Regulation of smooth muscle delayed rectifier K+ channels by protein kinase A

We identified voltage-activated K⁺ channels in freshly dispersed smooth muscle cells from the circular layer of the canine colon in patch-clamp experiments using 200 nM charybdotoxin to suppress 270-pS Ca²⁺-activated K⁺ channels (BK channels). Three channel types were distinguished in symmetrical 140 mM KCl solutions: 19.5 ± 1.7 pS channels (K_DR1), 90.6 ± 5.4 pS channels (K_DR2) and 149 ± 4 pS intermediate-conductance Ca²⁺-activated K⁺ channels (IK channels). All three types showed an increase in open probability with membrane depolarization. Ensemble average current from K_DR1 channels inactivated with a time constant of 1.7 ± 0.1 s at +60 mV test potential, while K_DR2 and IK channels did not show inactivation. IK channels were activated by free cytoplasmic Ca²⁺] (10⁻⁶M) but were insensitive to 4-aminopyridine (4-AP, 10 mM) and intracellular tetraethylammonium (TEA, 1 mM). K_DR1 channels were sensitive to 4-AP (10 mM) and intracellular TEA (1–10 mM) but not to Ca²⁺. K_DR2 channels did not have a consistent pharmacological profile, suggesting that this class may be comprised of several subtypes. At +40 mV membrane potential, the catalytic subunit of protein kinase A (PKA) increased the open probability of K_DR1 channels 3.4-fold and of K_DR2 channels 3.9-fold, but had no effect on IK channels. In the absence of Mg-ATP, PKA did not affect channel open probabilities. At physiological membrane potentials (−60 mV) only openings of K_DR1 channels could be induced by PKA, suggesting that these 4-AP-sensitive 20-pS K⁺ channels are primarily responsible for the cAMP-mediated hyperpolarization of colonic smooth muscle cells. Received: 20 June 1995/Received after revision: 25 January 1996/Accepted: 7 February 1996