Figure 2.

A hyperpolarization-activated I_h current is present in zebrafish cardiac myocytes. (A) Activation of I_h current by hyperpolarization. An individual dissociated cardiomyocyte was voltage-clamped from a holding potential of −40 mV to various hyperpolarizing voltages (−70 to −130 mV in 10-mV increments) once every 15 s. Each hyperpolarization was followed by a step to −130 mV to determine the current at a constant voltage. The current amplitude immediately after the voltage step to −130 mV was then plotted as a function of the activation voltage. The data were fitted with a Boltzmann function. The amplitude of the I_h current was given by the net amplitude of the Boltzmann function. The leak current was determined by the baseline of the fit and was not part of the I_h determination. Current traces were filtered at 1 kHz. (B) Tail current analysis of the I_h reversal potential. After I_h was activated for 1 s by a step from 0 mV to −130 mV, another step was applied to a new voltage (from −50 to +30 mV in 10-mV increments). The tail current amplitude immediately after the voltage clamp settled is plotted as a function of the tail current voltage. The tail currents reversed at −14 mV, as determined by the line fitted to the data points. A perfectly potassium-selective channel would have a reversal potential of −49 mV in the solutions used. The dihydropyridine niguldipine (1 μM) was included to block the calcium currents present in these cells. If the cell was held at 0 mV and not hyperpolarized to activate the I_h current, no tail currents were observed (data not shown). Current traces were filtered at 2 kHz. The voltage protocol was delivered once every 4 s. (C) Inhibition of I_h by cesium. I_h currents with and without 2 mM CsCl applied extracellularly, in response to a step from −40 mV to −130 mV. Current traces were filtered at 1 kHz.