No abstract

1 Two sodium channel toxins, BgII and BgIII, have been isolated and puri®ed from the sea anemone Bunodosoma granulifera. Combining dierent techniques, we have investigated the electrophysiological properties of these toxins. 2 We examined the eect of BgII and BgIII on rat ventricular strips. These toxins prolong action potentials with EC 50 values of 60 and 660 nM and modify the resting potentials. 3 The eect on Na + currents in rat cardiomyocytes was studied using the patch-clamp technique. BgII and BgIII slow the rapid inactivation process and increase the current density with EC 50 values of 58 and 78 nM, respectively. 4 On the cloned hH1 cardiac Na + channel expressed in Xenopus laevis oocytes, BgII and BgIII slow the inactivation process of Na + currents (respective EC 50 values of 0.38 and 7.8 mM), shift the steady-state activation and inactivation parameters to more positive potentials and the reversal potential to more negative potentials. 5 The amino acid sequences of these toxins are almost identical except for an asparagine at position 16 in BgII which is replaced by an aspartic acid in BgIII. In all experiments, BgII was more potent than BgIII suggesting that this conservative residue is important for the toxicity of sea anemone toxins. 6 We conclude that BgII and BgIII, generally known as neurotoxins, are also cardiotoxic and combine the classical eects of sea anemone Na + channels toxins (slowing of inactivation kinetics, shift of steady-state activation and inactivation parameters) with a striking decrease on the ionic selectivity of Na + channels.

Results from this study are similar to data described in other countries. HLA-B27 allele distribution in controls is lower than other reports in Caucasian populations.

Sodium ions have been reported to alter the permeation properties of L- and N-type Ca2+ channels. Here in frog atrial cardiomyocytes under whole-cell patch-clamp conditions, we have examined the effects of lowering the external Na+ concentration on the amplitude of T-type Ca2+ current, ICaT, and on the relief of its steady-state inactivation by large depolarizing prepulses, ICaT facilitation. A partial reduction in Na+ ion concentration did not significantly alter ICaT amplitude elicited at -50 mV. However, after a large depolarization, low- Na+ solutions enhanced the relief of inactivation and induced ICaT facilitation. This facilitation occurred independently of the divalent charge carrier, high intracellular Ca2+ buffering or the intracellular Na+ content. Its effects were additional to the beta-adrenergic effects mediated by a decrease of Gi/o-protein inhibitory tone. In Ca2+-free solution the very large T-type current, then carried by Na+ ions, showed only a weak relief of inactivation. In conclusion, ICaT facilitation--which, as previously reported, is modulated by the transient voltage-dependent relief of Gi-protein inhibitory tone--is further enhanced in a low-Na+ solution. In Ca2+-free solution, relief of inactivation due to re-openings dependent on the divalent charge carrier is improbable. It thus appears that for a short while after a large depolarization, external Na+ compete with Ca2+ ions on permeation-controlling sites, so as to modulate channel re-openings and thus the amplitude of voltage-facilitated ICaT independently of the control exerted by the inhibitory G-protein.

In long term treatment, thiazide diuretics such as hydrochlorothiazide (HCTZ) lower blood pressure by decreasing peripheral resistance rather than by their diuretic effect. This action has been attributed to the opening of Ca2+-activated K+ channels in vascular smooth muscle cells. However, little is known about their cardiac cellular actions. Here we investigated the possible actions of HCTZ on action potential and contraction of rat ventricular muscle strips and on the ionic currents of isolated rat ventricular cardiomyocytes. HCTZ depressed ventricular contraction with an IC30 of 1.85 microM (60% decrease at 100 microM). Action potential duration at -60 mV and maximal rate of depolarization were, however, only slightly decreased by 12% and 22%, respectively, at 100 microM. At the single cell level, HCTZ (100 microM) depressed the fast Na+ current (INa) and the L-type Ca2+ current (ICaL) by 30% and 20%, respectively. The effects on ICaL were not voltage-or frequency-dependent. In cells intracellularly perfused with 50 microM cyclic adenosine, monophosphate HCTZ reduced ICaL by 33%. The transient (Ito), the delayed rectifier and the inward rectifier potassium currents were decreased by 20% at 100 microM HCTZ. The effects on Ito were voltage-dependent. In conclusion, HCTZ at high concentrations possesses a negative inotropic action that could be in part due to its blocking action on INa and ICaL. The actions of HCTZ on multiple cardiac ionic currents could explain its weak effect on action potential duration.