scholarly journals Permeation and interaction of divalent cations in calcium channels of snail neurons.

1985 ◽  
Vol 85 (4) ◽  
pp. 491-518 ◽  
Author(s):  
L Byerly ◽  
P B Chase ◽  
J R Stimers
Keyword(s):  
Mole Fraction ◽  
Surface Potential ◽  
Divalent Cation ◽  
Lymnaea Stagnalis ◽  
Divalent Cations ◽  
Ca Channel ◽  
Ca Current ◽  
Channel Current ◽  
Gating Mechanism ◽  
Ca Channel Current

We have studied the current-carrying ability and blocking action of various divalent cations in the Ca channel of Lymnaea stagnalis neurons. Changing the concentration or species of the permeant divalent cation shifts the voltage dependence of activation of the Ca channel current in a manner that is consistent with the action of the divalent cation on an external surface potential. Increasing the concentration of the permeant cation from 1 to 30 mM produces a twofold increase in the maximum Ca current and a fourfold increase in the maximum Ba current; the maximum Ba current is twice the size of the maximum Ca current for 10 mM bulk concentration. Correcting for the changing surface potential seen by the gating mechanism, the current-concentration relation is almost linear for Ba2+, and shows only moderate saturation for Ca2+; also, Ca2+, Ba2+, and Sr2+ are found to pass through the channel almost equally well. These conclusions are obtained for either of two assumptions: that the mouth of the channel sees (a) all or (b) none of the surface potential seen by the gating mechanism. Cd2+ blocks Lymnaea and Helix Ca channels at concentrations 200 times smaller than those required for Co2+ or Ni2+. Ca2+ competes with Cd2+ for the blocking site; Ba2+ binds less strongly than Ca2+ to this site. Mixtures of Ca2+ and Ba2+ produce an anomalous mole fraction effect on the Ca channel current. After correction for the changing surface potential (using either assumption), the anomalous mole fraction effect is even more prominent, which suggests that Ba2+ blocks Ca current more than Ca2+ blocks Ba current.

scholarly journals Inactivation of calcium channel current in the calf cardiac Purkinje fiber. Evidence for voltage- and calcium-mediated mechanisms.

1984 ◽  
Vol 84 (5) ◽  
pp. 705-726 ◽  
Author(s):  
R S Kass ◽  
M C Sanguinetti
Keyword(s):  
Charge Carrier ◽  
Divalent Cations ◽  
Purkinje Fiber ◽  
Ca Channel ◽  
Channel Current ◽  
Zero Current ◽  
Dependent Inactivation ◽  
Voltage Dependent ◽  
Ca Channel Current ◽  
Dependent Mechanism

We have studied the influence of divalent cations on Ca channel current in the calf cardiac Purkinje fiber to determine whether this current inactivates by voltage- or Ca-mediated mechanisms, or by a combination of the two. We measured the reversal (or zero current) potential of the current when Ba, Sr, or Ca were the permeant divalent cations and determined that depletion of charge carrier does not account for time-dependent relaxation of Ca channel current in these preparations. Inactivation of Ca channel current persists when Ba or Sr replaces Ca as the permeant divalent cation, but the voltage dependence of the rate of inactivation is markedly changed. This effect cannot be explained by changes in external surface charge. Instead, we interpret the results as evidence that inactivation is both voltage and Ca dependent. Inactivation of Sr or Ba currents reflects a voltage-dependent process. When Ca is the divalent charge carrier, an additional effect is observed: the rate of inactivation is increased as Ca enters during depolarizing pulses, perhaps because of an additional Ca-dependent mechanism.

scholarly journals Hydrogen ion modulation of Ca channel current in cardiac ventricular cells. Evidence for multiple mechanisms.

1988 ◽  
Vol 91 (5) ◽  
pp. 641-657 ◽  
Author(s):  
D S Krafte ◽  
R S Kass
Keyword(s):  
Cell Surface ◽  
Potential Theory ◽  
Surface Potential ◽  
Current Amplitude ◽  
Hydrogen Ion ◽  
Ca Channel ◽  
Additional Mechanism ◽  
Channel Current ◽  
Ventricular Cells ◽  
Ca Channel Current

We have investigated the effects of H ions on (L-type) Ca channel current in isolated ventricular cells. We find that the current amplitude is enhanced in solutions that are alkaline relative to pH 7.4 and reduced in solutions acidic to this pH. We measured pH0-induced shifts in channel gating and analyzed our results in terms of surface potential theory. The shifts are well described by changes in surface potential caused by the binding of H ions to negative charges on the cell surface. The theory predicts a pK of 5.8 for this binding. Gating shifts alone cannot explain all of our observations on modulation of current amplitude. Our results suggest that an additional mechanism contributes to modification of the current amplitude.

scholarly journals Two kinds of calcium channels in canine atrial cells. Differences in kinetics, selectivity, and pharmacology.

1985 ◽  
Vol 86 (1) ◽  
pp. 1-30 ◽  
Author(s):  
B P Bean
Keyword(s):  
Single Channel ◽  
Cardiac Cells ◽  
Fluctuation Analysis ◽  
Ca Channel ◽  
Ca Channels ◽  
Ca Current ◽  
Channel Current ◽  
Atrial Cells ◽  
Cell Recording ◽  
Ca Channel Current

Currents through Ca channels were recorded in single canine atrial cells using whole-cell recording with patch pipettes. Two components of Ca channel current could be distinguished. One ("Ifast") was present only if cells were held at negative potentials, was most prominent for relatively small depolarizations, and inactivated within tens of milliseconds. The other ("Islow"), corresponding to the Ca current previously reported in single cardiac cells, persisted even at relatively positive holding potentials, required stronger depolarizations for maximal current, and inactivated much more slowly. Both currents were unaffected by tetrodotoxin and both were reduced by Co. Ifast had the same size and kinetics when Ca was exchanged for Ba, while Islow was bigger and slower with Ba as the charge carrier. In isotonic BaCl2, fluctuation analysis showed that Ifast had a smaller single channel current than Islow. Islow was much more sensitive to block by nitrendipine than was Ifast; also, Islow, but not Ifast, was increased by the dihydropyridine drug BAY K8644. Isoproterenol produced large increases in Islow but had no effect on Ifast.

scholarly journals Modulation of voltage-dependent Ca channel current by arachidonic acid and other long-chain fatty acids in rabbit intestinal smooth muscle.

1992 ◽  
Vol 100 (1) ◽  
pp. 27-44 ◽  
Author(s):  
T Shimada ◽  
A P Somlyo
Keyword(s):  
Fatty Acids ◽  
Arachidonic Acid ◽  
Smooth Muscle ◽  
Ca Channel ◽  
Long Chain Fatty Acids ◽  
Channel Current ◽  
Kinase C ◽  
Voltage Dependent ◽  
Ca Channel Current ◽  
Long Chain

The effects of arachidonic acid (AA) and other long-chain fatty acids on voltage-dependent Ca channel current (ICa) were investigated, with the whole cell patch clamp method, in longitudinal smooth muscle cells of rabbit ileum. 10-30 microM AA caused a gradual depression of ICa. The inhibitory effect of AA was not prevented by indomethacin (10 microM) (an inhibitor of cyclooxygenase) or nordihydroguaiaretic acid (10 microM) (an inhibitor of lipoxygenase). 1-(5-Isoquinolinesulfonyl)-2-methylpiperazine (H7; 25-50 microM) or staurosporine (2 microM) (inhibitors of protein kinase C) did not block the AA-induced inhibition of ICa, and application of phorbol ester (a protein kinase C activator) (phorbol-12,13-dibutyrate, 0.2 microM) did not mimic the AA action. Some other cis-unsaturated fatty acids (palmitoleic, linoleic, and oleic acids) were also found to depress ICa, while a trans-unsaturated fatty acid (linolelaidic acid) and saturated fatty acids (capric, lauric, myristic, and palmitic acids) had no inhibitory effects on ICa. Myristic acid consistently increased the amplitude of ICa at negative membrane potentials. The present results suggest the possible role of AA, and perhaps other fatty acids, in the physiological and/or pathological modulation of ICa in smooth muscle.

scholarly journals The dihydropyridine-sensitive calcium channel subtype in cone photoreceptors.

1996 ◽  
Vol 107 (5) ◽  
pp. 621-630 ◽  
Author(s):  
M F Wilkinson ◽  
S Barnes
Keyword(s):  
Channel Blocker ◽  
Channel Blockers ◽  
Ca Channel ◽  
Ca Channels ◽  
Cone Photoreceptors ◽  
Channel Current ◽  
Ca Channel Blockers ◽  
Voltage Dependent ◽  
P Type ◽  
Ca Channel Current

High-voltage activated Ca channels in tiger salamander cone photoreceptors were studied with nystatin-permeabilized patch recordings in 3 mM Ca2+ and 10 mM Ba2+. The majority of Ca channel current was dihydropyridine sensitive, suggesting a preponderance of L-type Ca channels. However, voltage-dependent, incomplete block (maximum 60%) by nifedipine (0.1-100 microM) was evident in recordings of cones in tissue slice. In isolated cones, where the block was more potent, nifedipine (0.1-10 microM) or nisoldipine (0.5-5 microM) still failed to eliminate completely the Ca channel current. Nisoldipine was equally effective in blocking Ca channel current elicited in the presence of 10 mM Ba2+ (76% block) or 3 mM Ca2+ (88% block). 15% of the Ba2+ current was reversibly blocked by omega-conotoxin GVIA (1 microM). After enhancement with 1 microM Bay K 8644, omega-conotoxin GVIA blocked a greater proportion (22%) of Ba2+ current than in control. After achieving partial block of the Ba2+ current with nifedipine, concomitant application of omega-conotoxin GVIA produced no further block. The P-type Ca channel blocker, omega-agatoxin IVA (200 nM), had variable and insignificant effects. The current persisting in the presence of these blockers could be eliminated with Cd2+ (100 microM). These results indicate that photoreceptors express an L-type Ca channel having a distinguishing pharmacological profile similar to the alpha 1D Ca channel subtype. The presence of additional Ca channel subtypes, resistant to the widely used L-, N-, and P-type Ca channel blockers, cannot, however, be ruled out.

Glutamate 172, essential for modulation of L247T α7 ACh receptors by Ca2+, lines the extracellular vestibule

2002 ◽  
Vol 283 (5) ◽  
pp. C1454-C1460 ◽  
Author(s):  
Donnie Eddins ◽  
Adrian D. Sproul ◽  
Lisa K. Lyford ◽  
James T. McLaughlin ◽  
Robert L. Rosenberg
Keyword(s):  
Crystal Structure ◽  
Divalent Cation ◽  
Lymnaea Stagnalis ◽  
Binding Protein ◽  
Divalent Cations ◽  
Receptor Activity ◽  
Ion Permeation ◽  
Ach Receptors

Neuronal α7 nicotinic ACh receptors (nAChRs) are permeable to and modulated by Ca2+, Ba2+, and Sr2+. These permeant divalent cations interact with slowly desensitizing L247T α7nAChRs to increase the potency and maximal efficacy of ACh, increase the efficacy of dihydro-β-erythroidine (DHβE), and increase agonist-independent activity. Mutation of glutamate 172 (E172) to glutamine or cysteine eliminated these effects of permeant divalent cations. 2-(Trimethylammonium)ethyl methanethiosulfonate (MTSET), a cysteine-modifying reagent directed at water-accessible thiols, inhibited ACh-evoked currents of E172C/L247T α7 nAChRs by >90%, demonstrating that E172 was accessible to permeant ions. The data are consistent with a model of α7receptors, derived from the crystal structure of the ACh binding protein (AChBP) from Lymnaea stagnalis, in which E172 projects toward the lumen of the extracellular vestibule. The observations that E172 was essential for divalent cation modulation of L247T α7 nAChRs and was accessible to permeating ions suggest that this residue participates in coupling ion permeation with modulation of receptor activity.

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