1. Properties of the pinacidil-sensitive K+ channel in the smooth muscle of the rabbit portal vein were investigated using cell-attached and inside- and outside-out patch clamp techniques. 2. In the cell-attached patch configuration, a K+ channel with a unitary conductance of 150 pS could be recorded when physiological salt solution (PSS) was in the pipette and high-K+ solution was in the bath. Tetraethylammonium (TEA; less than 1 mM) and charybdotoxin (CTX; greater than 50 nM) inhibited the 150 pS K+ channel from the outside of the membrane. This channel was activated by an increase in the concentrations of intracellular Ca2+ but not by pinacidil (less than or equal to 500 microM). 3. In the cell-attached patch configuration, bath application of pinacidil (greater than 3 microM) activated a K+ channel (ATP-sensitive K+ channel) with a unitary conductance of 15 pS and the enhancing action of pinacidil was blocked by glibenclamide. However, in the cell-free patch configuration, pinacidil (100 microM) failed to open the 15 pS K+ channel. With pinacidil in the pipette, the 15 pS K+ channel was completely inactivated within 5 s of the excision of the membrane. Opening of the 15 pS K+ channel also disappeared after saponin treatment (50 micrograms/ml). 4. In the cell-free patch configuration, application of guanosine 5'-diphosphate (GDP; greater than 100 microM) re-activated the inactivated 15 pS K+ channel only when pinacidil was present either in the pipette or bath. GDP increased the mean open time and open probability of the 15 pS K+ channel in a concentration-dependent manner. Simultaneous application of MgCl2 (less than or equal to 1 mM) with GDP did not modify the GDP-induced activation. Neither GDP nor GTP (1 mM) had any effect on the 150 pS K+ channel. 5. Guanosine 5'-triphosphate (GTP; 1 mM) activated the 15 pS K+ channel to a lesser extent that did GDP. Other guanine nucleotides (guanosine 5'-monophosphate, GMP, 1 mM; guanosine 5'-O-(3-thiotriphosphate), GTP gamma S, 100 microM; and guanosine 5'-O-(2-thiodiphosphate), GDP beta S, 1 mM) failed to activate the 15 pS K+ channel. However, GDP beta S, but not GMP or GTP gamma S, inhibited this channel when it was activated by 1 mM-GDP. 6. In the presence of pinacidil, adenosine 5'-triphosphate (ATP; greater than or equal to 10 microM) inhibited the ATP-sensitive K+ channel when it was activated by 1 mM-GDP.(ABSTRACT TRUNCATED AT 400 WORDS)