1 The effect of the bradycardic agent S 16257 on the main ionic mechanisms of diastolic depolarization in sinoatrial node cells isolated from rabbit heart, was investigated by the patch-clamp technique in whole-cell and macro-patch recordings. 4 A high concentration of S 16257 (10 yM) had no detectable effect on T-type calcium current and slightly decreased L-type calcium current (-18.12+0.66%), without significant use-dependent blockade. 5 S 16257 had no effect on the delayed outward potassium current IK at 3 pM and slightly decreased it only at high concentrations, -16.3+ 1.2% at 10 gM. In contrast, zatebradine, another bradycardic agent, reduced IK by 20.3+2.5% at 3 gM. 6 In conclusion, S 16257 may lower heart rate without significant negative inotropic action. In comparison with zatebradine, S 16257 had less effect on IK suggesting less prolongation of repolarization time.

This investigation provides positive evidence of the asymmetry of the His-Purkinje system of the adult mouse, and the first patch-clamp recording data on murine cardiac Purkinje cells. This mouse model opens up new perspectives for investigating the contribution of specific genes to the morphology and function of the His-Purkinje system.

Background and purpose: 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. Experimental approach: 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. Key results: The CFTR channel activator benzo[c]quinolizinium MPB-104 inhibited TRPM4 current with an IC 50 in the range of 2 Â 10 À5 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. Conclusions and implications: 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.

TRPM4 is functionally expressed in SAN cells and may be a key player in the generation and/or perturbation of heart rhythm.