Mytilus inhibitory peptide (MIP) induces a Na+-activated K+-current in snail neurons

Two microelectrode voltage-clamp and single-channel recordings were perform ed on D-cluster neurons of snail right parietal ganglion in order to study the properties of MIF-activated potassium current. It was found that the oc tapeptide member of the MIP-family, ASHIPRFVa elicits an outward current, w hich possesses all the properties characteristic for the hexapeptide(s) inw ard membrane response. The main component of the peptide elicited response is highly [K+](o) dependent, however the response was attenauted in Na-free extracellular saline. The peptide elicited response was mimicked by raisin g the [Na+](i) by pressure injection of Na+ into the cell. Single channel r ecordings indicated that MIP-induced outward K-current is Na-dependent. The probability to find a channel in open state increases with increasing intr acellular Na+-concentration. Excised inside-out parches obtained from D-neu rons contained I-K(Na) channels could be activated by exposure of the cytop lasmic face of the patch membrane to 40 mM Na+, and 40 mM Li+, as well. The single channel current amplitude at -60 mV is 15 pA and the single channel conductance is 212 pS between -80 and 0 mV. It was concluded that MIP's ac tivate a novel type of K+-current in the snail neurons. This current is the Na-activated K+-current. The single channel properties of the MIP activate d channel is in concert with I-K(Na) data obtained on different vertebrate and invertebrate preparations.