BETA-ADRENOCEPTOR ACTIVATION AND PKA REGULATE DELAYED RECTIFIER K-MUSCLE CELLS( CHANNELS OF VASCULAR SMOOTH)

Macroscopic 4-aminopyridine (4-AP)-sensitive, delayed rectifier K+ cur rent of vascular smooth muscle cells is increased during beta-adrenoce ptor activation with isoproterenol via a signal transduction pathway i nvolving adenylyl cyclase and cAMP-dependent protein kinase (PKA) (Aie llo, E. A., M. P. Walsh, and W. C. Cole. Am. J. Physiol. 268 (Heart Ci rc. Physiol. 37): H926-H934, 1995.). In this study, we identified the single delayed rectifier K+ (K-DR) channel(s) of rabbit portal vein my ocytes affected by treatment with isoproterenol or the catalytic subun it of PKA. 4-AP-sensitive K-DR channels of 15.3 +/- 0.6 pS (n = 5) and 14.8 +/- 0.6 pS (n = 5) conductance, respectively, were observed in i nside-out (I-O) and cell-attached (C-A) membrane patches in symmetrica l KCl recording conditions. The kinetics of activation (time constant of 10.7 +/- 3.02 ms) and inactivation (fast and slow time constants of 0.3 and 2.5 s, respectively) of ensemble currents produced by these c hannels mimicked those reported for inactivating, 4-AP-sensitive whole cell K-DR current of vascular myocytes. Under control conditions, the open probability (NP0) of K-DR channels of C-A membrane patches at -4 0 mV was 0.014 +/- 0.005 (n = 8). Treatment with 1 mu M isoproterenol caused a significant, approximately threefold increase in NP0 to 0.041 +/- 0.02 (P < 0.05). K-DR channels of I-O patches exhibited rundown a fter similar to 5 min, which was not affected by ATP (5 mM) in the bat h solution. Treatment with the purified catalytic subunit of PKA (50 n M; 5 mM ATP) restored K-DR channel activity and caused NP0 to increase from 0.011 +/- 0.003 to 0.138 +/- 0.03 (P < 0.05; n = 11). These data indicate that small-conductance, 15-pS K-DR channels are responsible for inactivating the macroscopic delayed rectifier K+ current of rabbi t portal vein myocytes and that the activity of these channels is enha nced by a signal transduction mechanism involving beta-adrenoceptors a nd phosphorylation by PKA at a membrane potential consistent with that observed in the myocytes in situ.