Protein kinase C inhibits delayed rectifier K+ current in rabbit vascular smooth muscle cells

The effect of protein kinase C (PKC) activation on 4-aminopyridine (4-AP)-sensitive delayed rectifier current (I-dK) was Studied in isolated rabbit portal vein smooth muscle cells by use of standard whole cell voltage clamp. The effects of the phorbol ester, 4 beta-phorbol 12,13-dibutyrate (PdBu, 100 nM) and diacylglycerol analogues, 1,2-dioctanoyl-sn-glycerol (1,2-diC(8), 10 mu M) and 1,3-dioctanoyl-sn-glycerol (1,3-diC(8), 10 mu M), on macroscopic whole cell I-dK were assessed in myocytes dialyzed with 10 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) and 5 mM ATP (20-22 mu C). Activation of PKC by 1,2-diC(8) or PdBu caused a decline in I-dK that was reversed with washout of drug. 1,2-diC(8) had no effect on outward current present after exposure to 4-AP (20 mM). The inactive analogue, 1,3-diC(8), did not affect I-dK, but subsequent exposure to the active analogue, 1,2-diC(8), caused a marked depression of the current. The inhibition of I-dK by 1,2-diC(8) was significantly reduced by intracellular dialysis with the inhibitors of PKC, chelerythrine (50 mu M) and calphostin C (1 mu M) Substitution of extracellular Ca2+ with Mg2+ in the presence of 10 mM intracellular BAPTA did not affect the suppression of I-dK by 1,2-diC(8), indicating the involvement of a Ca2+-independent isoform of PKC. This study suggests a novel signal transduction mechanism for inhibition of 4-AP-sensitive I-dK involving a phosphotransferase reaction catalyzed by PKC in vascular smooth muscle myocytes.