Unraveling of a Novel Cation Current in Cardiac Myocytes using Fenamates

Abstract

Interest in non-selective channels has increased recently following the discovery of transient receptor potential (TRP) proteins, which underlie many of these channels. We used the whole-cell patch-clamp technique on isolated ventricular myocytes to investigate the effect of fenamates on membrane ion currents. With voltage-dependent and other ion channels inhibited, all cells that were challenged with either N-(p-amylcinnamoyl)anthranilic acid (ACA, ≥ 3μM), ONO-RS-082 (≥ 100μM) or flufenamic acid (≥ 100μM) responded with an increase in currents (induced current: −0.8±0.06 pA/pF at −120 mV with 30 μM ACA; n=9). ACA was the most potent (K0.5 =13 μM) of all drugs tested. The induced current reversed at +43±2.2 mV (n=9) and its inward but not outward component was suppressed in Na+-free extracellular conditions (Na+ replaced by NMDG+). The current and its reversal potential (Erev) were unaffected by lowering extracellular Cl- concentration or by the removal of extracellular Ca2+ and Mg2+. The current could not be induced by other non-fenamate anti-inflammatory drugs such as diclofenac, nor by non-fenamate phospholipase-A2 inhibitors such as bromoenol lactone and bromophenacyl bromide. Muscarinic or α-adrenergic receptor activation or application of diacylglycerol failed to induce or enhance the current. The lack of effect of removing extracellular divalent cations and the fact that the induced current could be obtained in the presence of high intracellular Mg2+ indicated that the channel implicated is not TRPM7. Given our experimental conditions, where Na+ is the only ion with an equilibrium potential close to the above Erev values, it is very likely that Na+ carries the novel current induced by fenamates.