Properties of SpIH channels. (A) Voltage protocol (V_m) and whole cell currents (I_m) recorded in control, untransfected (top), and SpIH-transfected HeLa cells in the absence (middle) or presence (bottom) of 50 μM cAMP. I_m were elicited by 2-s voltage pulses delivered from a holding potential of 0 mV to test potentials between −20 and −120 mV, in 20-mV increments, and returning to a tail potential of +50 mV. I_m increased in SpIH-transfected cells when cAMP was present in the pipette solution. (B) Average of tail current densities measured after voltage step to V_m = −100 mV from control cells (0.3 ± 0.05 pA/pF, n = 7), SpIH-transfected cells (5.3 ± 0.7 pA/pF, n = 13), and SpIH-transfected cells with 50 μM cAMP in the pipette (28.6 ± 3.1 pA/pF, n = 13). Data represent means ± SEM. (C) Dependence of the SpIH current on intracellular cAMP concentration. Solid line represents best fit of data to the Hill equation with E_C50 = 7.1 μM, Hill slope = 2.01. Data represent means ± SEM, obtained from a total 48 cells at V_m = −100 mV. The experiments were performed in the presence of IBMX and 2’,5′ dideoxyadenosine.