The conductance changes underlying agonist-evoked depolarization in human airway smooth muscle (ASM) were examined using single ASM cells liberated enzymatically from noncarcinomatous bronchi and studied using patch-clamp techniques. Step commands to potentials at or more positive than the resting membrane potential evoked outward current, which was predominantly delayed rectifier K+ current with some Ca(2+)-dependent K+ current Caffeine (5 mM) evoked depolarization and contraction lasting several minutes. During voltage clamp at -60 mV, caffeine evoked inward current with a latency of approximately equal to 1 s, mean amplitude of 320 +/- 65 pA, and a duration of approximately equal to 5 s (even though agonist application exceeded this duration). With the use of the perforated-path configuration, these responses could be evoked repeatedly at 4-min intervals for up to 30 min; rupture of the membrane and dialysis of the cytosol, however, abrogated the responses to caffeine. The current was outwardly rectifying with mean reversal potential (Vrev) of -31 +/- 4 mV. When K+ conductances were blocked by Ca+, the current-voltage (I-V) relationship was linear (i.e., an outwardly-rectifying component was eliminated) and Vrev was displaced in the positive direction to +2 +/- 1 mV. Changes in the CL- equilibrium potential were accompanied by a displacement of Vrev in a manner predicted by the Nernst equation for a Cl- current. The effects of caffeine were mimicked by acetylcholine; in addition, acetylcholine and caffeine each occluded the response to the other agonist. Spasmogens also caused a prolonged suppression of K+ currents (both Ca(2+)--dependent and delayed rectifier). We conclude that, in human ASM, acetylcholine and caffeine cause a transient activation of Ca(2+)--dependent Cl- current (due to release of internal Ca2+) and prolonged suppression of K+ currents, leading to depolarization and contraction.