Patch-clamp techniques were used to examine the effect of substance P on acetylcholine-induced current in bovine chromaffin cells. Cells had been enzymatically isolated and kept in short-term culture. Experiments were performed at 22 degrees C. Under whole-cell voltage-clamp conditions substance P alone (2-10 microM) did not induce ionic currents. Acetylcholine (ACh, 20 microM) at -60 mV induced an inward current that desensitized in the continued presence of ACh. The time course of desensitization was somewhat variable from cell to cell. In most cases it could be fitted by a single exponential with time constant of 8-10 s. Substance P (2-50 microM) applied simultaneously with ACh induced what appeared to be an acceleration of the desensitization process. The time course in the presence of 10 microM-substance P (20 microM-ACh) was best fitted by the sum of two exponentials with time constants of 0.6 s and 5 s respectively. The effect was reversible. The recovery of ACh-induced current from desensitization was not affected by substance P. The time constant for recovery was approximately 7 s in the presence or absence of substance P. Single-channel records showed that the conductance of individual channels was not changed by substance P. The mean open time of single channels was shortened by substance P both at high (20 microM) and at low (0.5 microM) concentrations of ACh. The inverse mean open time varied linearly with substance P concentration. Single-channel responses appeared in bursts and clusters after almost complete desensitization at 20 microM-ACh, as was previously observed in frog skeletal muscle. Substance P dramatically reduced ACh current by increasing interburst intervals while decreasing burst duration and the number of openings per burst. We conclude that substance P inhibits ACh-induced depolarization of chromaffin cells either by increasing the rate of desensitization or by inducing channel blockade, which indirectly enhances desensitization. Possible models of desensitization in the absence and presence of substance P are discussed.