Figure 6. The voltage dependence of I_Cat activated by histamine and methacholine is equivalent.

A, traces show activation of I_Cl(Ca) (partially shown) and I_Cat by histamine (left) and methacholine (right). Voltage pulses of 1 s duration from −60 mV to potentials from −120 to 80 mV were applied before and during activation of I_Cat. Note that I_Cl(Ca) had decayed completely before test potentials were imposed. Both cells were recorded using the perforated patch clamp method. B, difference currents were obtained from the voltage clamp protocol shown (top trace) by subtraction of the current before agonist application from the currents obtained during I_Cat activation. Note that current relaxations of either the histamine- or methacholine-induced I_Cat were not observed at hyperpolarizing potentials. C, the I–V relationship for the histamine- and methacholine-activated cation currents shown in above. The currents were taken from the end of the voltage step and are not normalized. Note the slight outward rectification of both currents.