stimulating sperm adenylate cyclase activity. The Ca2+ augmentation of the stimulatory effect of Zn2+, Co2+, Mg2+ and Mn2+ appeared to be specific.     

Rapid induction of acetylcholine receptor aggregates by a neural factor and extracellular Ca2+

A soluble fetal brain extract (EBX) induces acetylcholine receptor (AChR) aggregation in cultured rat myotubes within 4 hr at 36 degrees C in a defined medium containing 1.8 mM (normal) extracellular Ca2+ (Olek et al., 1983). The activity of EBX was Ca2+ dependent; reducing extracellular Ca2+ significantly inhibited EBX-induced AChR aggregation and a 15-50% increase in extracellular Ca2+ synergistically enhanced the activity of EBX. Synergism was specific for Ca2+ as increases in other divalent cations (Ba2+, Co2+, Mg2+, Mn2+, Sr2+) had no effect. A large increase (300-500%) in extracellular Ca2+ alone also induced AChR aggregation within 4 hr at 36 degrees C. An equivalent increase in other cations (Ba2+, Co2+, Mg2+, Mn2+, Sr2+) did not promote AChR aggregation. An initial 15-min pulse of increased extracellular Ca2+ alone or with EBX was adequate to induce AChR aggregation. Aggregates induced by EBX, Ca2+ alone, or EBX/Ca2+ were found predominantly on the top surface of the myotube. These treatments did not detectably alter preexisting aggregates present at substrate contact sites on the bottom surface of myotubes. AChR aggregation induced by any treatment was not inhibited by cycloheximide, Ca2+ channel blockers, or protease inhibitors but was blocked by Co2+ and sodium azide.     

High-affinity Ca2+-binding site inhibiting Ca2+ release from sarcoplasmic reticulum

Ca2+ release from sarcoplasmic reticulum membranes, activated by alkaline pH occurs only when EGTA is present in the release medium. Addition of very low concentrations of Ca2+ to the medium inhibits Ca2+ release. The concentration of free Ca2+ required for 50% inhibition ranges from between 5 and 20 nM in different experiments and/or membrane preparations, irrespective of whether the free Ca2+ concentration is controlled by EGTA or CDTA. Other divalent cations such as Mn2+, Ba2+, Cu2+, Cd2+ and Mg2+ also exert an inhibitory effect on Ca2+ release, with higher or lower potency than that of Ca2+. The inactivation of Ca2+ release by Ca2+ is reversible. We suggest the involvement of high-affinity Ca2+-binding sites in the control of Ca2+ release.     

Stringent requirement for Ca2+ in the removal of Z-lines and alpha-actinin from isolated myofibrils by Ca2+-activated neutral proteinase

Treatment of isolated myofibrils with Ca2+-activated neutral proteinase (CANP) results in specific removal of Z-line and of alpha-actinin. To investigate the ionic requirement for these processes, we measured Z-line removal by phase-contrast and interference microscopy and alpha-actinin removal by sodium dodecyl sulphate/polyacrylamide-gel electrophoretic analysis of myofibrillar proteins. The proteolytic digestion of native purified proteins was measured directly on polyacrylamide gels and by the fluorescamine technique. We found that the removal of Z-line and alpha-actinin as well as the release of proteolytic degradation products from isolated myofibrils by CANP occur only in the presence of Ca2+; Sr2+, Ba2+, Mn2+, Mg2+, Co2+ and Zn2+ are all ineffective. In contrast with this stringent requirement for Ca2+, the proteolytic activity of CANP measured with denatured casein, native and denatured haemoglobin, native actin and tropomyosin also occurs in the presence of other bivalent cations, in the following order: Ca2+ greater than Sr2+ greater than Ba2+. These data suggest that only Ca2+ can produce the conformational change in myofibrils that renders them susceptible to the action of CANP, whereas its proteolytic activity is stimulated by several bivalent ions.     

A role for Na+/Ca2+ exchange in the generation of superoxide radicals by human neutrophils

A Na+/Ca2+ exchange mechanism has been recently described in human neutrophils that constitutes the principal pathway for Ca2+ influx into resting cells. The potential role of this system in regulating the respiratory burst in response to activation by the chemotactic tripeptide N-formyl-methionyl-leucyl-phenylalanine was explored. In the presence of 1 mM Ca2+, a variety of di- and trivalent cations suppressed the generation of O(-2) radicals in a series of decreasing efficacy: La3+ approximately Zn2+ much greater than Sr2+ approximately Cd2+ greater than Ba2+ greater than Co2+ greater than Ni2+ approximately Mg2+. This sequence is similar to their rank order of activity in inhibiting 45Ca2+ influx via Na+/Ca2+ counter-transport. Benzamil, phenamil, and 2',4'-dichlorobenzamil, analogues of amiloride which selectively block Na+/Ca2+ exchange in neutrophils, likewise suppressed the release of O(-2) with apparent Ki values of approximately 30 microM. The effect of the cations was competitive with Ca2+, while the interaction between the benzamil derivatives and Ca2+ appeared to be noncompetitive in nature. Both the divalent cations and benzamil also inhibited the rise in cytoplasmic Ca2+ as monitored by fura-2 fluorescence: these agents reduced peak cytosolic Ca2+ levels after N-formyl-methionyl-leucyl-phenylalanine stimulation to values seen in the absence of extracellular Ca2+. These results are compatible with the hypothesis that the influx of Ca2+ via Na+/Ca2+ exchange contributes to the transient elevation in intracellular free Ca2+. The polyvalent cations block the entry of critical Ca2+ ions by competing with Ca2+ for binding to the translocation site on the exchange carrier, while benzamil acts by lowering the maximal transport rate. These studies emphasize that Na+/Ca2+ exchange through its effects on cytoplasmic Ca2+ plays a major regulatory role in activation of the respiratory burst in chemotactic factor-stimulated neutrophils.     

Studies of gastric Ca2+-stimulated adenosine triphosphatase. I. characterization and general properties

Gastric microsomes do not contain any significant Ca2+-stimulated ATPase activity. Trypsinization of pig gastric microsomes in presence of ATP results in significant (2-3 fold) increase in the basal (with Mg2+ as the only cation) ATPase activity, with virtual elimination of the K+-stimulated component. Such treatment causes unmasking of latent Mg2+-dependent Ca2+-stimulation ATPase. Other divalent cations such as Sr2+, Ba2+, Zn2+, and Mn2+ were found ineffective as a substitute for Ca2+. Moreover, those divalent cations acted as inhibitors of the Ca2+-stimulated ATPase activity. The pH optimum of the enzyme is around 6.8. The enzyme has a Km of 70 microM for ATP and the Ka values for Mg2+ and Ca2+ are about 4 x 10(-4) and 10(-7) M, respectively. Studies with inhibitors suggest the involvement of sulfhydryl and primary amino groups in the operation of the enzyme. Possible roles of the enzyme in gastric H+ transport have been discussed.     

Lanthanide ions and Cd2+ are able to substitute for Ca2+ in regulating phosphorylase kinase

Trivalent lanthanide ions and Cd2+ were found to mimic effectively the stimulatory action of Ca2+ on rabbit muscle phosphorylase kinase. In the range of concentrations tested, Cd2+ and lanthanides (Tb3+, Gd3+, Pr3+, Ce3+) could substitute for Ca2+ in activating the enzyme to about 60% and 70% respectively of the maximal level seen with Ca2+, at pH 8.2. The effect induced by Cd2+ was biphasic (stimulation followed by inhibition with increasing metal cation concentration). Similar results were obtained at pH 6.8. Cd2+ and Tb3+ were also able to replace Ca2+ required for the stimulation of phosphorylase kinase activity at pH 8.2 by exogenous calmodulin. Maximal stimulation induced by calmodulin in presence of Cd2+ was significantly higher than that in presence of Ca2+ or Tb3+.     

Cooperative effects of Ca2+ and Sr2+ on sarcoplasmic reticulum adenosine triphosphatase

The intrinsic fluorescence of purified Ca-ATPase from skeletal sarcoplasmic reticulum was measured in the presence of various concentrations of Ca2+, Sr2+, and Ba2+. Ca2+ and Sr2+ induce positive cooperative fluorescence enhancement, whereas Ba2+ does not change the fluorescence of ATPase. ATP does not seem to modify the kinetic parameters of Ca2+ and Sr2+ binding to ATPase. Nevertheless, p-nitrophenylphosphate hydrolysis, activated by Ca2+ or Sr2+ at various pHs, changes the affinity and the cooperative behavior for both cations and two components appear in the Hill plots. For Ca2+, nH of 1.6 to 3.5 were obtained, and 1.06 to 1.83 for Sr2+; nH changes of the second component seem to be pH dependent. Differences in the ratio between rates of Ca2+ transport and substrate hydrolysis by sarcoplasmic reticulum were found, i.e., two for ATP and one for p-nitrophenylphosphate. For Sr2+ this ratio was one for either ATP or p-nitrophenylphosphate.     

Dependence of the inhibitory effect of metal cations on rat liver mitochondrial Ca2+ uptake on their physico-chemical properties

It has been found that Sr2+, La3+ Mn2+ (10-50 microM) inhibit Ca2+ transport into mitochondria in a competitive manner. Cd2+ ions show the mixed type inhibition of this transport. The inhibitory constants (Ki, microM) of the metals cations effect on Ca2+ transport increases in such a sequence: La3+ (2,11), Cd2+ (10,36), Mn2+ (49,29), Sr2+ (66,43). The metals cations inhibitory effect has an insignificant dependence on their ionic radii. But it is good correlated with the series of metals cations, based on the stability constants of their complexes with acetate (r = -0.96), aspartic (r = -0.91) and glutaminic acids and their hydratation enthalpy (r = -0.78). These data reveal that hydratation of metals cations and their interaction with carboxyles of Ca(2+)-uniporter plays an important role in the process of Ca2+ transport into mitochondrial matrix space and its inhibition by the metals cations. The mixed type inhibition of mitochondrial Ca2+ uptake by Cd2+ seems to be caused by the partial de-energization of mitochondria owing to Cd2+ interaction with SH-containing respiratory chain components and pore-forming ligands of mitochondrial membrane.     

Divalent cation conduction in the ryanodine receptor channel of sheep cardiac muscle sarcoplasmic reticulum

The conduction properties of the alkaline earth divalent cations were determined in the purified sheep cardiac sarcoplasmic reticulum ryanodine receptor channel after reconstitution into planar phospholipid bilayers. Under bi-ionic conditions there was little difference in permeability among Ba2+, Ca2+, Sr2+, and Mg2+. However, there was a significant difference between the divalent cations and K+, with the divalent cations between 5.8- and 6.7-fold more permeant. Single-channel conductances were determined under symmetrical ionic conditions with 210 mM Ba2+ and Sr2+ and from the single-channel current-voltage relationship under bi-ionic conditions with 210 mM divalent cations and 210 mM K+. Single-channel conductance ranged from 202 pS for Ba2+ to 89 pS for Mg2+ and fell in the sequence Ba2+ greater than Sr2+ greater than Ca2+ greater than Mg2+. Near-maximal single-channel conductance is observed at concentrations as low as 2 mM Ba2+. Single-channel conductance and current measurements in mixtures of Ba(2+)-Mg2+ and Ba(2+)-Ca2+ reveal no anomalous behavior as the mole fraction of the ions is varied. The Ca(2+)-K+ reversal potential determined under bi-ionic conditions was independent of the absolute value of the ion concentrations. The data are compatible with the ryanodine receptor channel acting as a high conductance channel displaying moderate discrimination between divalent and monovalent cations. The channel behaves as though ion translocation occurs in single file with at most one ion able to occupy the conduction pathway at a time.     

Cation and sequence effects on stability of intermolecular pyrimidine-purine-purine triplex.

A differential effect is found of various bivalent cations (Ba2+, Ca2+, Mg2+, Cd2+, Co2+, Mn2+, Ni2+, Zn2+ and Hg2+) on stability of intermolecular Py-Pu-Pu triplex with different sequence of base triads. Ca2+, Mg2+, Cd2+, Co2+, Mn2+, Ni2+ and Zn2+ do stabilize the d(C)n d(G)n d(G)n triplex whereas Ba2+ and Hg2+ do not. Ba2+, Ca2+, Mg2+ and Hg2+ destabilize the d(TC)n d(GA)n d(AG)n triplex whereas Cd2+, Co2+, Mn2+, Ni2+ and Zn2+ stabilize it. The complexes we observe are rather stable because they do not dissociate during time of gel electrophoresis in the co-migration experiments. Chemical probing experiments with dimethyl sulfate as a probe indicate that an arbitrary homopurine-homopyrimidine sequence forms triplex with corresponding purine oligonucleotide in the presence of Mn2+ or Zn2+, but not Mg2+. In the complex the purine oligonucleotide has antiparallel orientation with respect to the purine strand of the duplex. Specifically, we have shown the formation of the Py-Pu-Pu triplex in a fragment of human papilloma virus HPV-16 in the presence of Mn2+.     

The effect of ions on the adhesion and internalization of Chlamydia trachomatis by HeLa cells

The adhesion and internalization of Chlamydia trachomatis by HeLa cells was unaffected by removal of K+, Mg2+, or glucose from the incubation medium, slightly reduced by removal of Na+, and significantly reduced by omission of Ca2+, Sr2+, Mg2+, and Mn2+ could replace Ca2+ in the adhesion but only Sr2+ supported internalization, and La3+, Co2+, Fe3+, Ba2+, and Zn2+ all reduced internalization more than adhesion. During initial infection there was no measurable difference in the uptake or release of 45Ca2+ or 86Rb+ between infected and noninfected HeLa monolayers. Infection was not prevented by pretreatment of the monolayers with the calcium channel blockers, verapamil, D600, and nitrendipine, or the calmodulin inhibitors, TMB-8 or trifluperazine. The results suggest that divalent cations are not essential for chlamydial infection but that the process of internalization is facilitated by the presence of cations, particularly Na+ and Ca2+.