Stimulatory Action of Telmisartan, an Antagonist of Angiotensin II Receptor, on Voltage-Gated Na⁺ Current: Experimental and Theoretical Studies

Investigating the Impact of Selective Modulators on the Renin-Angiotensin-Aldosterone System: Unraveling Their Off-Target Perturbations of Transmembrane Ionic Currents

The renin-angiotensin-aldosterone system (RAAS) plays a crucial role in maintaining various physiological processes in the body, including blood pressure regulation, electrolyte balance, and overall cardiovascular health. However, any compounds or drugs known to perturb the RAAS might have an additional impact on transmembrane ionic currents. In this retrospective review article, we aimed to present a selection of chemical compounds or medications that have long been recognized as interfering with the RAAS. It is noteworthy that these substances may also exhibit regulatory effects in different types of ionic currents. Apocynin, known to attenuate the angiotensin II-induced activation of epithelial Na+ channels, was shown to stimulate peak and late components of voltage-gated Na+ current (INa). Esaxerenone, an antagonist of the mineralocorticoid receptor, can exert an inhibitory effect on peak and late INa directly. Dexamethasone, a synthetic glucocorticoid, can directly enhance the open probability of large-conductance Ca2+-activated K+ channels. Sparsentan, a dual-acting antagonist of the angiotensin II receptor and endothelin type A receptors, was found to suppress the amplitude of peak and late INa effectively. However, telmisartan, a blocker of the angiotensin II receptor, was effective in stimulating the peak and late INa along with a slowing of the inactivation time course of the current. However, telmisartan's presence can also suppress the erg-mediated K+ current. Moreover, tolvaptan, recognized as an aquaretic agent that can block the vasopressin receptor, was noted to suppress the amplitude of the delayed-rectifier K+ current and the M-type K+ current directly. The above results indicate that these substances not only have an interference effect on the RAAS but also exert regulatory effects on different types of ionic currents. Therefore, to determine their mechanisms of action, it is necessary to gain a deeper understanding.

Concerted suppressive effects of carisbamate, an anti-epileptic alkyl-carbamate drug, on voltage-gated Na+ and hyperpolarization-activated cation currents

Carisbamate (CRS, RWJ-333369) is a new anti-seizure medication. It remains unclear whether and how CRS can perturb the magnitude and/or gating kinetics of membrane ionic currents, despite a few reports demonstrating its ability to suppress voltage-gated Na⁺ currents. In this study, we observed a set of whole-cell current recordings and found that CRS effectively suppressed the voltage-gated Na⁺ (I_Na) and hyperpolarization-activated cation currents (I_h) intrinsically in electrically excitable cells (GH₃ cells). The effective IC₅₀ values of CRS for the differential suppression of transient (I_Na(T)) and late I_Na (I_Na(L)) were 56.4 and 11.4 μM, respectively. However, CRS strongly decreased the strength (i.e., Δarea) of the nonlinear window component of I_Na (I_Na(W)), which was activated by a short ascending ramp voltage (V_ramp); the subsequent addition of deltamethrin (DLT, 10 μM) counteracted the ability of CRS (100 μM, continuous exposure) to suppress I_Na(W). CRS strikingly decreased the decay time constant of I_Na(T) evoked during pulse train stimulation; however, the addition of telmisartan (10 μM) effectively attenuated the CRS (30 μM, continuous exposure)-mediated decrease in the decay time constant of the current. During continued exposure to deltamethrin (10 μM), known to be a pyrethroid insecticide, the addition of CRS resulted in differential suppression of the amplitudes of I_Na(T) and I_Na(L). The amplitude of I_h activated by a 2-s membrane hyperpolarization was diminished by CRS in a concentration-dependent manner, with an IC₅₀ value of 38 μM. For I_h, CRS altered the steady-state I-V relationship and attenuated the strength of voltage-dependent hysteresis (Hys_(V)) activated by an inverted isosceles-triangular V_ramp. Moreover, the addition of oxaliplatin effectively reversed the CRS-mediated suppression of Hys_(V). The predicted docking interaction between CRS and with a model of the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel or between CRS and the hNa_V1.7 channel reflects the ability of CRS to bind to amino acid residues in HCN or hNa_V1.7 channel via hydrogen bonds and hydrophobic interactions. These findings reveal the propensity of CRS to modify I_Na(T) and I_Na(L) differentially and to effectively suppress the magnitude of I_h. I_Na and I_h are thus potential targets of the actions of CRS in terms of modulating cellular excitability.

Effects of Sesamin, the Major Furofuran Lignan of Sesame Oil, on the Amplitude and Gating of Voltage-Gated Na+ and K+ Currents

Sesamin (SSM) and sesamolin (SesA) are the two major furofuran lignans of sesame oil and they have been previously noticed to exert various biological actions. However, their modulatory actions on different types of ionic currents in electrically excitable cells remain largely unresolved. The present experiments were undertaken to explore the possible perturbations of SSM and SesA on different types of ionic currents, e.g., voltage-gated Na⁺ currents (I_Na), erg-mediated K⁺ currents (I_K(erg)), M-type K⁺ currents (I_K(M)), delayed-rectifier K⁺ currents (I_K(DR)) and hyperpolarization-activated cation currents (I_h) identified from pituitary tumor (GH₃) cells. The exposure to SSM or SesA depressed the transient and late components of I_Na with different potencies. The IC₅₀ value of SSM needed to lessen the peak or sustained I_Na was calculated to be 7.2 or 0.6 μM, while that of SesA was 9.8 or 2.5 μM, respectively. The dissociation constant of SSM-perturbed inhibition on I_Na, based on the first-order reaction scheme, was measured to be 0.93 μM, a value very similar to the IC₅₀ for its depressant action on sustained I_Na. The addition of SSM was also effective at suppressing the amplitude of resurgent I_Na. The addition of SSM could concentration-dependently inhibit the I_K(M) amplitude with an IC₅₀ value of 4.8 μM. SSM at a concentration of 30 μM could suppress the amplitude of I_K(erg), while at 10 μM, it mildly decreased the I_K(DR) amplitude. However, the addition of neither SSM (10 μM) nor SesA (10 μM) altered the amplitude or kinetics of I_h in response to long-lasting hyperpolarization. Additionally, in this study, a modified Markovian model designed for SCN8A-encoded (or Na_V1.6) channels was implemented to evaluate the plausible modifications of SSM on the gating kinetics of Na_V channels. The model demonstrated herein was well suited to predict that the SSM-mediated decrease in peak I_Na, followed by increased current inactivation, which could largely account for its favorable decrease in the probability of the open-blocked over open state of Na_V channels. Collectively, our study provides evidence that highlights the notion that SSM or SesA could block multiple ion currents, such as I_Na and I_K(M), and suggests that these actions are potentially important and may participate in the functional activities of various electrically excitable cells in vivo.