Pharmacology of Slo2.1 Channel Activation and Block

Abstract

Intracellular sodium regulated Slo2.1 channels are selectively expressed in the brain and the heart. They are closed under normal physiological and ionic conditions, but markedly activated after elevation of [Na+]i, for example during myocardial ischemia. These channels are also activated by fenamates without elevation of intracellular sodium. Fenamates (N-arylated derivatives of anthranilic acid with different substituents on the aryl ring) are non-steroidal anti-inflammatory drugs that function as mixed agonists for Slo2.1 channels. We performed Quantitative Structure Activity Relationship (QSAR) studies on these drugs and established the requirement of a minimal pharmacophore defined as fenamic acid (2-(phenylamino) benzoic acid) for Slo2.1 channel activation. Meclofenamic acid was found to be the most potent activator of Slo2.1 channels expressed in Xenopus oocytes with an EC50 of 90 μM. We further report that verapamil, an open-state dependent blocker, blocks Slo2.1 (IC50 = 15 μM) and has free access to its binding site even in the non-activated state of the channel. The permanently charged, membrane impermeant analogue of verapamil, D890 was used to address sidedness of block and location of the binding site in the channel protein. These observations indicate that unlike other potassium channels, the canonical S6 bundle crossing in Slo2.1 channels is always in an open configuration and does not function as the primary activation gate.