REGULATION OF SMOOTH-MUSCLE DELAYED RECTIFIER K-KINASE-A( CHANNELS BYPROTEIN)

We identified voltage-activated K+ channels in freshly dispersed smoot h muscle cells from the circular layer of the canine colon in patch-cl amp experiments using 200 nM charybdotoxin to suppress 270-pS Ca2+-act ivated K+ channels (BK channels). Three channel types were distinguish ed in symmetrical 140 mM KCl solutions: 19.5 +/- 1.7 pS channels (K-DR 1), 90.6 +/- 5.4 pS channels (K-DR2) and 149 +/- 4 pS intermediate-con ductance Ca2+-activated K+ channels (IK channels). All three types sho wed an increase in open probability with membrane depolarization. Ense mble average current from K-DR1 channels inactivated with a time const ant of 1.7 +/- 0.1 s at +60 mV test potential, while K-DR2 and IK chan nels did not show inactivation. IK channels were activated by free cyt oplasmic [Ca2+] (10(-6) M) but were insensitive to 4-aminopyridine (4- AP, 10 mM) and intracellular tetraethylammonium (TEA, 1 mM). K-DR1 cha nnels were sensitive to 4-AP (10 mM) and intracellular TEA (1-10 mM) b ut not to Ca2+. K-DR2 channels did not have a consistent pharmacologic al profile, suggesting that this class may be comprised of several sub types. At +40 mV membrane potential, the catalytic subunit of protein kinase A (PKA) increased the open probability of K-DR1 channels 3.4-fo ld and of K-DR2 channels 3.9-fold, but had no effect on IK channels. I n 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 hyp erpolarization of colonic smooth muscle cells.