Verapamil hydrochloride

Ammonium bicarbonate, ammonium formate, and formic acid 99% were purchased from VWR Chemicals (Leuven, Belgium). Milli-Q water from a Merck milli-Q pump. Methanol HPLC gradient grade was purchased from J.T. Baker (Deventer, Netherlands).
Standard compounds: 2,2′-bipyridine, 4-nitrophenol, ibuprofen, metoclopramide, papaverine, verapamil hydrochloride from TCI; pindolol from Abcam (Rozenburg, Netherlands).

Isoprenaline hydrochloride, atenolol and verapamil hydrochloride were purchased from Abcam (Cambridge, UK). Dobutamine hydrochloride, digoxin and flecainide acetate were purchased from Tocris Bioscience (Oxford, UK). Stock solutions (1–10 mM) of all compounds were made and were stored at −20 °C. digoxin and dobutamine were dissolved in dimethyl sulfoxide (DMSO) to make the stock solution. The final concentration of DMSO during experimentation was less than 0.01%. Verapamil, atenolol, flecainide and isoprenaline were all dissolved in ultra-pure water to make the stock solution. All other reagents were purchased from either Sigma Aldrich (Dorset, UK) or Fisher Scientific (Loughborough, UK) unless stated otherwise.