ACETYLCHOLINE AND CAFFEINE ACTIVATE CL- AND SUPPRESS K-MUSCLE( CONDUCTANCES IN HUMAN BRONCHIAL SMOOTH)

The conductance changes underlying agonist-evoked depolarization in hu man airway smooth muscle (ASM) were examined using single ASM cells li berated enzymatically from noncarcinomatous bronchi and studied using patch-clamp techniques. Step commands to potentials at or more positiv e than the resting membrane potential evoked outward current, which wa s predominantly delayed rectifier K+ current with some Ca2+-dependent K+ current. Caffeine (5 mM) evoked depolarization and contraction last ing several minutes. During voltage clamp at -60 mV, caffeine evoked i nward current with a latency of approximate to 1 s, mean amplitude of 320 +/- 65 pA, and a duration of approximate to 5 s (even though agoni st application exceeded this duration). With the use of the perforated -patch configuration, these responses could be evoked repeatedly at 4- min intervals for up to 30 min; rupture of the membrane and dialysis o f the cytosol, however, abrogated the responses to caffeine. The curre nt was outwardly rectifying with mean reversal potential (V-rev) of -3 1 +/- 4 mV. When K+ conductances were blocked by Cs+, the current-volt age (I-V) relationship was linear (i.e., an outwardly-rectifying compo nent was eliminated) and V-rev was displaced in the positive direction to +2 +/- 1 mV. Changes in the Cl- equilibrium potential were accompa nied by a displacement of V-rev in a manner predicted by the Nernst eq uation for a Cl- current. The effects of caffeine were mimicked by ace tylcholine; in addition, acetylcholine and caffeine each occluded the response to the other agonist. Spasmogens also caused a prolonged supp ression of K+ currents (both Ca2+ dependent and delayed rectifier). We conclude that, in human ASM, acetylcholine and caffeine cause a trans ient activation of Ca2+-dependent Cl- current (due to release of inter nal Ca2+) and prolonged suppression of K+ currents, leading to depolar ization and contraction.