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Department of Physiology, University of Nevada School of Medicine, Reno 89557-0046, USA.

1. We investigated the pharmacology and voltage-dependent activation and inactivation kinetics of the 'delayed rectifier' K+ current, IdK, in canine colonic myocytes and developed protocols which separate this current into three distinct components that differ in their kinetics and pharmacology. 2. Block of IdK by TEA or 4-aminopyridine (4-AP) alone was incomplete. Maximal concentrations of TEA or 4-AP blocked 76% (EC50 = 2.6 mM) and 51% (EC50 = 69 mM) of current, respectively. In the presence of 10 mM 4-AP, IdK could be blocked completely by TEA. 3. TEA and 4-AP had distinct effects on current activation: time constants for activation of IdK at +10 mV were 25.6 +/- 4.4 ms under control conditions, 40.3 +/- 7.6 ms in the presence of 10 mM 4-AP and 16.7 +/- 2.3 ms with 10 mM TEA in the bath solution. 4-AP block and removal of block were use dependent, but no frequency dependence or voltage dependence of steady-state block could be detected. These data are consistent with the presence of a rapidly activating 4-AP-sensitive current, IdK(f), and a more slowly activating TEA-sensitive current component, IdK(s). 4. A third component of the delayed rectifier current, IdK(n), was revealed when 10 mM TEA was included in the pipette solution. IdK(n) was rapidly activating, had a membrane potential at half-maximal inactivation (V1/2) for steady-state inactivation 13 mV negative of that for the mixed IdK, was completely insensitive to 4-AP (10 mM) and was blocked by external TEA with an EC50 of 7.7 mM. 5. These data demonstrate that the delayed rectifier current in canine colonic smooth muscle is composed of three currents, IdK(f), IdK(s) and IdK(n). All three currents are insensitive to charybdotoxin (100 nM).

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