Variants of the Drosophila melanogaster Ih-channel are generated by different splicing

We isolated splice variants of the DMIH cDNA encoding members of the I(h)-channel family from Drosophila melanogaster by means of polymerase chain reaction and homology screening. Splicing at four different sites generates a great variety of different channel transcripts. The variants so obtained code for ion channel proteins with long or short N-termini and variations in the length of the interloop regions between the membrane-spanning domains S3-S4 and S4-S5. The multiple variants of DMIH coded by a single gene thus might form the molecular basis for a variety of I(h)-channels. Functional expression of one of the DMIH variants with a long N-terminus in HEK293 cells produced unitary currents that were preferentially selective for potassium over sodium ions and were activated by hyperpolarizing voltage steps. Cyclic nucleotides shifted the voltage activation curve to more positive membrane potentials. The current kinetics and modulatory influence of cyclic nucleotides resemble closely those of other invertebrate I(h)-channels, but activation by hyperpolarizing voltage steps had a V(1/2) of 123 mV, a more negative value than those of other recombinantly expressed insect I(h)-channels with a short N-terminus.