Role of Ca2+-activated K+ channels in catecholamine release from in vivo rat adrenal medulla

Aluminum alters excitability by inhibiting calcium, sodium, and potassium currents in bovine chromaffin cells

Aluminum (Al³⁺ ) has long been related to neurotoxicity and neurological diseases. This study aims to describe the specific actions of this metal on cellular excitability and neurotransmitter release in primary culture of bovine chromaffin cells. Using voltage-clamp and current-clamp recordings with the whole-cell configuration of the patch clamp technique, online measurement of catecholamine release, and measurements of [Ca²⁺ ]_c with Fluo-4-AM, we have observed that Al³⁺ reduced intracellular calcium concentrations around 25% and decreased catecholamine secretion in a dose-dependent manner, with an IC₅₀ of 89.1 μM. Al³⁺ blocked calcium currents in a time- and concentration-dependent manner with an IC₅₀ of 560 μM. This blockade was irreversible since it did not recover after washout. Moreover, Al³⁺ produced a bigger blockade on N-, P-, and Q-type calcium channels subtypes (69.5%) than on L-type channels subtypes (50.5%). Sodium currents were also inhibited by Al³⁺ in a time- and concentration-dependent manner, 24.3% blockade at the closest concentration to the IC₅₀ (399 μM). This inhibition was reversible. Voltage-dependent potassium currents were low affected by Al³⁺ . Nonetheless, calcium/voltage-dependent potassium currents were inhibited in a concentration-dependent manner, with an IC₅₀ of 447 μM. This inhibition was related to the depression of calcium influx through voltage-dependent calcium channels subtypes coupled to BK channels. In summary, the blockade of these ionic conductance altered cellular excitability that reduced the action potentials firing and so, the neurotransmitter release and the synaptic transmission. These findings prove that aluminum has neurotoxic properties because it alters neuronal excitability by inhibiting the sodium currents responsible for the generation and propagation of impulse nerve, the potassium current responsible for the termination of action potentials, and the calcium current responsible for the neurotransmitters release.

Pro-arrhythmogenic effects of Trypanosoma cruzi conditioned medium proteins in a model of bovine chromaffin cells

Asymptomatic sudden death is the principal cause of mortality in Chagas disease. There is little information about molecular mechanisms involved in the pathophysiology of malignant arrhythmias in Chagasic patients. Previous studies have involved Trypanosoma cruzi secretion proteins in the genesis of arrhythmias ex vivo, but the molecular mechanisms involved are still unresolved. Thus, the aim was to determine the effect of these secreted proteins on the cellular excitability throughout to test its effects on catecholamine secretion, sodium-, calcium-, and potassium-conductance and action potential (AP) firing. Conditioned medium was obtained from the co-culture of T. cruzi and Vero cells (African green monkey kidney cells) and ultra-filtered for concentrating immunogenic high molecular weight parasite proteins. Chromaffin cells were assessed with the parasite and Vero cells control medium. Parasite-secreted proteins induce catecholamine secretion in a dose-dependent manner. Additionally, T. cruzi conditioned medium induced depression of both calcium conductance and calcium and voltage-dependent potassium current. Interestingly, this fact was related to the abolishment of the hyperpolarization phase of the AP produced by the parasite medium. Taken together, these results suggest that T. cruzi proteins may be involved in the genesis of pro-arrhythmic conditions that could influence the appearance of malignant arrhythmias in Chagasic patients.

The residue I257 at S4-S5 linker in KCNQ1 determines KCNQ1/KCNE1 channel sensitivity to 1-alkanols

AIM

KCNQ1 and KCNE1 form a complex in human ventricular cardiomyocytes, which are important in maintaining a normal heart rhythm. In the present study we investigated the effects of a homologous series of 1-alkanols on KCNQ1/KCNE1 channels expressed in Xenopus oocytes.

METHODS

ECG recording was made in rats injected with ethanol-containing solution (0.3 mL, ip). Human KCNQ1 channel and its auxiliary subunit KCNE1 were heterologously coexpressed in Xenopus oocytes, which were superfused with ND96 solution; 1-alkanols (ethanol, 1-butanol and 1-hexanol) were delivered through a gravity-driven perfusion device. The slow-delayed rectifier potassium currents IKs (KCNQ1/KCNE1 currents) were recorded using a two-electrode voltage clamp method. Site-directed mutations (I257A) were made in KCNQ1.

RESULTS

In ECG recordings, a low concentration of ethanol (3%, v/v) slightly increased the heart rate of rats, whereas the higher concentrations of ethanol (10%, 50%, v/v) markedly reduced it. In oocytes coexpressing KCNQ1/KCNE1 channels, ethanol, 1-butanol and 1-hexanol dose-dependently inhibited IKs currents with IC50 values of 80, 11 and 2.7 mmol/L, respectively. Furthermore, the 1-alkanols blocked the KCNQ1 channel in both open and closed states, and a four-state model could adequately explain the effects of 1-alkanols on the closed-state channel block. Moreover, the mutation of I257A at the intracellular loop between S4 and S5 in KCNQ1 greatly decreased the sensitivity to 1-alkanols; and the IC50 values of ethanol, 1-butanol and 1-hexanol were increased to 634, 414 and 7.4 mmol/L, respectively. The mutation also caused the ablation of closed-state channel block.

CONCLUSION

These findings provide new insight into the intricate mechanisms of the blocking effects of ethanol on the KCNQ1 channel.

Myocardial interstitial serotonin and its major metabolite, 5-hydroxyindole acetic acid levels determined by microdialysis technique in rat heart

AIMS

The aim of this study was to elucidate myocardial interstitial serotonin (5-HT) kinetics in the heart, including 5-HT reuptake and enzymatic degradation to 5-hydroxyindole acetic acid (5-HIAA) via monoamine oxidase (MAO).

MAIN METHODS

Using microdialysis technique in anesthetized rats, we simultaneously monitored myocardial interstitial levels of 5-HT and its major metabolite, 5-HIAA, in the left ventricle and examined the effects of local administration of a MAO inhibitor, pargyline, or a 5-HT uptake inhibitor, fluoxetine.

KEY FINDINGS

Pargyline increased dialysate 5-HT concentration from 1.8±0.3 at baseline to 3.9±0.5nM but decreased dialysate 5-HIAA concentration from 20.7±1.0 at baseline to 15.8±1.4nM at 60-80min of administration. Fluoxetine increased dialysate 5-HT concentration from 1.9±0.4 at baseline to 6.5±0.9nM at 60-80min of administration, but did not change dialysate 5-HIAA concentration. Local administration of ADP (100mM) increased dialysate 5-HT and 5-HIAA concentrations. Pargyline did not affect ADP-induced increase in dialysate 5-HT concentration but suppressed ADP-induced increase in dialysate 5-HIAA concentration during 60min of ADP administration. Fluoxetine increased dialysate 5-HT concentration at 40-60min of ADP administration, but did not affect ADP-induced increase in dialysate 5-HIAA concentration.

SIGNIFICANCE

Simultaneous monitoring of myocardial interstitial 5-HT and 5-HIAA levels provides valuable information on 5-HT kinetics including reuptake and enzymatic degradation by MAO, which play a role in the regulation of myocardial interstitial 5-HT levels at baseline and when 5-HT levels are elevated.

Increased catecholamine secretion from single adrenal chromaffin cells in DOCA-salt hypertension is associated with potassium channel dysfunction

The mechanism of catecholamine release from single adrenal chromaffin cells isolated from normotensive and DOCA-salt hypertensive rats was investigated. These cells were used as a model for sympathetic nerves to better understand how exocytotic release of catecholamines is altered in this model of hypertension. Catecholamine secretion was evoked by local application of acetylcholine (1 mM) or high K+ (70 mM), and continuous amperometry was used to monitor catecholamine secretion as an oxidative current. The total number of catecholamine molecules secreted from a vesicle, the total number of vesicles fusing and secreting, and the duration of secretion in response to a stimulus were all significantly greater for chromaffin cells from hypertensive rats as compared to normotensive controls. The greater catecholamine secretion from DOCA-salt cells results, at least in part, from functionally impaired large conductance, Ca2+-activated (BK) and ATP-sensitive K+ channels. This work reveals that there is altered vesicular release of catecholamines from these cells (and possibly from perivascular sympathetic nerves) and this may contribute to increased vasomotor tone in DOCA-salt hypertension.

Methylmercury decreases cellular excitability by a direct blockade of sodium and calcium channels in bovine chromaffin cells: an integrative study

Methylmercury, a potent environmental pollutant responsible for fatal food poisoning, blocked calcium channels of bovine chromaffin cells in a time- and concentration-dependent manner with an IC50 of 0.93 μM. This blockade was not reversed upon wash-out and was greater at more depolarising holding potentials (i.e. 21 % at -110 mV and 60 % at -50 mV, after 3 min perfusion with methylmercury). In ω-toxins-sensitive calcium channels, methylmercury caused a higher blockade of I Ba than in ω-toxins-resistant ones, in which a lower blockade was detected. The sodium current was also blocked by acute application of methylmercury in a time- and concentration-dependent manner with an IC50 of 1.05 μM. The blockade was not reversed upon wash-out of the drug. The drug inhibited sodium current at all test potentials and shows a shift of the I-V curve to the left of about 10 mV. Intracellular dialysis with methylmercury caused no blockade of calcium or sodium channels. Voltage-dependent potassium current was not affected by methylmercury. Calcium- and voltage-dependent potassium current was also drastically depressed. This blockade was related to the prevention of Ca(2+) influx through voltage-dependent calcium channels coupled to BK channels. Under current-clamp conditions, the blockade of ionic current present during the generation and termination of action potentials led to a drastic alteration of cellular excitability. The application of methylmercury greatly reduced the shape and the number of electrically evoked action potentials. Taken together, these results point out that the neurotoxic action evoked by methylmercury may be associated to alteration of cellular excitability by blocking ionic currents responsible for the generation and termination of action potentials.

H2S induces catecholamine secretion in rat adrenal chromaffin cells

Hydrogen sulfide (H(2)S) is recognized as an important gaseous signaling molecule in mammalian tissues and exerts its modulating functions of different systems via targeting different ion channels and receptors. H(2)S can be synthesized from l-cysteine by cystathionine β-synthetase (CBS) or cystathionine γ-lyase (CSE). It has been reported recently that H(2)S can be synthesized and released in rat adrenal medulla chromaffin cells (AMCs) which play a critical role in the regulation of stress response by releasing catecholamine (CA). In the present study, we combined amperometry and whole-cell patch-clamp recording to explore the direct effect of exogenous H(2)S on CA release in AMCs and the underlying ionic mechanism. Amperometry showed that local application of NaHS, the H(2)S donor, evoked CA release from AMCs. Furthermore, the CA secretory response to NaHS was totally blocked by removing extracellular Ca(2+). Whole-cell patch-clamp experiments showed that H(2)S-induced CA release is produced by membrane depolarization generated by an inhibition of Ca(2+)-activated K(+) current [I(K(Ca)) current]. We conclude that H(2)S is capable of directly inducing CA release by inhibiting the I(K(Ca)) current. This conclusion indicates that H(2)S may involve in the response of adrenal medulla to stress by modulating I(K(Ca)) current and CA release in mammalian animals.