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.
