Regulation of resting ionic conductances in frog skeletal muscle

Role of physiological ClC-1 Cl- ion channel regulation for the excitability and function of working skeletal muscle

Electrical membrane properties of skeletal muscle fibers have been thoroughly studied over the last five to six decades. This has shown that muscle fibers from a wide range of species, including fish, amphibians, reptiles, birds, and mammals, are all characterized by high resting membrane permeability for Cl⁻ ions. Thus, in resting human muscle, ClC-1 Cl⁻ ion channels account for ∼80% of the membrane conductance, and because active Cl⁻ transport is limited in muscle fibers, the equilibrium potential for Cl⁻ lies close to the resting membrane potential. These conditions—high membrane conductance and passive distribution—enable ClC-1 to conduct membrane current that inhibits muscle excitability. This depressing effect of ClC-1 current on muscle excitability has mostly been associated with skeletal muscle hyperexcitability in myotonia congenita, which arises from loss-of-function mutations in the CLCN1 gene. However, given that ClC-1 must be drastically inhibited (∼80%) before myotonia develops, more recent studies have explored whether acute and more subtle ClC-1 regulation contributes to controlling the excitability of working muscle. Methods were developed to measure ClC-1 function with subsecond temporal resolution in action potential firing muscle fibers. These and other techniques have revealed that ClC-1 function is controlled by multiple cellular signals during muscle activity. Thus, onset of muscle activity triggers ClC-1 inhibition via protein kinase C, intracellular acidosis, and lactate ions. This inhibition is important for preserving excitability of working muscle in the face of activity-induced elevation of extracellular K⁺ and accumulating inactivation of voltage-gated sodium channels. Furthermore, during prolonged activity, a marked ClC-1 activation can develop that compromises muscle excitability. Data from ClC-1 expression systems suggest that this ClC-1 activation may arise from loss of regulation by adenosine nucleotides and/or oxidation. The present review summarizes the current knowledge of the physiological factors that control ClC-1 function in active muscle.

Protein kinase A modulates A-type potassium currents of larval zebrafish (Danio rerio) white muscle fibres

AIMS

Potassium (K(+)) channels are involved in regulating cell excitability and action potential shape. To our knowledge, very little is known about the modulation of A-type K(+) currents in skeletal muscle fibres. Therefore, we sought to determine whether K(+) currents of zebrafish white skeletal muscle were modulated by protein kinase A (PKA).

METHODS

Pharmacology and whole-cell patch clamp were used to examine A-type K(+) currents and action potentials associated with zebrafish white skeletal muscle fibres.

RESULTS

Activation of PKA by a combination of forskolin + 3-isobutyl-1-methylxanthine (Fsk + IBMX) decreased the peak current density by approximately 60% and altered the inactivation kinetics of A-type K(+) currents. The specific PKA inhibitor H-89 partially blocked the Fsk + IBMX-induced reduction in peak current density, but had no effect on the change in decay kinetics. Fsk + IBMX treatment did not shift the activation curve, but it significantly reduced the slope factor of activation. Activation of PKA by Fsk + IBMX resulted in a negative shift in the V(50) of inactivation. H-89 prevented all Fsk + IBMX-induced changes in the steady-state properties of K(+) currents. Application of Fsk + IBMX increased action potential amplitude, but had no significant effect on action potential threshold, half width or recovery rate, when fibres were depolarized with single pulses, paired pulses or with high-frequency stimuli.

CONCLUSION

PKA modulates the A-type K(+) current in zebrafish skeletal muscle and affects action potential properties. Our results provide new insights into the role of A-type K(+) channels in muscle physiology.

Regulation of the human skeletal muscle chloride channel hClC-1 by protein kinase C

1. The regulation of a recombinant human muscle chloride channel, hClC-1, by protein kinase C (PKC) was investigated in human embryonic kidney (HEK 293) cells. 2. External application of 4beta-phorbol esters (4beta-PMA) reduced the instantaneous whole-cell current amplitude over the entire voltage range tested. This effect was abolished when the cells were intracellularly perfused with a specific protein kinase C inhibitor, chelerythine. Inactive 4alpha-phorbolesters did not affect the chloride currents. We conclude that the effect of 4beta-phorbol esters is mediated by protein kinase C (PKC). 3. Activation of PKC resulted in changes in macroscopic current kinetics. The time course of current deactivation determined in the presence and absence of 4beta-phorbol esters could be fitted with the sum of two exponentials and a constant value. In the presence of phorbol esters, the fast time constants and the minimum value of the fraction of non-deactivating current were increased, whereas the voltage dependence of all fractional current amplitudes remained unchanged. PKC-induced phosphorylation had only small effects on the voltage dependence of the relative open probability and the maximum absolute open probability was unaffected by treatment with 4beta-PMA, as shown by non-stationary noise analysis. 4. The kinetic changes indicate that phosphorylation alters functional properties of active channels. Since the absolute open probability is not reduced, the observed macroscopic current reduction implies alterations of the ion permeation process. 5. Phosphorylation by PKC appears to affect ion transfer and gating processes. It is postulated that the phosphorylation site may be located at the cytoplasmic vestibule face of the pore.

Chloride current in toad skeletal muscle and its modification by the histidine-modifying reagent diethylpyrocarbonate

Cl- currents were measured in short fibres in the toad lumbricalis muscle with a two-microelectrode voltage clamp. Membrane Cl- conductance increased markedly when external pH was raised. At pH 7 or higher, the Cl- current fell during a hyperpolarizing voltage pulse and the rate of inactivation was directly proportional to the voltage change. The histidinemodifying reagent diethylpyrocarbonate (DEPC, 1 mM) which carbethoxylates histidil residues in proteins, suppressed the inactivation of Cl- currents at pH 7.5. On the other hand, no apparent changes in the kinetics of the currents at pH 5 were seen. No3- currents, which are independent of the extracellular pH and time, were not affected by DEPC. Our results support the notion that the inactivation of Cl- currents at pH 7.5 represents a membrane permeability change and that DEPC interferes with this process. Protonation of histidine groups associated with Cl- channels may be the controlling reaction for the pH -dependent Cl- response.