Voltage dependence of intracellular and extracellular PhTx block at saturating and subsaturating cGMP concentrations. (A) Normalized current transients elicited by stepping membrane voltage from −80 to 120 mV in the absence (top) or presence (bottom) of 0.3 µM of intracellular PhTx and of 2 mM (black) or 20 µM cGMP (blue; average of 20 consecutive traces from the same patch). The current in 20 µM cGMP is sevenfold smaller than that in 2 mM. (B) Normalized current transients, elicited by stepping membrane voltage from 120 to −80 mV, in the presence of 0.3 µM of intracellular PhTx and of 2 mM (black; average of five consecutive traces from the same patch) or 20 µM cGMP (blue; average of 10 consecutive traces from the same patch). (C) Normalized current transients elicited by stepping membrane voltage from 100 to −150 mV in the presence of 0.1 µM of extracellular PhTx and of 2 mM (black) or 20 µM cGMP (blue; average of 20 consecutive traces from the same patch). (D) Normalized current transients elicited by stepping membrane voltage from −200 to −50 mV in the presence of 0.1 µM of intracellular PhTx and of 2 mM (black) or 20 µM cGMP (blue; average of 20 consecutive traces from the same patch).