Multiple Actions of Rotenone, an Inhibitor of Mitochondrial Respiratory Chain, on Ionic Currents and Miniature End-Plate Potential in Mouse Hippocampal (mHippoE-14) Neurons

BACKGROUND/AIMS

Rotenone (Rot) is known to suppress the activity of complex I in the mitochondrial chain reaction; however, whether this compound has effects on ion currents in neurons remains largely unexplored.

METHODS

With the aid of patch-clamp technology and simulation modeling, the effects of Rot on membrane ion currents present in mHippoE-14 cells were investigated.

RESULTS

Addition of Rot produced an inhibitory action on the peak amplitude of INa with an IC50 value of 39.3 µM; however, neither activation nor inactivation kinetics of INa was changed during cell exposure to this compound. Addition of Rot produced little or no modifications in the steady-state inactivation curve of INa. Rot increased the amplitude of Ca2+-activated Cl- current in response to membrane depolarization with an EC50 value of 35.4 µM; further addition of niflumic acid reversed Rot-mediated stimulation of this current. Moreover, when these cells were exposed to 10 µM Rot, a specific population of ATP-sensitive K+ channels with a single-channel conductance of 18.1 pS was measured, despite its inability to alter single-channel conductance. Under current clamp condition, the frequency of miniature end-plate potentials in mHippoE-14 cells was significantly raised in the presence of Rot (10 µM) with no changes in their amplitude and time course of rise and decay. In simulated model of hippocampal neurons incorporated with chemical autaptic connection, increased autaptic strength to mimic the action of Rot was noted to change the bursting pattern with emergence of subthreshold potentials.

CONCLUSIONS

The Rot effects presented herein might exert a significant action on functional activities of hippocampal neurons occurring in vivo.

Activity of Ca -activated Cl channels contributes to regulating receptor- and store-operated Ca entry in human pulmonary artery smooth muscle cells

Intracellular Ca²⁺ plays a fundamental role in regulating cell functions in pulmonary arterial smooth muscle cells (PASMCs). A rise in cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) triggers pulmonary vasoconstriction and stimulates PASMC proliferation. [Ca²⁺]_cyt is increased mainly by Ca²⁺ release from intracellular stores and Ca²⁺ influx through plasmalemmal Ca²⁺-permeable channels. Given the high concentration of intracellular Cl^- in PASMCs, Ca²⁺-activated Cl^-(Cl_Ca) channels play an important role in regulating membrane potential and cell excitability of PASMCs. In this study, we examined whether activity of Cl_Ca channels was involved in regulating [Ca²⁺]_cyt in human PASMCs via regulating receptor- (ROCE) and store- (SOCE) operated Ca²⁺ entry. The data demonstrated that an angiotensin II (100 nM)-mediated increase in [Ca²⁺]_cyt via ROCE was markedly attenuated by the Cl_Ca channel inhibitors, niflumic acid (100 μM), flufenamic acid (100 μM), and 4,4’-diisothiocyanatostilbene-2,2’-disulfonic acid (100 μM). The inhibition of Cl_Ca channels by niflumic acid and flufenamic acid significantly reduced both transient and plateau phases of SOCE that was induced by passive depletion of Ca²⁺ from the sarcoplasmic reticulum by 10 μM cyclopiazonic acid. In addition, ROCE and SOCE were abolished by SKF-96365 (50 μM) and 2-aminoethyl diphenylborinate (100 μM), and were slightly decreased in the presence of diltiazem (10 μM). The electrophysiological and immunocytochemical data indicate that Cl_Ca currents were present and TMEM16A was functionally expressed in human PASMCs. The results from this study suggest that the function of Cl_Ca channels, potentially formed by TMEM16A proteins, contributes to regulating [Ca²⁺]_cyt by affecting ROCE and SOCE in human PASMCs.

Limited role of Ca2+-activated Cl- current in early afterdepolarisations

OBJECTIVES

The proarrhythmic, early afterdepolarisations during phase two of the action potential (phase-2 EADs) are associated with secondary Ca²⁺ release from the sarcoplasmic reticulum. This makes it probable that the Ca²⁺-activated Cl^- current (I_Cl(Ca)) may contribute to phase-2 EADs. Activation of I_Cl(Ca) during phase two of the action potential will result in a repolarising current and may thus be expected to prevent excessive depolarisation of phase-2 EADs. The present study was designed to test this hypothesis.

METHODS

The contribution of I_Cl(Ca) during phase-2 EADs was studied in enzymatically isolated sheep ventricular myocytes using the patch-clamp methodology. EADs were induced at a stimulus frequency of 0.5 Hz by exposure of the myocytes to 1 μM noradrenaline.

RESULTS

The I_Cl(Ca) blocker 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS, 0.5 mM) abolished phase-1 repolarisation of the action potential in all myocytes tested. This indicates that I_Cl(Ca) is present in all myocytes. However, DIDS had no effect on phase-2 EAD characteristics.

CONCLUSION

In sheep ventricular myocytes, I_Cl(Ca) contributes to phase-1 repolarisation of the action potential, but plays a limited role in phase-2 EADs.