Fig. 5.

Estimation of ion channel numbers in a sinus node cell. a Estimated number of ion channels in a sinus node cell based on mass spectrometry. Calculation of ion channel numbers is based on measured relative ion channel protein abundance taking numbers of subunits into consideration; absolute ion channel number was then calculated by normalising to the number of HCN4 channels estimated at 6255 HCN4 channels per nodal cell from the Markov chain model below. Error bars indicate mean +/− standard deviation. Data points indicate underlying single values. Mean protein intensity values are based on three biological replicates in each group, each containing pooled tissue sections from ten mice. Source data are provided as a Source Data file. b–e Markov chain model of HCN4 current. b Unitary current traces (green; 9th, 13th, 24th, 25th, 38th, 54th, 69th, 73th and 79th sweeps) and ensemble average current of 100 consecutive sweeps (blue) during a hyperpolarising pulse from a holding potential of −30 mV to −90 mV for 2000 ms (red). c Current–voltage relationship of the unitary HCN4 channel. Solid circles show the maximum current when the channel was open during ten sweeps. The slope of the line shows the unitary channel conductance to be ~1.0 pS. d Whole cell current from 6255 unitary HCN4 channels with a holding potential of −30 mV and test potentials from −140 mV to −40 mV. e Whole cell HCN4 current–voltage relationship of the whole cell HCN4 current. The maximum current was recorded at 10 s after the application of the test pulse