A macro cell-attached patch-clamp study of the properties of the Na current in the vicinity of the motor endplate region of frog single interosseal skeletal muscle fibres

Mechanisms of the sensory effects of tacrolimus on the skin

BACKGROUND

Tacrolimus is an immunosuppressant drug currently used for the treatment of atopic dermatitis and pruritus. This topical therapy is effective and safe, but transient burning, stinging and itch are frequently reported.

OBJECTIVES

To understand the mechanisms underlying these burning sensations.

METHODS

We examined the impact of tacrolimus on substance P (SP) release in an in vitro model of cutaneous neurogenic inflammation. Because phosphorylation of TRPV1 (transient receptor potential subtype vanilloid 1) plays a role in the induction of pain, we investigated whether tacrolimus regulates the phosphorylation state of TRPV1. Finally, we used a macropatch to evaluate the impact of tacrolimus on voltage-gated calcium currents of sensory neurons.

RESULTS

Tacrolimus was able to induce initial SP release by extracellular calcium influx and inhibited SP release induced by capsaicin after 1, 24 and 72 h of pretreatment. Analysis of TRPV1 phosphorylation by Western blot confirmed the capacity of tacrolimus to favour phosphorylation. An electrophysiological study showed inhibitory effects on calcium currents.

CONCLUSIONS

The efficacy of tacrolimus in pruritus, as well as the sensory side-effects, could be explained by a direct effect on neurons through an effect on calcineurin, possibly by a desensitization of TRPV1 and calcium currents through the PIP(2) regulation pathway.

Characterization of the voltage-activated currents in cultured atrial myocytes isolated from the heart of the common oysterCrassostrea gigas

Using the macro-patch clamp technique, we show that cardiac myocytes isolated from the heart of the oyster Crassostrea gigas possess several types of voltage-activated ionic currents. (1) A classical non-inactivating potassium current of the IK type that is inhibited by tetraethyl ammonium and shows an outward rectification and a slow activation. (2) A potassium current of the IA type that shows rapid activation and inactivation, and is blocked by 4-amino pyridine or preliminary depolarisation. (3) A potassium calcium-dependent current that is inhibited by charybdotoxin, activated by strong depolarisations and shows a large conductance. (4) A calcium inward current of the L-type that is inhibited by verapamil, cobalt and high concentrations of cadmium. This current is identified in most cells, but a T-type calcium current and classical fast sodium current are only identified in few cells, and only after a strong hyperpolarizing pulse. This suggests that these channels are normally inactivated in cultured cells and are not involved in the spontaneous activity of these cells. When they exist, the fast sodium channel is blocked by tetrodotoxin. The L-type calcium conductance is increased by serotonin. The identification in cultured oyster atrial cells of classical ionic currents,which are observed in most vertebrate species but only in a few species of molluscs, demonstrates that these cells are an interesting model. Moreover the viability and the electrophysiological properties of these cells are not significantly modified by freezing and thawing, thus increasing their usefulness in various bioassays.

Differential effects of alkaloids on sodium currents of isolated single skeletal muscle fibers

The effects of the alkaloids ajmaline, lupanine, sparteine, serpentine, strychnine, and yohimbine were studied with the loose patch clamp technique on sodium currents of isolated single skeletal muscle fibers. The IC50 values for half-maximal blocking of the sodium currents were 6.6 microM for ajmaline, 55.7 microM for quinidine, 168.8 microM for sparteine, and 1.2 mM for lupanine. The observed Na+ channel inhibition is in accordance with the use of ajmaline, quinidine and sparteine as antiarrhythmic drugs. The interference of alkaloids with Na+ channels can also be interpreted as a means to strongly interfere with neuronal transmission in herbivores. Alkaloids thus serve as chemical defense compounds for the plants producing them.

Poneratoxin, a new toxin from an ant venom, reveals an interconversion between two gating modes of the Na channels in frog skeletal muscle fibres

The effects of synthetic poneratoxin (PoTX), a new toxin isolated from the venom of the ant Paraponera clavata, were studied under current- and voltage-clamp conditions in frog skeletal muscle fibres. PoTX induces a concentration-dependent (10(-9) M-5 x 10(-6) M) prolongation of the action potentials and, at saturating concentration, a slow repetitive activity developing at negative potentials. PoTX specifically acts on voltage-dependent Na channels by decreasing the peak Na current (INa) and by simultaneously inducing a slow INa which starts to activate at -85 mV and inactivates very slowly. Both the fast and the slow components of INa are suppressed by tetrodotoxin and reverse at the same potential corresponding to the equilibrium potential for Na ions. The fast component of INa has voltage dependence, activation and steady-state inactivation almost similar to those of the control INa. The voltage dependence of the slow Na conductance is 40 mV more negative than that of the fast one. The results suggest that PoTX affects all the Na channels and that the fast and the slow INa components originate from a possible PoTX-induced interconversion between a fast and a slow operating mode of the Na channels.