Chloride currents activated by caffeine in rat intestinal smooth muscle cells.

1. Current responses to caffeine in single smooth muscle cells isolated from rat intestine were studied with the whole‐cell patch clamp technique. Intracellular calcium concentration, [Ca2+]i, was simultaneously monitored with fura‐2 (0.1 mM) introduced into the cell through a patch pipette. 2. With a potassium‐containing pipette solution, caffeine (10 mM) produced an outward current at a holding potential of 0 mV and an inward current at ‐60 mV, both of which were accompanied by parallel increases in [Ca2+]i. The outward current response disappeared after the removal of K+ from pipette solutions, indicating that caffeine activates a Ca(2+)‐activated K+ conductance. 3. When NaCl was present in both pipette and external solutions as the major constituent, caffeine evoked an inward current at ‐60 mV simultaneously with a rise in [Ca2+]i. The reversal potential (Er) of this current was about 0 mV. 4. Substitution of Tris+ or choline+ for external Na+ did not alter the Er. When external Cl‐ was replaced by thiocyanate‐, iodide‐ or glutamate‐, the Er changed to respectively ‐55, ‐38 and +35 mV. 5. The current response to caffeine decreased with increasing concentration of EGTA in the pipette solution. The caffeine‐induced current and the intracellular Ca2+ transient was still observed for a few minutes after exposure of the cells to Ca(2+)‐free external solution containing 2 mM EGTA. Caffeine failed to produce an inward current and Ca2+ transient after treatment with extracellular ryanodine. 6. It is concluded that caffeine caused an increase in membrane Cl‐ conductance and in K+ conductance resulting from a rise in [Ca2+]i derived from ryanodine‐sensitive intracellular Ca2+ stores in isolated smooth muscle cells of the rat intestine.