scholarly journals Actions of chiriquitoxin on frog skeletal muscle fibers and implications for the tetrodotoxin/saxitoxin receptor.

1992 ◽  
Vol 100 (4) ◽  
pp. 609-622 ◽  
Author(s):  
L Yang ◽  
C Y Kao
Keyword(s):  
Skeletal Muscle ◽  
Sodium Channel ◽  
Binding Site ◽  
Muscle Fiber ◽  
Salt Bridge ◽  
Costa Rican ◽  
Frog Skeletal Muscle ◽  
Glycine Residue ◽  
Chemical States ◽  
Available Information

Chiriquitoxin (CqTX) from the Costa Rican frog Atelopus chiriquensis differs from tetrodoxin (TTX) only in that a glycine residue replaces a methylene hydrogen of the C-11 hydroxymethyl function. On the voltage-clamped frog skeletal muscle fiber, in addition to blocking the sodium channel and unrelated to such an action, CqTX also slows the activation of the fast potassium current in approximately 40% of the muscle fiber population. At pH 7.25, CqTX is as potent as TTX in blocking the sodium channel, with an ED50 of 3.8 nM. Its ED50's at pH 6.50 and 8.25 are 6.8 and 2.3 nM, contrasted with 3.8 and 4.3 nM for TTX. These differences are attributable to changes in the chemical states in the glycine residue. The equipotency of CqTX with TTX at pH 7.25 is explainable by an intramolecular salt bridge between the amino and carboxyl groups of the glycine function, all other surface groups in TTX and CqTX being the same. From available information on these groups and those in saxitoxin (STX), the TTX/STX binding site is deduced to be in a pocket 9.5 A wide, 6 A high, and 5 A deep. The glycine residue of CqTX probably projects out of the entrance to this pocket. Such a view of the binding site could also account for the actions of STX analogues, including the C-11 sulfated gonyautoxins and the 21-sulfocarbamoyl analogues. In the gonyautoxins the sulfate groups are equivalently placed as the glycine in CqTX, whereas in the sulfocarbamoyl toxins the sulfate groups extend the carbamoyl side-chain, leading to steric hinderance to productive binding.

Inaction of saxitoxin-oximes on the sodium channel of frog skeletal muscle fibers

Toxicon ◽  
1987 ◽  
Vol 25 (2) ◽  
pp. 159-165 ◽  
Author(s):  
S.L. Hu ◽  
C.Y. Kao ◽  
F.E. Koehn ◽  
H.K. Schnoes
Keyword(s):  
Skeletal Muscle ◽  
Sodium Channel ◽  
Muscle Fibers ◽  
Frog Skeletal Muscle ◽  
Skeletal Muscle Fibers

scholarly journals Interactions of neosaxitoxin with the sodium channel of the frog skeletal muscle fiber.

1991 ◽  
Vol 97 (3) ◽  
pp. 561-578 ◽  
Author(s):  
S L Hu ◽  
C Y Kao
Keyword(s):  
Skeletal Muscle ◽  
Sodium Channel ◽  
Relative Abundance ◽  
Sodium Current ◽  
Ph Dependence ◽  
Hydroxyl Group ◽  
Dimensional Structure ◽  
Hydroxyl Groups ◽  
Frog Skeletal Muscle ◽  
Relative Potencies

Neosaxitoxin (neoSTX) differs structurally from saxitoxin (STX) in that the hydrogen on N-1 is replaced by a hydroxyl group. On single frog skeletal muscle fibers in the vaseline-gap voltage clamp, the concentrations for reducing the maximum sodium current by 50% (ED50) at pH's 6.50, 7.25, and 8.25 are, respectively, 4.9, 5.1, and 8.9 nM for STX and 1.6, 2.7, and 17.2 nM for neoSTX. The relative potencies of STX at the different pH's closely parallel the relative abundance of the protonated form of the 7,8,9 guanidinium function, but the relative potencies of neoSTX at the same pH's vary with the relative abundance of the deprotonated N-1 group. In constant-ratio mixtures of the two toxins, the observed ED50's are consistent with the notion that the two toxins compete for the same site. At pH's 6.50 and 7.25, the best agreement between observed and computed values is obtained when the efficacy term (epsilon) for either toxin is 1. At pH 8.25 the best agreement is obtained if the efficacy is 1 for STX but 0.75 for neo-STX. The marked pH dependence of the actions of neoSTX probably reflects the presence of a site in the receptor that interacts with the N-1 -OH, in addition to those interacting with the 7,8,9 guanidinium and the C-12 hydroxyl groups. Considering the three-dimensional structure of the STX and neoSTX molecules, the various site points are probably located in a fold or a crevice of the channel protein, where the extracellular orifice of the sodium channel is located.

scholarly journals Sodium channel gating currents in frog skeletal muscle.

1983 ◽  
Vol 82 (5) ◽  
pp. 679-701 ◽  
Author(s):  
D T Campbell
Keyword(s):  
Skeletal Muscle ◽  
Sodium Channel ◽  
Time Course ◽  
Channel Gating ◽  
Total Charge ◽  
Charge Movement ◽  
Frog Skeletal Muscle ◽  
Gating Currents ◽  
Gating Mechanism ◽  
Charge Movements

Charge movements similar to those attributed to the sodium channel gating mechanism in nerve have been measured in frog skeletal muscle using the vaseline-gap voltage-clamp technique. The time course of gating currents elicited by moderate to strong depolarizations could be well fitted by the sum of two exponentials. The gating charge exhibits immobilization: at a holding potential of -90 mV the proportion of charge that returns after a depolarizing prepulse (OFF charge) decreases with the duration of the prepulse with a time course similar to inactivation of sodium currents measured in the same fiber at the same potential. OFF charge movements elicited by a return to more negative holding potentials of -120 or -150 mV show distinct fast and slow phases. At these holding potentials the total charge moved during both phases of the gating current is equal to the ON charge moved during the preceding prepulse. It is suggested that the slow component of OFF charge movement represents the slower return of charge "immobilized" during the prepulse. A slow mechanism of charge immobilization is also evident: the maximum charge moved for a strong depolarization is approximately doubled by changing the holding potential from -90 to -150 mV. Although they are larger in magnitude for a -150-mV holding potential, the gating currents elicited by steps to a given potential have similar kinetics whether the holding potential is -90 or -150 mV.

scholarly journals Modified kinetics and selectivity of sodium channels in frog skeletal muscle fibers treated with aconitine.

1982 ◽  
Vol 80 (5) ◽  
pp. 713-731 ◽  
Author(s):  
D T Campbell
Keyword(s):  
Skeletal Muscle ◽  
Sodium Channel ◽  
Sodium Channels ◽  
State Level ◽  
Steady State Level ◽  
Channel Block ◽  
Frog Skeletal Muscle ◽  
Channel Kinetics ◽  
Ionic Selectivity ◽  
Channel Selectivity

The effect of the plant alkaloid aconitine on sodium channel kinetics, ionic selectivity, and blockage by protons and tetrodotoxin (TTX) has been studied in frog skeletal muscle. Treatment with 0.25 or 0.3 mM aconitine alters sodium channels so that the threshold of activation is shifted 40-50 mV in the hyperpolarized direction. In contrast to previous results in frog nerve, inactivation is complete for depolarizations beyond about -60 mV. After aconitine treatment, the steady state level of inactivation is shifted approximately 20 mV in the hyperpolarizing direction. Concomitant with changes in channel kinetics, the relative permeability of the sodium channel to NH4,K, and Cs is increased. This altered selectivity is not accompanied by altered block by protons or TTX. The results suggest that sites other than those involved in channel block by protons and TTX are important in determining sodium channel selectivity.

Nalorphine: actions at an opiate receptor on frog skeletal muscle fibers

1983 ◽  
Vol 61 (11) ◽  
pp. 1361-1367
Author(s):  
Thomas E. Ary ◽  
George B. Frank
Keyword(s):  
Skeletal Muscle ◽  
Electrical Stimulation ◽  
Muscle Fiber ◽  
Partial Agonist ◽  
Opiate Receptor ◽  
Skeletal Muscle Fiber ◽  
Frog Skeletal Muscle ◽  
Full Agonist ◽  
Opiate Antagonists ◽  
Sodium And Potassium

Intracellular microelectrode studies were conducted to investigate the actions of the partial agonist – antagonist nalorphine at an opiate receptor on functional frog skeletal muscle fiber membranes. In high bath concentrations (≥ 10−4 M), nalorphine alone produces agonist actions similar to the "full" opiate agonists. These actions were (i) to depress both the sodium and potassium (gNa and gK) conductance increases due to electrical stimulation by a nonspecific local anestheticlike mechanism and (ii) to depress gNa by a specific opiate receptor mediated mechanism. In a much lower bath concentration (1 × 10−8 M) nalorphine acts to antagonize the specific opiate receptor mediated depression of gNa produced by the "full" agonist meperidine. Thus in this preparation nalorphine, "the partial antagonist," has the same actions as naloxone, which is often considered to be a full antagonist. The quantitative differences observed in the effects of these two opiate antagonists are discussed.

scholarly journals Effects of low myoplasmic Mg2+ on calcium binding by parvalbumin and calcium uptake by the sarcoplasmic reticulum in frog skeletal muscle.

1992 ◽  
Vol 100 (1) ◽  
pp. 115-135 ◽  
Author(s):  
V Jacquemond ◽  
M F Schneider
Keyword(s):  
Skeletal Muscle ◽  
Sarcoplasmic Reticulum ◽  
Muscle Fiber ◽  
Calcium Binding ◽  
Calcium Uptake ◽  
Calcium Transient ◽  
Calcium Transients ◽  
Frog Skeletal Muscle ◽  
Fura 2 ◽  
Calcium Removal

The effects of low intracellular free Mg2+ on the myoplasmic calcium removal properties of skeletal muscle were studied in voltage-clamped frog skeletal muscle fibers by analyzing the changes in intracellular calcium and magnesium due to membrane depolarization under various conditions of internal free [Mg2+]. Batches of fibers were internally equilibrated with cut end solutions containing two calcium indicators, antipyrylazo III (AP III) and fura-2, and different concentrations of free Mg2+ (25 microM-1 mM) obtained by adding appropriate total amounts of ATP and magnesium to the solutions. Changes in AP III absorbance were used to monitor [Ca2+] and [Mg2+] transients, whereas fura-2 fluorescence was mostly used to monitor resting [Ca2+]. Shortly after applying an internal solution containing less than 60 microM free Mg2+ to the cut ends of depolarized fibers most of the fibers exhibited spontaneous repetitive movements, suggesting that free internal Mg2+ might affect the activity of the sarcoplasmic reticulum (SR) calcium channels at rest. The spontaneous contractions generally subsided. In polarized fibers the maximal amplitude of the calcium transient elicited by a depolarizing pulse was about the same whatever the internal [Mg2+], but its decay after the end of the pulse slower in low [Mg2+]. In low [Mg2+] (less than 0.14 mM), the mean rate constant of decay obtained from fitting a single exponential plus a constant to the decay of the calcium transients was approximately 30% of its value in the control fibers (1 mM internal [Mg2+]). A model characterizing the main calcium removal properties of a frog skeletal muscle fiber, including the SR pump and the Ca-Mg sites on parvalbumin, was fitted to the decay of the calcium transients. Results of the fits show that in low internal [Mg2+] the slowing of the decay of the calcium transient can be well predicted by both a decreased rate of SR calcium uptake and an expected decreased resting magnesium occupancy of parvalbumin leading to a reduced contribution of parvalbumin to the overall rate of calcium removal. These results are thus consistent with the known properties of parvalbumin as a Ca-Mg buffer and furthermore suggest that in an intact portion of a muscle fiber, the activity of the SR calcium pump can be affected by the level of free Mg2+.

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