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PubMed1979

Microbial transformation of A23187, a divalent cation ionophore antibiotic.

A23187 is an ionophore antibiotic that forms dimeric complexes with divalent cations such an Mn2+ and Ca2+. Over 200 randomly selected soil microorganisms were incubated with A23187. None of these cultures was capable of transforming this compound. In contrast, many microorganisms were able to modify the methyl ester of A23187. The transformation products produced by one culture, Streptomyces chartreusis, were isolated and identified as 16-hydroxy-N-demethyl A23187 methyl ester, 16-hydroxy-A23187 methyl ester, and N-demethyl A23187 methyl ester. These ester derivatives lack most of the ionophore properties of the acids and cannot readily form dimeric complexes with divalent cations. However, they could be hydrolyzed by a mild treatment with ethanolic KOH to free acids that possess good ionophore activity. The use of the ester substrate in conjunction with the hydrolysis procedure is, at the present time, the only known method for microbiologically producing A23187 derivatives.

Anti-Bacterial Agents↗
PubMed1995

Divalent cations control cell-substrate adhesion and laminin expression in normal and malignant human melanocytes in early and late stages of cellular differentiation.

Integrins are a class of adhesion molecules that depends on divalent cations for proper function. This study examined whether human normal melanocytes and malignant (metastatic) melanocytes with early and late stages of cellular differentiation (G361 and SK-MEL-23, respectively) would differ in integrin-mediated adhesion to fibronectin, laminin, as well as collagens type I and type IV, and whether divalent cations could influence the strength of adhesion ability. Integrin subunit expression was determined by flow cytometry using integrin subunit-specific antibodies as probes. Integrin-specific adhesion was determined using soluble glycine-arginine-glycine-asparagine-serine peptide and integrin subunit-specific antibodies as functional blocking agents. This study shows that both normal and malignant melanocytes adhere to extracellular matrices in a divalent cation-dependent manner, and adhesion strength varies with the cation species. Integrins can be rapidly activated by small alterations in cation concentration, manganese being the most potent. There were marked differences in substrate adhesion between normal melanocytes and metastatic malignant melanoma cells, but these differences were not related to the stage of cellular differentiation. All the three cell types, however, expressed the same integrin subunits at approximately the same levels. This suggests that substrate adhesion of melanocytes and melanoma cells might involve some integrin-independent mechanisms as well. Manganese, in particular, appears to cause adhesion by activating both integrin-dependent and -independent mechanisms.

Amino Acid Sequence↗
PubMed1983

Large divalent cations and electrostatic potentials adjacent to membranes. Experimental results with hexamethonium.

A simple extension of the Gouy-Chapman theory predicts that the ability of a divalent cation to screen charges at a membrane-solution interface decreases significantly if the distance between the charges on the cation is comparable with the Debye length. We tested this prediction by investigating the effect of hexamethonium on the electrostatic potential adjacent to negatively charged phospholipid bilayer membranes. The distance between the two charges of an extended hexamethonium molecule is approximately 1 nm, which is the Debye length in the 0.1 M monovalent salt solutions used in these experiments. Six different experimental approaches were utilized. We measured the electrophoretic mobility of multilamellar vesicles to determine the zeta potential, the line width of the 31P nuclear magnetic resonance (NMR) signal from sonicated vesicles to calculate the change in potential at the phosphodiester moiety of the lipid, and the conductance of planar bilayer membranes exposed to either carriers (nonactin) or pore formers (gramicidin) to estimate the change in potential within the membrane. We also measured directly the effect of hexamethonium on the potential above a monolayer formed from negative lipids, and attempted to calculate the change in the surface potential of a bilayer membrane from capacitance measurements. With the exception of the capacitance calculations, each of the techniques gave comparable results: hexamethonium exerts a smaller effect on the potential than that predicted by the classic screening theory. The results are consistent with the predictions of the extended Gouy-Chapman theory and are relevant to the interpretation of physiological and pharmacological experiments that utilize hexamethonium and other large divalent cations.

Animals↗
PubMed1988

Tubulin exchanges divalent cations at both guanine nucleotide-binding sites.

The tubulin heterodimer binds a molecule of GTP at the nonexchangeable nucleotide-binding site (N-site) and either GDP or GTP at the exchangeable nucleotide-binding site (E-site). Mg2+ is known to be tightly linked to the binding of GTP at the E-site (Correia, J. J., Baty, L. T., and Williams, R. C., Jr. (1987) J. Biol. Chem. 262, 17278-17284). Measurements of the exchange of Mn2+ for bound Mg2+ (as monitored by atomic absorption and EPR) demonstrate that tubulin which has GDP at the E-site possesses one high affinity metal-binding site and that tubulin which has GTP at the E-site possesses two such sites. The apparent association constants are 0.7-1.1 x 10(6) M-1 for Mg2+ and approximately 4.1-4.9 x 10(7) M-1 for Mn2+. Divalent cations do bind to GDP at the E-site, but with much lower affinity (2.0-2.3 x 10(3) M-1 for Mg2+ and 3.9-6.6 x 10(3) M-1 for Mn2+). These data suggest that divalent cations are involved in GTP binding to both the N- and E-sites of tubulin. The N-site metal exchanges slowly (kapp = 0.020 min-1), suggesting a mechanism involving protein "breathing" or heterodimer dissociation. The N-site metal exchange rate is independent of the concentration of protein and metal, an observation consistent with the possibility that a dynamic breathing process is the rate-limiting step. The exchange of Mn2+ for Mg2+ has no effect on the secondary structure of tubulin at 4 degrees C or on the ability of tubulin to form microtubules. These results have important consequences for the interpretation of distance measurements within the tubulin dimer using paramagnetic ions. They are also relevant to the detailed mechanism of divalent cation release from microtubules after GTP hydrolysis.

Animals↗
PubMed1984

Effects of divalent cations on biphasic potassium contractures and on contractile inactivation in low calcium solutions in frog single twitch muscle fibers.

When the concentration of external Ca2+ was reduced for 30 sec in a single twitch muscle fiber of a frog, the peak tension of the initial component of biphasic 80 mM K+ contractures was potentiated, whereas that of the secondary component was markedly inhibited, despite the fact that in the early stage it was potentiated usually in case of contractures in 60 and 40 nM K+ but rarely in 80 mM K+. These changes were not observed, however, in the presence of 0.5-1 nM Mn2+, i.e., Mn2+ having been substituted for Ca2+. The foregoing result and the authors' previously reported data indicated the following. First, the concentrations of divalent cations having an equal effect in bringing about the peak tensions of both components are 3 mM Mg2+, 0.7 mM Mn2+, 0.5 mM Ni2+, and 1.8 mM Ca2+. Secondly, this sequence constitutes their increasing order of effectiveness on the time course of the secondary component. Meanwhile, a similar order was found to exist in another experiment concerning the effectiveness in inhibiting the inactivation of K contractures facilitated by lowering the concentration of external Ca2+. All these findings indicated that the divalent cations act on the activation processes of both components in a stabilizing manner, differing from the way in which they act on the inactivation process of the secondary component. The mechanisms in which the peak tensions of the initial and secondary components are inhibited in a low Ca2+ solution and the divalent cations act on both components are discussed. Finally, another experiment made in the absence and presence of Ca2+ revealed that the effect of high concentrations of Mn2+ in the initial component is different from that on the secondary component.

Animals↗
PubMed1994

Divalent cations and radiation induced lipid peroxidation.

Response of Co2+, Cu2+, Fe2+, Ni2+ and Mn2+ against radiation induced lipid peroxidation has been examined. Liposomes prepared from L-alpha-lecithin were irradiated with or without divalent cations, Co2+ and Cu2+ inhibited lipid peroxidation in concentration dependent manner. On the other hand Fe2+, Ni2+ and Mn2+ enhanced the peroxidation. The enhancement of peroxidation due to Ni2+ was biphasic in nature. Higher concentration of Mn2+ decreased the peroxidation. Except Cu2+ no other cation studied could change the non-linear pattern of radiation induced lipid peroxidation. The pro- or antioxidant effect of divalent cations may be due to their reactivity towards oxygen and oxygen free radicals, and at least in part to their hard/soft acid character.

Cations, Divalent↗
PubMed1982

Effects of divalent cations on toad end-plate channels.

Miniature end-plate currents (MEPCs) and acetylcholine-induced current fluctuations were recorded in voltage-clamped, glycerol-treated toad sartorius muscle fibers in control solution and in solutions with added divalent cations. In isosmotic solutions containing 20 mM Ca or Mg, MEPCs had time constants of decay (tau D) which were about 30% slower than normal. In isotonic Ca solutions (Na-free), greater increases in both tau D and channel lifetime were seen; the null potential was -34 mV, and single-channel conductance decreased to approximately 5 pS. Zn or Ni, at concentrations of 0.1-5 mM, were much more effective in increasing tau D than Ca or Mg, although they did not greatly affect channel conductance. The normal temperature and voltage sensitivity of tau was not significantly altered by any of the added divalent cations. Surface potential shifts arising from screening of membrane fixed charge by divalent cations cannot entirely explain the observed increases in tau, especially when taken together with changes in channel conductance.

Acetylcholine↗
PubMed2001

Photosystem II of peas: effects of added divalent cations of Mn, Fe, Mg, and Ca on two kinetic components of P(+)(680) reduction in Mn-depleted core particles.

The catalytic Mn cluster of the photosynthetic oxygen-evolving system is oxidized via a tyrosine, Y(Z), by a photooxidized chlorophyll a moiety, P(+)(680). The rapid reduction of P(+)(680) by Y(Z) in nanoseconds requires the intactness of an acid/base cluster around Y(Z) with an apparent functional pK of <5. The removal of Mn (together with bound Ca) shifts the pK of the acid/base cluster from the acid into the neutral pH range. At alkaline pH the electron transfer (ET) from Y(Z) to P(+)(680) is still rapid (<1 micros), whereas at acid pH the ET is much slower (10-100 micros) and steered by proton release. In the intermediate pH domain one observes a mix of these kinetic components (see R. Ahlbrink, M. Haumann, D. Cherepanov, O. Bögershausen, A. Mulkidjanian, W. Junge, Biochemistry 37 (1998)). The overall kinetics of P(680)(+) reduction by Y(Z) in Mn-depleted photosystem II (PS II) has been previously shown to be slowed down by divalent cations (added at >10 microM), namely: Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+) (C.W. Hoganson, P.A. Casey, O. Hansson, Biochim. Biophys. Acta 1057 (1991)). Using Mn-depleted PS II core particles from pea as starting material, we re-investigated this phenomenon at nanosecond resolution, aiming at the effect of divalent cations on the particular kinetic components of P(+)(680) reduction. To our surprise we found only the slower, proton steered component retarded by some added cations (namely Co(2+)/Zn(2+)>Fe(2+)>Mn(2+)). Neither the fast component nor the apparent pK of the acid/base cluster around Y(Z) was affected. Apparently, the divalent cations acted (electrostatically) on the proton release channel that connects the oxygen-evolving complex with the bulk water, but not on the ET between Y(Z) and P(+)(680), proper. Contrastingly, Ca(2+) and Mg(2+), when added at >5 mM, accelerated the slow component of P(+)(680) reduction by Y(Z) and shifted the apparent pK of Y(Z) from 7.4 to 6.6 and 6.7, respectively. It was evident that the binding site(s) for added Ca(2+) and Mg(2+) were close to Y(Z) proper. The data obtained are discussed in relation to the nature of the metal-binding sites in photosystem II.

Binding Sites↗
PubMed1982

Phosphorus-31 and carbon-13 nuclear magnetic resonance studies of divalent cation binding to phosphatidylserine membranes: use of cobalt as a paramagnetic probe.

The paramagnetic divalent cation cobalt has large and well-understood effects on NMR signals from ligands bound in the first coordination sphere, i.e., inner-sphere ligands, and we have used these effects to identify divalent cation binding sites at the surface of phosphatidylserine membranes. 31P NMR results show that 13% of the bound cobalt ions are involved in inner-sphere complexes with the phosphodiester group, while 13C NMR results show that 54% of the bound cobalt ions are involved in unidentate inner sphere complexes with the carboxyl group. No evidence is found for cobalt binding to the carbonyl groups, but proton release studies suggest that 32% of the bound cobalt ions are involved in chelate complexes that contain both the carboxyl and the amine groups. All (i.e., 13% + 54% + 32% = 99%) of the bound cobalt ions can thus be accounted for in terms of inner sphere complexes with the phosphodiester group or the carboxyl group. We suggest that the unidentate inner-sphere complex between cobalt and the carboxyl group of phosphatidylserine and the inner-sphere complex between cobalt and the phosphodiester group of phosphatidylserine provide reasonable models for complexes between alkaline earth cations and phosphatidylserine membranes.

Animals↗
PubMed1994

Role of the invertebrate electrogenic 2Na+/1H+ antiporter in monovalent and divalent cation transport.

In recent years, an electrogenic 2Na+/1H+ antiporter has been identified in a variety of invertebrate epithelial brush-border membranes of gut, kidney and gill tissues. The antiporter differs significantly in its physiological properties from the electroneutral 1Na+/1H+ antiporter proposed for vertebrate cells. In all invertebrate cells examined, the antiporter displayed a 2:1 transport stoichiometry, responded to an induced transmembrane potential and exhibited a high binding affinity for the divalent cation Ca2+, which acted as a competitive inhibitor of Na+ transport. A monoclonal antibody specific for the crustacean electrogenic antiporter inhibited 2Na+/1H+ exchange, but was without effect on Na(+)-dependent D-glucose transport. Immunoreactivity was localized at hepatopancreatic brush-border and vacuolar membranes, antennal gland coelomosac podocytes and posterior gill epithelial cells-all locations were published reports described unique cation exchange kinetics. Significant fractions of Ca2+ transport into invertebrate cells across brush-border membranes occurred by an electrogenic, amiloride-sensitive exchange process, probably by the 2Na+/1H+ antiporter, and this transport was markedly inhibited by exogenous zinc and cadmium. A recently identified electroneutral, amiloride-sensitive, hepatopancreatic epithelial basolateral Na+/H+ antiporter was uninfluenced by the brush-border monoclonal antibody, exhibited an apparent 1:1 transport stoichiometry and possessed a minimal divalent cation specificity. Calcium transport at this epithelial pole occurred by the combination of a Ca2+/Na+ antiporter, an ATP-dependent Ca(2+)-ATPase and a verapamil-sensitive calcium channel. These crustacean brush-border and basolateral transporters may play significant roles in calcification and heavy metal detoxification.

Animals↗
PubMed1999

Membrane fusion and the lamellar-to-inverted-hexagonal phase transition in cardiolipin vesicle systems induced by divalent cations.

The polymorphic phase behavior of bovine heart cardiolipin (CL) in the presence of different divalent cations and the kinetics of CL vesicle fusion induced by these cations have been investigated. (31)P-NMR measurements of equilibrium cation-CL complexes showed the lamellar-to-hexagonal (L(alpha)-H(II)) transition temperature (T(H)) to be 20-25 degrees C for the Sr(2+) and Ba(2+) complexes, whereas in the presence of Ca(2+) or Mg(2+) the T(H) was below 0 degrees C. In the presence of Sr(2+) or Ba(2+), CL large unilamellar vesicles (LUVs) (0.1 microm diameter) showed kinetics of destabilization, as assessed by determination of the release of an aqueous fluorescent dye, which strongly correlated with the L(alpha)-H(II) transition of the final complex: at temperatures above the T(H), fast and extensive leakage, mediated by vesicle-vesicle contact, was observed. On the other hand, mixing of vesicle contents was limited and of a highly transient nature. A different behavior was observed with Ca(2+) or Mg(2+): in the temperature range of 0-50 degrees C, where the H(II) configuration is the thermodynamically favored phase, relatively nonleaky fusion of the vesicles occurred. Furthermore, with increasing temperature the rate and extent of leakage decreased, with a concomitant increase in fusion. Fluorescence measurements, involving incorporation of N-NBD-phosphatidylethanolamine in the vesicle bilayer, demonstrated a relative delay in the L(alpha)-H(II) phase transition of the CL vesicle system in the presence of Ca(2+). Freeze-fracture electron microscopy of CL LUV interaction products revealed the exclusive formation of H(II) tubes in the case of Sr(2+), whereas with Ca(2+) large fused vesicles next to H(II) tubes were seen. The extent of binding of Ca(2+) to CL in the lamellar phase, saturating at a binding ratio of 0.35 Ca(2+) per CL, was close to that observed for Sr(2+) and Ba(2+). It is concluded that CL LUVs in the presence of Ca(2+) undergo a transition that favors nonleaky fusion of the vesicles over rapid collapse into H(II) structures, despite the fact that the equilibrium Ca(2+)-CL complex is in the H(II) phase. On the other hand, in the presence of Sr(2+) or Ba(2+) at temperatures above the T(H) of the respective cation-CL complexes, CL LUVs rapidly convert to H(II) structures with a concomitant loss of vesicular integrity. This suggests that the nature of the final cation-lipid complex does not primarily determine whether CL vesicles exposed to the cation will initially undergo a nonleaky fusion event or collapse into nonvesicular structures.

4-Chloro-7-nitrobenzofurazan↗
PubMed1982

Queuine-containing isoacceptor of tyrosine tRNA in Drosophila melanogaster. Alteration of levels by divalent cations.

Dietary cadmium causes the queuine-containing, Q(+), isoacceptors to increase relative to the guanine-containing, Q(-), ones of tRNATyr, tRNAHis and tRNAAsp of Drosophila melanogaster. Of the other divalent cations examined, Sr2+, Ni2+, Cu2+, Zn2+ and Hg2+, only Hg2+ failed to cause an increase in Q(+)tRNATyr. For these results, all pre-adult stages of the organism were spent on media containing the divalent ions. Adult flies that had developed on a normal diet also responded to divalent ions; Hg2+ as well as Cd2+, Sr2+ and Zn2+ caused an increase in Q(+)tRNATyr in 4 days. Using adult flies, the rate of the response was measured; when placed on a Cd2+-containing diet, they formed significantly more Q(+)tRNATyr within 24 h as compared to adults on a normal diet. Whether the queuine is derived from the diet or from de novo synthesis is yet to be determined. Since the metal ions represent a range of values in the 'hard-soft' classification, different sites of reaction are expected, yet for Drosophila a common result is an alteration in the ratio of Q(+) and Q(-) isoacceptors of these tRNAs. The transition to Q(+)tRNA may be an early indication of the metabolic imbalances resulting from the presence of the divalent cation.

Animals↗
PubMed1987

Intensification of rheological destruction of rat fibrosarcoma KMT 17 cells by elimination of divalent cations.

In an effort to find a method to intensify the rheological destruction of tumor cells, rat fibrosarcoma KMT 17 cells in ascitic form were exposed to rheological stresses in test solutions; modified Eagle medium with EDTA, and phosphate buffer solution without divalent cations or with verapamil. KMT 17 cells were exposed to a uniform shear stress produced by the rotation of a rotating cone plate viscometer for 1 to 2 hours and to the strong deformation by the passage through Nuclepores of 10, 8 and 5 micrometer. KMT 17 cells suspended in the test solutions were more effectively destroyed by the stresses than those suspended in normal solutions containing divalent cations without any other calcium-suppressing agents. These results suggest that the elimination of divalent cations and/or the block of calcium ion channels of cell membranes intensify the rheological destruction of tumor cells migrating in the circulatory system.

Animals↗
PubMed1984

Effects of guanine nucleotides and divalent cations on forskolin activation of rabbit luteal adenylyl cyclase: evidence for the existence of an inhibitory guanine nucleotide-binding regulatory component.

The effects of guanine nucleotides and divalent cations on the activation of rabbit luteal adenylyl cyclase by the diterpene forskolin were investigated. Saturating concentrations of forskolin elicited 10- to 15-fold stimulation of adenylyl cyclase activity in the absence of added guanine nucleotide. No lag was observed in the time course of forskolin-induced activation. Addition of 10 microM guanosine triphosphate (GTP) and guanyl-5'-yl imidodiphosphate [GMP-P(NH)P] inhibited forskolin activation by 10-15% and 30-40%, respectively, in the presence of 3.0 mM MgCl2. GMP-P(NH)P was more potent than GTP in inhibiting forskolin activation of adenylyl cyclase having an IC50 of 36 nM compared to 610 nM for GTP. In contrast, the Kact for stimulating adenylyl cyclase activity by both GMP-P(NH)P and GTP were similar, 1.00 and 0.86 microM, respectively. GMP-P(NH)P-induced inhibition of the forskolin-activated enzyme was not due to the hysteretic nature of GMP-P(NH)P activation of luteal adenylyl cyclase, as addition of GMP-P(NH)P to an enzyme that had been treated 5 min earlier with forskolin resulted in the immediate inhibition of enzymatic activity. Addition of GMP-P(NH)P to a concentration-effect curve for forskolin increased the Kact value for forskolin form 7.18 to 26.8 microM. There was a different MgCl2 concentration requirement for maximal stimulation of luteal cyclase by GMP-P(NH)P (8 mM MgCl2) and maximal inhibition of forskolin-stimulated activity by GMP-P(NH)P (0.5-0.6 mM MgCl2). Further, MnCl2 concentrations above 1.0 mM completely abolished the inhibitory action of GMP-P(NH)P on forskolin activation of luteal cyclase. In fact, at 2.0 mM MnCl2, adenylyl cyclase activity in the presence of GMP-P(NH)P plus forskolin was greater than that of forskolin alone. Thus taken together, these findings suggest that the rabbit corpus luteum contains an inhibitory guanine nucleotide-binding regulatory component in addition to a stimulatory regulatory component. Further, these components demonstrate differing requirements for both guanine nucleotides and divalent cations in order to interact with the catalytic moiety of adenylyl cyclase. The existence of such an inhibitory component suggests the presence of an inhibitory receptor in the corpus luteum which could negatively regulate adenylyl cyclase resulting in the inhibition of cAMP production and reduced progesterone output from the corpus luteum under normal physiological conditions.

Adenylyl Cyclases↗
PubMed1992

Differential sensitivity of pneumolysin-induced channels to gating by divalent cations.

The induction of channels across planar lipid bilayers by purified, recombinant pneumolysin (a hemolytic protein from Streptococcus pneumoniae) has been studied by measuring increases in electrical conductivity. Pneumolysin-induced channels exhibit a wide range of single channel conductances (less than 50 pS to greater than 1 nS at 0.1 M KCl). Channels can be categorized on the basis of their K+:Cl- selectivity: the smallest channels are strongly cation selective, with t+ (the cation transference number) approaching 1.0; the largest channels are unselective (t+ approximately 0.5). Channels tend to remain open at all voltages (-150 to 150 mV); only the smallest channels exhibit any rectification. In the presence of divalent cations (1-5 mM Zn2+; 10-20 mM Ca2+), small (less than 50 pS) and medium-sized (50 pS to 1 nS) channels are closed in a voltage-dependent manner (more closure at higher voltages); at 0 voltage channels reopen. Overall selectivity is reduced by divalent cations, compatible with small, selective channels being closed preferentially to large, nonselective ones. It is concluded that a single molecular species (pneumolysin) induces multiple-sized channels that can be categorized by cation:anion selectivity and by their sensitivity to closure by divalent cations.

Bacterial Proteins↗
PubMed1982

Electrical measurement of electroneutral fluxes of divalent cations through charged planar phospholipid membranes.

The voltage-sensitive channel-former monazomycin is used as a conductance probe to monitor changes in the trans electrostatic surface potentials of negatively-charged planar phospholipid bilayers. Cis-to-trans electroneutral fluxes of divalent cations mediated by ionophores A23187 and X537A are sensed via the effect of transported divalent cations on the trans surface potentials. Quantitative determinations of neutral Ca2+ and Mg2+ fluxes are made and related to ionophore function.

Anti-Bacterial Agents↗
PubMed1984

The effect of divalent cations on the rosette formation via C3 receptors.

The indicator cells with the specificity for the various C3 receptors were prepared using highly purified complement components. The effect of EDTA and divalent cations on the rosette formation of these indicators with human blood cells and lymphoblastoid cells were investigated. B lymphocytes form rosettes with EACl-3b, EACl-3bH, EACl-3bi and EACl-3d irrespective of the presence of cations. PMN and monocytes form rosettes with EACl-3b and EACl-3bH independent of cations, but their formation with EAC1-3bi needs the presence of magnesium. In this case calcium shows a cooperative effect with magnesium. PMN and monocytes do not react with EACl-3d even in the presence of magnesium. Although Raji cells do not react with EACl-3b, they react with EACl-3bH independent of divalent cations. EACm has a similar reactivity to EACl-3bi, and by the treatment with trypsin it becomes similar to EACl-3d.

Calcium↗
PubMed1975

Divalent cations: effects on post-synaptic pharmacology of invertebrate synapses.

(1) The effects of divalent cations (Ca++, Mg++, Sr++ and Co++) were studied on the post-synaptic responses of crustacean neuromuscular junctions and identified molluscan neurons to bath and iontophoretic application of putative transmitters. (2) The glutamate response of the crustacean muscle was parabolically dependent on [Ca++]0, while the ACh response of an identified molluscan neuron was inversely dependent on[Ca++]0. Elevated [Ca++]0 depressed both glutamate and ACh depolarizations in a concentration-dependent, reversible manner. Low concentrations of Co++ also depressed both depolarizations in a concentration-dependent, reversible manner. (3) Double-reciprocal plot analyses of the Ca++ and Co++ depressions indicate that these agents were apparently not acting to reduce the affinity of the receptor for the agonist. Elevated concentrations of both Ca++ and Co++ shifted the inversion potential of the ACh response in a hyperpolarizing direction, suggesting a preferential block of the receptor-coupled Na+ conductance. (4) Neither Ca++ nor Co++ depressed Cl- or K+-dependent responses coupled to the putative transmitters GABA, glutamate, dopamine or ACh. (5) The selective inhibition of the ACh and glutamate responses by the general anesthetic pentobarbital was examined as a function of[Ca++]0. Decreasing [Ca++]0 by 5-fold decreased the pentobarbital inhibition by about 50% while increasing [Ca++]0 by 5-fold produced an insignificant increase in the inhibition. (6) The data indicate that divalent cations, like general anesthetics, selectively depress post-synaptic excitatory responses that are primarily Na+-dependent. This selective depression by Ca++ could contribute to its anesthetic and anticonvulsant properties when present in elevated concentrations in the ventricular fluid. The mechanism by which divalent ions and general anesthetics selectively depress receptor-coupled conductances appear to be different: divalent ions preferentially attack the Na+ component while anesthetics block Na+ and K+ conductance equally (possibly by affecting the kinetics of the mechanism).

Acetylcholine↗