Protein phosphorylation/dephosphorylation and the transient, voltage-dependent potassium conductance in Hermissenda crassicornis.

Recent interest in the modulation of ionic conductances in excitable membranes, particularly K+ conductances, has focused on biochemical mechanisms such as protein phosphorylation. Several distinct types of voltage-dependent K+ currents (e.g. transient, delayed, and Ca2+-dependent) can be separated by electrophysiological and pharmacological means. We have found that two treatments, 4-aminopyridine and high external K+, which preferentially reduce the transient K+ current also reduce the level of 32P incorporation in a 25,000 molecular weight phosphoprotein band in eyes and ganglia of the nudibranch mollusc Hermissenda crassicornis. The effect of agents which reduce the transient K+ current is relatively specific because 32P incorporation in this band is not significantly affected by Ba2+ which preferentially blocks the delayed K+ current, or by Ni2+, which blocks the Ca2+ channel and in turn inhibits the Ca2+-dependent K+ current. Additional studies indicate that a change in endogenous impulse activity is not required for the expression of the 4-aminopyridine effect on protein phosphorylation. The concentration dependence and time course of the 4-aminopyridine effect on 32P incorporation are in reasonable agreement with those reported for the blockade of the transient K+ current by 4-aminopyridine. Following removal of 4-aminopyridine, 32P incorporation increases in the 25,000 molecular weight phosphoprotein band and in a 23,000 molecular weight Ca2+-stimulated phosphoprotein band. The data presented here suggest that there is a relationship between the transient K+ current and the state of phosphorylation of specific neuronal proteins in Hermissenda and point to further studies to identify the phosphoproteins and the mechanism which leads to the observed changes in protein phosphorylation.