Phloretin affects the fast potassium channels in frog nerve fibres

The effect of phloretin (20-100 microM), a dipolar organic compound, on the voltage clamp currents of the frog node of Ranvier has been investigated. The Na currents are simply reduced in size but not otherwise affected. Phloretin has no effect on the slow 4-aminopyridine-resistant K channels. However, the voltage dependence and time course of the fast K conductance (gK) is markedly altered. The gK (E) curve, determined by measuring fast tail currents at different pulse potentials, normally exhibits a bend at -50 mV, indicating the existence of two types of fast K channels. Phloretin shifts the gK (E) curve to more positive potentials, reduces its slope and its maximum and abolishes the distinction between the two types of fast K channels. The effect becomes more pronounced with time. Phloretin also markedly slows the opening of the fast K channels, but has much less effect on the closing. Opening can be accelerated again by a long depolarizing prepulse which presumably removes part of the phloretin block. It is concluded that phloretin selectively affects the fast K channels of the nodal membrane. The results are compared with similar observations on the squid giant axon.