9-phenanthrol inhibits human TRPM4 but not TRPM5 cationic channels.

TRPM4 and TRPM5 are calcium-activated non-selective cation channels with almost identical characteristics. TRPM4 is detected in several tissues including heart, kidney, brainstem, cerebral artery and immune system whereas TRPM5 expression is more restricted. Determination of their roles in physiological processes requires specific pharmacological tools. TRPM4 is inhibited by glibenclamide, a modulator of ATP binding cassette proteins (ABC transporters), such as the cystic fibrosis transmembrane conductance regulator (CFTR). We took advantage of this similarity to investigate the effect of hydroxytricyclic compounds shown to modulate ABC transporters, on TRPM4 and TRPM5. Experiments were conducted using HEK-293 cells permanently transfected to express human TRPM4 or TRPM5. Currents were recorded using the whole-cell and inside-out variants of the patch-clamp technique. The CFTR channel activator benzo[c]quinolizinium MPB-104 inhibited TRPM4 current with an IC(50) in the range of 2 x 10(-5) M, with no effect on single-channel conductance. In addition, 9-phenanthrol, lacking the chemical groups necessary for CFTR activation, also reversibly inhibited TRPM4 with a similar IC(50). Channel inhibition was voltage independent. The IC(50) determined in the whole-cell and inside-out experiments were similar, suggesting a direct effect of the molecule. However, 9-phenanthrol was ineffective on TRPM5, the most closely related channel within the TRP protein family. We identify 9-phenanthrol as a TRPM4 inhibitor, without effects on TRPM5. It could be valuable in investigating the physiological functions of TRPM4, as distinct from those of TRPM5.