Phorbol Ester-Induced Inhibition of Potassium Currents in Rat Sensory Neurons Requires Voltage-Dependent Entry of Calcium

The whole cell patch-clamp technique was used to examine the effects of protein kinase C (PKC) activation (via the phorbol ester, phorbol 12,13 dibutyrate, PDBu) on the modulation of potassium currents ( I K) in cultured capsaicin-sensitive neurons isolated from dorsal root ganglia from embryonic rat pups and grown in culture. PDBu, in a concentration- and time-dependent manner, reduced I K measured at +60 mV by ∼30% if the holding potential ( V h) was −20 or −47 mV but had no effect if V h was −80 mV. The PDBu-induced inhibition of I K was blocked by pretreatment with the PKC inhibitor bisindolylmaleimide I and I K was unaffected by 4-α phorbol, indicating that the suppression of I Kwas mediated by PKC. The inhibition of I K by 100 nM PDBu at a V h of −50 mV was reversed over several minutes if V h was changed to −80 mV. In addition, intracellular perfusion with 5 mM bis-( o-aminophenoxy)- N,N,N′,N′-tetraacetic acid (BAPTA) or pretreatment with ω-conotoxin GVIA or Cd2+-Ringer, but not nifedipine, prevented the PDBu-induced suppression of I K at −50 mV, suggesting that a voltage-dependent influx of calcium through N-type calcium channels was necessary for the activation of PKC. The potassium channel blockers tetraethylammonium (TEA, 10 mM) and 4-aminopyridine (4-AP, 3 mM and 30 μM) reduced I K, but only TEA attenuated the ability of PDBu to further inhibit the current, suggesting that the I K modified by PDBu was sensitive to TEA. Interestingly, in the presence of 3 mM or 30 μM 4-AP, 100 nM PDBu inhibited I K when V h was −80 mV. Thus 4-AP promotes the capacity of PDBu to reduce I K at −80 mV. We find that activation of PKC inhibits I K in rat sensory neurons and that voltage-dependent calcium entry is necessary for the development and maintenance of this inhibition.