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PubMed1998

The role of an alpha subtype M2-M3 His in regulating inhibition of GABAA receptor current by zinc and other divalent cations.

Sensitivity of GABAA receptors (GABARs) to inhibition by zinc and other divalent cations is influenced by the alpha subunit subtype composition of the receptor. For example, alpha6beta3gamma2L receptors are more sensitive to inhibition by zinc than alpha1beta3gamma2L receptors. We examined the role of a His residue located in the M2-M3 extracellular domain (rat alpha6 H273) in the enhanced zinc sensitivity conferred by the alpha6 subtype. The alpha1 subtype contains an Asn (N274) residue in the equivalent location. GABA-activated whole-cell currents were obtained from L929 fibroblasts after transient transfection with expression vectors containing GABAA receptor cDNAs. Mutation of alpha1 (alpha1(N274H)) or alpha6 (alpha6(H273N)) subtypes did not alter the GABA EC50 of alphabeta3gamma2L receptors. alpha1(N274H)beta3gamma2L receptor currents were as sensitive to zinc as alpha6beta3gamma2L receptor currents, although alpha6(H273N)beta3gamma2L receptor currents had the reduced zinc sensitivity of alpha1beta3gamma2L receptor currents. We also examined the activity of other inhibitory divalent cations with varying alpha subtype dependence: nickel, cadmium, and copper. alpha6beta3gamma2L receptor currents were more sensitive to nickel, equally sensitive to cadmium, and less sensitive to copper than alpha1beta3gamma2L receptor currents. Studies with alpha1 and alpha6 chimeric subunits indicated that the structural dependencies of the activity of some of these cations were different from zinc. Compared with alpha6beta3gamma2L receptor currents, alpha6(H273N)beta3gamma2L receptor currents had reduced sensitivity to cadmium and nickel, but the sensitivity to copper was unchanged. Compared with alpha1beta3gamma2L receptor currents, alpha1(N274H)beta3gamma2L receptor currents had increased sensitivity to nickel, but the sensitivity to cadmium and copper was unchanged. These findings indicate that H273 of the alpha6 subtype plays an important role in determining the sensitivity of recombinant GABARs to the divalent cations zinc, cadmium, and nickel, but not to copper. Our results also suggest that the extracellular N-terminal domain of the alpha1 subunit contributes to a regulatory site(s) for divalent cations, conferring high sensitivity to inhibition by copper and cadmium.

Amino Acid Sequence↗
PubMed1979

Electric birefringence of DNA and chromatin. Influence of divalent cations.

The effects of divalent cations on the DNA and chromatin conformation have been investigated by electric birefringence and birefringence relaxation measurements at low and constant ionic strength (0.001). An important decrease of the intrinsic optical anisotropy of DNA has been found in the presence of Mn2+ and Cu2+, but not with Mg2+. A complex variation of the mean relaxation time with the ratio I/P of ion to DNA-phosphate molar concentration has been evidenced in the presence of Mn2+ and Cu2+, while the mean relaxation time monotonously decreased in the presence of Mg2+. These observations are interpreted in terms of a specific organization of DNA in a compact, rigid structure, in the presence of Mn2+ and Cu2+, and a non-specific coiling in the presence of Mg2+. Drastic conformational changes encountered by chromatin in the presence of Mg2+ and Mn2+ cations have also been evidenced through electric birefringence measurements. They are interpreted by the formation of a superhelical compact arrangement of nucleosome strings which yielded a reversal of the birefringence sign with respect to the negative anisotropy observed in the presence of Na+ ions. The removal of the histone H1 prevented the appearance of this quaternary structure. More extended fragments of the chromatin chain obtained by ECTHAM chromatography of sonicated chromatin could not afford such compact arrangements.

Birefringence↗
PubMed1995

Permeation and block by internal and external divalent cations of the catfish cone photoreceptor cGMP-gated channel.

The ability of the divalent cations calcium, magnesium, and barium to permeate through the cGMP-gated channel of catfish cone outer segments was examined by measuring permeability and conductance ratios under biionic conditions and by measuring their ability to block current carried by sodium when presented on the cytoplasmic or extracellular side of the channel. Current carried by divalent cations in the absence of monovalent cations showed the typical rectification pattern observed from these channels under physiological conditions (an exponential increase in current at both positive and negative voltages). With calcium as the reference ion, the relative permeabilities were Ca > Ba > Mg, and the chord conductance ratios at +50 mV were in the order of Ca approximately Mg > Ba. With external sodium as the reference ion, the relative permeabilities were Ca > Mg > Ba > Na with chord conductance ratios at +30 mV in the order of Na >> Ca = Mg > Ba. The ability of divalent cations presented on the intracellular side to block the sodium current was in the order Ca > Mg > Ba at +30 mV and Ca > Ba > Mg at -30 mV. Block by external divalent cations was also investigated. The current-voltage relations showed block by internal divalent cations reveal no anomalous mole fraction behavior, suggesting little ion-ion interaction within the pore. An Eyring rate theory model with two barriers and a single binding site is sufficient to explain both these observations and those for monovalent cations, predicting a single-channel conductance under physiological conditions of 2 pS and an inward current at -30 mV carried by 82% Na, 5% Mg, and 13% Ca.

Animals↗
PubMed1991

Thermodynamics of actin polymerization; influence of the tightly bound divalent cation and nucleotide.

Previous work by this laboratory has shown that the tightly bound divalent cation of actin affects the enthalpy of the polymerization reaction for ATP-actin (Selden et al. (1986) J. Muscle Res. Cell Motil. 7, 215-224). In the present study, we have measured the temperature dependence of polymerization for actin containing ATP or ADP as the bound nucleotide and Mg2+ or Ca2+ (Mg-actin or Ca-actin) as the tightly bound divalent cation. In contrast to the marked effect of the tightly bound divalent cation on enthalpy and entropy changes for the polymerization of ATP-actin, ADP-actin polymerization is affected very little by the tightly bound divalent cation. The Arrhenius and van't Hoff plots for polymerization of Ca-ATP-, Mg-ADP- and Ca-ADP-actin were found to be non-linear. The free energy data for actin polymerization have been analyzed as a second order function of absolute temperature (Osborne et al. (1976) Biochemistry 15, 317-320). The values of the enthalpy change and activation enthalpy change for Ca-ATP-, Mg-ADP- and Ca-ADP-actin polymerization were found to be temperature-dependent, in contrast to those for Mg-ATP-actin, which were nearly constant over the temperature range studied. These results suggest that (1) polymerization of actin which does not contain both Mg2+ and ATP may be a multi-step reaction including a rate-limiting step and (2) Mg-ATP-actin has a unique conformation which enhances its ability to polymerize.

Actins↗
PubMed1983

Large divalent cations and electrostatic potentials adjacent to membranes. A theoretical calculation.

We have extended the Gouy-Chapman theory of the electrostatic diffuse double layer by considering the finite size of divalent cations in the aqueous phase adjacent to a charged surface. The divalent cations are modeled as either two point charges connected by an infinitely thin, rigid "rod" or two noninteracting point charges connected by an infinitely thin, flexible "string." We use the extended theory to predict the effects of a cation of length 10 A (1 nm) on the zeta and surface potentials of phospholipid bilayer membranes. The predictions of the rod and string models are similar to one another but differ markedly from the predictions of the Gouy-Chapman theory. Specifically, the extended model predicts that a large divalent cation will have a smaller effect on the potential adjacent to a negatively charged bilayer membrane than a point divalent cation, that the magnitude of this discrepancy will decrease as the Debye length increases, and that a large divalent cation will produce a negative zeta potential on a membrane formed from zwitterionic lipids. These predictions agree qualitatively with the experimental results obtained with the large divalent cation hexamethonium. We discuss the biological relevance of our calculations in the context of the interaction of cationic drugs with receptor sites on cell membranes.

Cations, Divalent↗
PubMed1980

Influence of divalent cations on the reconstituted ADP, ATP exchange.

1. Divalent cations cause a decrease in the exchange activity of the reconstituted ADP,ATP translocator from beef heart mitochondria. This effect is due to complex formation with the adenine nucleotides. 2. It is confirmed that only the free nucleotides are transported. A possible competition of free adenine nucleotides and the Mg2+-complexes for the binding site at the carrier protein is excluded. 3. The stability constants (Kn) for the cation-nucleotide complexes are derived from these experiments. For Mg2+-ATP, Kn = 0.8 x 10(4) M-1 and for Mg2+-ADP, Kn = 0.8 x 10(3) M-1 is obtained. 4. The carrier system was reconstituted with the neutral phospholipids phosphatidylcholine and phosphatidylethanolamine. Interaction of the divalent cations with these phospholipids seem not to be important for the exchange suppression.

Adenosine Diphosphate↗
PubMed1981

Polymorphonuclear leucocyte membrane-stimulated oxidative metabolic activity---the effect of divalent cations and cytochalasins.

The effect of divalent cations and cytochalasins on the oxidative metabolic response of polymorphonuclear leucocytes (PMNL) following membrane stimulation by opsonized zymosan, phorbol myristate acetate (PMA), digitonin and n-formyl-methionyl-leucyl-phenylalanine (FMLP) was investigated using several techniques. For optimal ferricytochrome C reduction, oxygen consumption, luminol- and lucigenin-dependent chemiluminescence by PMNL on exposure to opsonized zymosan, extracellular magnesium was necessary. In contrast FMLP- and digitonin-induced PMNL metabolic activities were less dependent on extracellular magnesium than on calcium. Phorbol myristate acetate-induced PMNL oxidative metabolic activity occurred independently of extracellular magnesium and calcium concentration. Incubation of PMNL with cytochalasins B and E had little effect on PMA-induced metabolic activity, enhanced FMLP-induced metabolic activity but inhibited digitonin-induced metabolic activity. Investigation of zymosan-induced PMNL metabolic activity after incubation with cytochalasins demonstrated a dissociation between the different parameters of metabolic activity measured. The results described in this study support the suggestion that PMNL oxidative metabolism can be activated by at least two different mechanisms depending on the stimulus used, and that these mechanisms can in part be differentiated by their dependence on extracellular divalent cations.

Adult↗
PubMed1984

Role of divalent cation bound to phosphoenzyme intermediate of sarcoplasmic reticulum ATPase.

Effect of divalent cations bound to the phosphoenzyme intermediate of the ATPase of sarcoplasmic reticulum was investigated at 0 degree C and pH 7.0 using the purified ATPase preparations. Our previous study (Shigekawa, M., Wakabayashi, S., and Nakamura, H. (1983) J. Biol. Chem. 258, 14157-14161) indicated that 1 mol of the ADP-sensitive phosphoenzyme (E1P) formed from CaATP has 3 mol of high affinity binding sites for Ca2+, of which two are transport sites for calcium while the remainder is the acceptor site for calcium derived from the substrate, CaATP ("substrate site"). When incubated with a chelator of divalent cation, E1P formed from CaATP released all of its bound calcium to form a divalent cation-free phosphoenzyme. Evidence was presented that calcium dissociation from the substrate site was faster than that from the transport sites and primarily responsible for the ADP sensitivity loss of E1P induced by the chelator. Divalent cation-free phosphoenzyme was kinetically stable but when treated with divalent cations, it behaved similarly to the ADP-insensitive phosphoenzyme (E2P) which is the normal reaction intermediate of ATP hydrolysis. 45Ca bound at the substrate site on E1P formed from 45CaATP exchanged readily with nonradioactive ionized Ca2+ in the reaction medium whereas 45Ca at the transport sites on E1P was displaced only at a very slow rate which was almost the same as that for the phosphoenzyme hydrolysis. It was suggested that calcium at the transport sites on E1P formed from CaATP is released only after the rate-limiting conformational transition of the phosphoenzyme from E1P to E2P and that removal of calcium by a chelator from the substrate site facilitates this conformational transition, thereby allowing calcium bound at the transport sites to be released readily from the phosphoenzyme.

Adenosine Triphosphatases↗
PubMed1985

Effect of divalent cations on the limited proteolysis of prothrombin by thrombin.

The inhibitory influence of divalent cations on the ability of bovine alpha-thrombin to hydrolyze prothrombin showed the trend Mn2+ much greater than Ca2+ greater than or equal to Mg2+ greater than Sr2+ much greater than Ba2+. This effect was not due to an inhibition of thrombin's catalytic activity as measured by hydrolysis of a specific synthetic substrate, H-D-Phe-pipecolyl-Arg-p-nitroanilide (D-PhePipArgNA). The presence of divalent cations did not inhibit thrombic proteolysis of gamma-carboxyglutamic acid (Gla)-domainless prothrombin. Prothrombin and Gla-domainless prothrombin were used as competitive inhibitors in the thrombic hydrolysis of D-PhePipArgNA. The apparent Ki value calculated for prothrombin was 18 microM. When either Ca2+ or Mn2+ were present, there was no inhibition. The apparent Ki value determined for Gla-domainless prothrombin was 28 microM in either the absence or presence of Ca2+. Addition of divalent cations to prothrombin, but not to Gla-domainless prothrombin, resulted in an altered protein conformation as measured by high-performance size-exclusion chromatography and ultraviolet difference spectroscopy. These results suggest that a conformational change secondary to the interaction of divalent cations with the Gla-containing domain of prothrombin is required for cation-dependent inhibition of thrombin hydrolysis.

Animals↗
PubMed1993

Mutation of putative divalent cation sites in the alpha 4 subunit of the integrin VLA-4: distinct effects on adhesion to CS1/fibronectin, VCAM-1, and invasin.

To investigate the functional significance of putative integrin divalent cation binding sites, several mutated alpha 4 subunit cDNAs were constructed. Mutants contained the conservative substitution of Glu for Asp or Asn at the third position in each of three putative divalent cation sites. Transfection of wild-type or mutated alpha 4 into K562 cells yielded comparable expression levels and immunoprecipitation profiles. However, for all three alpha 4 mutants, adhesion to CS1/fibronectin was greatly diminished in either the presence or absence of the stimulatory anti-beta 1 mAb TS2/16. Constitutive adhesion to vascular cell adhesion molecule (VCAM) 1 was also diminished but, unlike CS1 adhesion, was restored upon TS2/16 stimulation. In contrast, adhesion to the bacterial protein invasin was minimally affected by any of the three mutations. For each of the mutants, the order of preference for divalent cations was unchanged compared to wild-type alpha 4, on CS1/fibronectin (Mn2+ > Mg2+ > Ca2+), on VCAM-1 (Mn2+ > Mg2+ = Ca2+) and on invasin (Mg2+ = Ca2+). However for the three mutants, the efficiency of divalent cation utilization was decreased. On VCAM-1, 68-108 microM Mn2+ was required to support half-maximal adhesion for the mutants compared with 14-18 microM for wild-type alpha 4. These results indicate (a) that three different ligands for VLA-4 show widely differing sensitivities to mutations within putative divalent cation sites, and (b) each of the three putative divalent cation sites in alpha 4 have comparable functional importance with respect to both divalent cation usage and cell adhesion.

Adhesins, Bacterial↗
PubMed1986

Changes in conformation, stability and condensation of DNA by univalent and divalent cations in methanol-water mixtures.

Circular dichroism spectroscopy, absorption spectroscopy, measurements of Tm values, sedimentation analysis and electron microscopy were used to study properties of calf thymus DNA in methanol-water mixtures as a function of monovalent cation (Na+ or Cs+) concentration and also in the presence of divalent cations Ca2+, Mg2+, and Mn2+. In the absence of divalent cations only slight conformational changes occurred and no condensation and/or aggregation could be detected. The Tm values depend on the amount of methanol and on the nature and concentration of cations. In methanol-water mixtures higher thermal stability was observed in solutions containing Cs+ ions. Up to 40% (v/v) methanol the addition of divalent ions leads to DNA stabilization. At methanol concentration higher than 50% the presence of divalent cations causes DNA condensation and denaturation even at room temperature. The denaturation is reversible with respect to EDTA addition indicating that no separation of complementary strands occurred and the resulting form of DNA is probably similar to the P form. DNA destacking appears to be a direct consequence of stronger cation binding by the condensed DNA in methanol-water mixtures.

Animals↗
PubMed1983

Regulation by divalent cations of 3H-baclofen binding to GABAB sites in rat cerebellar membranes.

When investigating the effects of divalent cations (Mg2+, Ca2+, Sr2+, Ba2+, Mn2+ and Ni2+) on 3H-baclofen binding to rat cerebellar synaptic membranes, we found that the specific binding of 3H-baclofen was not only dependent on divalent cations, but was increased dose-dependently in the presence of these cations. The effects were in the following order of potency: Mn2+ congruent to Ni2+ greater than Mg2+ greater than Ca2+ greater than Sr2+ greater than Ba2+. Scatchard analysis of the binding data revealed a single component of the binding sites in the presence of 2.5 mM MgCl2, 2.5 mM CaCl2 or 0.3 mM MnCl2 whereas two components appeared in the presence of 2.5 mM MnCl2 or 1 mM NiCl2. In the former, divalent cations altered the apparent affinity (Kd) without affecting density of the binding sites (Bmax). In the latter, the high-affinity sites showed a higher affinity and lower density of the binding sites than did the single component of the former. As the maximal effects of four cations (Mg2+, Ca2+, Mn2+ and Ni2+) were not additive, there are probably common sites of action of these divalent cations. Among the ligands for GABAB sites, the affinity for (-), (+) and (+/-) baclofen, GABA and beta-phenyl GABA increased 2-6 fold in the presence of 2.5 mM MnCl2, in comparison with that in HEPES-buffered Krebs solution (containing 2.5 mM CaCl2 and 1.2 mM MgSO4), whereas that for muscimol was decreased to one-fifth. Thus, the affinity of GABAB sites for its ligands is probably regulated by divalent cations, through common sites of action.

Animals↗
PubMed1990

Modulation of sodium channel gating by external divalent cations: differential effects on opening and closing rates.

The effects of external divalent cations on the steady-state open probability (Po), opening and closing rates, and conductance of batrachotoxin (BTX)-activated Na channels were studied in planar lipid bilayers. External divalent cations shifted the midpoint of the Po versus membrane potential (Vm) relation (gating curve) to more depolarized potentials. Of the group IIA divalent cations tested, Ca2+ caused the largest depolarizing shift followed by Ba2+, Sr2+ and Mg2+ in that order. In contrast, the order of decreasing efficacy for block of the single channel conductance was Ca2+, Mg2+, Sr2+ and Ba2+. Thus, because it is a relatively good shifter but a poor blocker, Ba2+ was used to study the effects of divalent cations on gating. External Ba2+ decreased the Na channel opening rate and increased the closing rate. At voltages close to the midpoint of the gating curve, the opening rate decreased by a factor of 1/2.8 while the closing rate increased by 2.0-fold. Although Ba2+ would be expected to increase the external surface potential by interacting with negatively charged residues on the channel, this preferential effect on the opening rate indicates that the effects of external Ba2+ cannot be attributed solely to an increase in the potential gradient across the voltage sensing apparatus.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1983

Alteration of susceptibility to EDTA, polymyxin B and gentamicin in Pseudomonas aeruginosa by divalent cation regulation of outer membrane protein H1.

Induction of outer membrane protein H1 in Pseudomonas aeruginosa results in decreased susceptibility to aminoglycosides, polymyxin B, and EDTA. We have previously shown that protein H1 can become the major cellular protein in cells grown in low (0.02 mM) Mg2+. The induction of protein H1 was prevented by supplementation of low Mg2+ medium with Mg2+, Ca2+, Mn2+, or Sr2+ (each at 0.5 mM), but not with Zn2+, Ba2+, Sn2+, Al3+ or Na+ (each at 0.5 mM). Only cells grown in the presence of those cations which failed to prevent H1 induction were resistant to the cationic antibiotics, polymyxin B and gentamicin, and to chelators of divalent cations. Cells grown in Ca2+, but not in Mg2+, were susceptible to outer membrane permeabilization by the Ca2+ specific chelator EGTA, whereas both were susceptible to EDTA. In agreement with this, cells grown in Mg2+, Ca2+, Mn2+, or Zn2+ showed enhanced levels of these cations respectively as their major cell envelope-associated cation. When cells were shifted from low to high Mg2+ medium, the time course of the decrease in the levels of protein H1 correlated well with the increase in sensitivity to EDTA and polymyxin B. These results support the hypothesis that protein H1 acts to replace divalent cations at a critical outer membrane site attacked by cationic antibiotics and chelators of divalent cations, and suggest that only a small proportion of the total divalent cation-binding sites in the outer membrane are susceptible to attack by these agents.

Bacterial Outer Membrane Proteins↗
PubMed1985

Divalent cations and the phosphatase activity of the (Na + K)-dependent ATPase.

Phosphatase activity of a kidney (Na + K)-ATPase preparation was optimally active with Mg2+ plus K+. Mn2+ was less effective and Ca2+ could not substitute for Mg2+. However, adding Ca2+ with Mg2+ or substituting Mn2+ for Mg2+ activated it appreciably in the absence of added K+, and all three divalent cations decreased apparent affinity for K+. Inhibition by Na+ decreased with higher Mg2+ concentrations, when Ca2+ was added, and when Mn2+ was substituted for Mg2+. Dimethyl sulfoxide, which favors E2 conformations of the enzyme, increased apparent affinity for K+, whereas oligomycin, which favors E1 conformations, decreased it. These observations are interpretable in terms of activation through two cases of cation sites. (i) At divalent cation sites, Mg2+ and Mn2+, favoring (under these conditions) E2 conformations, are effective, whereas Ca2+, favoring E1, is not, and monovalent cations complete. (ii) At monovalent cation sites divalent cations compete with K+, while Na+ at these sites favors E1 conformations. K+ increases the Km for substrate, but both Ca2+ and Mn2+ decrease it, perhaps by competing with K+. On the other hand, phosphatase activity in the presence of Na+ plus K+ is stimulated by dimethyl sulfoxide, by higher concentrations of Mg2+ and Mn2+, but not by adding Ca2+; this is consistent with stimulation occurring through facilitation of an E1 to E2 transition, perhaps an E1-P to E2-P step like that in the (Na + K)-ATPase reaction sequence. However, oligomycin stimulates phosphatase activity with Mg2+ plus Na+ alone or Mg2+ plus low K+: this effect of oligomycin may reflect acceleration, in the absence of adequate K+, of an alternative E2-P to E1 pathway bypassing the monovalent cation-activated steps in the hydrolytic sequence.

Animals↗
PubMed1984

Apparent saturation kinetics of divalent cation uptake in yeast caused by a reduction in the surface potential.

The concentration dependence of the uptake rate of divalent cations in yeast can be described by a simple diffusion process after accounting for the effect of the surface potential upon the divalent cation concentration near the membrane. It is also necessary to correct for the effect of the cell pH upon the rate of translocation. The apparent saturation kinetics is ascribed to the fact that the quotient of the concentration of the divalent cations near the cell membrane and the bulk aqueous phase concentration is reduced on increasing the divalent cation concentration in the medium. The diffusion process regulated by the surface potential even mimics the saturation kinetics of a two-carrier transport system. The selectivity found between Ca2+ and Sr2+ uptake can probably be traced to differences in their affinity for the negative groups on the cell membrane determining the surface potential rather than to differences in their affinity for a transport system. The enhancement of divalent cation uptake by loading the cells with phosphate is probably due to the concomitant increase in the net negative charge of the cell membrane.

Biological Transport, Active↗
PubMed1991

Blockage and permeation of divalent cations through the cyclic GMP-activated channel from tiger salamander retinal rods.

1. Blockage and permeation of divalent cations through channels activated by guanosine 3',5'-cyclic monophosphate (cyclic GMP) were studied in membrane patches excised from retinal rods of the tiger salamander Ambystoma tigrinum by rapidly changing the ionic medium bathing the intracellular side of the excised membrane. 2. The Na+ current, observed when 110 mM-NaCl was present on both sides of the membrane patch, was reduced by the addition of 1 mM of the chloride salts of Ca2+, Mg2+, Sr2+, Ba2+ or Mn2+ to the bathing medium. The sequence of blocking potency at +60 mV was Mg2+ greater than Mn2+ approximately Ba2+ greater than Ca2+ greater than Sr2+, while at -60 mV it was Ba2+ greater than Ca2+ greater than Sr2+ greater than Mn2+ approximately Mg2+. For all divalent cations the blocking effect depended, in a complex way, on the membrane potential. 3. The blocking effect of Ca2+ and Mg2+ increased when the concentration of cyclic GMP was reduced from 100 to 5 microM. At -60 mV 1 mM-Ca2+ blocked about 34% of the Na+ current in the presence of 100 microM-cyclic GMP, while in the presence of 5 microM-cyclic GMP, 1 mM-Ca2+ blocked about 56% of the Na+ current. 4. When, in the presence of 100 microM-cyclic GMP, 110 mM-NaCl at the intracellular side was replaced by equiosmolar amounts of chloride salts of divalent cations (73.3 mM) a small outward current carried by divalent cations could be observed at large positive membrane potentials. At +60 mV the ratio between the current carried by Na+, Sr2+, Ca2+, Ba2+, Mg2+ and Mn2+ was 83.3:1.4:1:0.58:0.33:0.25. 5. In agreement with previous observations the dependence of the Na+ current on the concentration of cyclic GMP shows a clear co-operativity among cyclic GMP molecules.4+ cyclic GMP-gated channel in excised patches is similar to but not identical to the selectivity sequence of divalent cations through the channel in intact rods.

Animals↗
PubMed1981

Effects of divalent cations on beta-cell electrical activity.

The effects of various divalent cations, added to the external medium, upon beta-cell action potential were studied using intracellular microelectrodes. Changes of spike peak potential, as a function of external cation concentration, indicate that Sr2+ or Ba2+ may substitute for Ca2+ as a charge carrier. Complete blockage by Mn2+ of electrical activity elicited by Sr2+, Ba2+, or Ca2+ suggests that these cations penetrate the membrane though the same Ca2+ channel. The increase of maximum rate of depolarization, dV(d)/dtmax, and decrease of maximum rate of repolarization, dV(r)/dtmax, when Sr2+ is substituted for Ca2+ suggest that Sr2+ penetrates more readily the Ca2+ channel but is less effective than Ca2+ in activating K permeability. Reversal of these effects by addition of equimolar Ca2+ to Sr2+ indicates that Ca2+ has a greater affinity than Sr2+ for the receptor site. The blockage of electrical activity by Ba2+ at a depolarized membrane level suggests that Ba2+ markedly reduces all K+ permeabilities. Analysis of dV(d)/dtmax at various Ca2+ concentrations, in the presence of nonpermeant divalent cations (Co2+, Mn2+, and Mg2+), shown that these cations bind competitively at the same receptor site with differing dissociation constants, For all of these divalent cations, the order of binding would be Co2+ greater than Mn2+ greater than Ca2+ greater than Sr2+, Mg2+.

Action Potentials↗