Fig. 4. 8-Br-cGMP markedly reduces the open probability of N channels in human neuroblastoma cells.A, Representative traces of N-channel activity recorded in a cell-attached patch containing more than one channel under control conditions (left) and during exposure to 400 μm 8-Br-cGMP (right). Nifedipine (5 μm) was present in the pipette solution to block L channels, and depolarization at +20 mV was delivered fromVh = −80 mV. B, NPo versus time before (●) and during application of 8-Br-cGMP (○). Data plotted refer to the same patch shown in A, andhorizontal bars indicate the selected traces presented in A. C, Averaged currents calculated over 10 sweeps in control and 40 sweeps with 8-Br-cGMP from the same patch shown in A and B. D, Mean changes in NPo induced by 400 μm 8-Br-cGMP in seven patches containing two or three N channels. Filled column shows NPo value obtained by averaging data collected during 1 min recording under control conditions before application of the test agent. The open columns indicate mean NPo obtained by averaging the data collected in the seven studied patches over 30 sec periods. E, NPo versus time for a representative patch recorded under control conditions (i.e., in absence of test drugs) shows the absence of significant rundown. Mean NPo is 0.37 ± 0.04 during the first minute and 0.33 ± 0.02 during the second through the sixth minutes. F, The 8-Br-cGMP-induced decrease in NPo is estimated with and without inclusion of null sweeps (59.3 and 42.9% reduction, respectively). InF–H, filled columnsindicate controls obtained by averaging data collected during 1 min recordings, and open columns show values obtained by averaging all the data collected from the second to the fifth minute of drug application. G, Marked increase in null sweep probability induced by 8-Br-cGMP with respect to control (0.29 ± 0.04 vs 0.06 ± 0.03). H, Mean values of NPo at +10, +20, and +30 mV, the percentage decrease induced by 8-Br-cGMP being 60.3, 59.3, and 52.9%, respectively.