CALCICLUDINE, A VENOM PEPTIDE OF THE KUNITZ-TYPE PROTEASE INHIBITOR FAMILY, IS A POTENT BLOCKER OF HIGH-THRESHOLD CA2-AFFINITY FOR L-TYPE CHANNELS IN CEREBELLAR GRANULE NEURONS( CHANNELS WITH A HIGH)

Calcicludine (CaC) is a 60-amino acid polypeptide from the venom of De ndroaspis angusticeps. It is structurally homologous to the Kunitz-typ e protease inhibitor, to dendrotoxins, which block K+ channels, and to the protease inhibitor domain of the amyloid beta protein that accumu lates in Alzheimer disease. Voltage-clamp experiments on a variety of excitable cells have shown that CaC specifically blocks most of the hi gh-threshold Ca2+ channels (L-, N-, or P-type) in the 10-100 nM range. Particularly high densities of specific I-125-labeled CaC binding sit es were found in the olfactory bulb, in the molecular layer of the den tate gyrus and the stratum oriens of CA3 field in the hippocampal form ation, and in the granular layer of the cerebellum. I-125-labeled CaC binds with a high affinity (K(d) = 15 pM) to a single class of noninte racting sites in rat olfactory bulb microsomes. The distribution of Ca C binding sites in cerebella of three mutant mice (Weaver, Reeler, and Purkinje cell degeneration) clearly shows that the specific high-affi nity labeling is associated with granule cells. Electrophysiological e xperiments on rat cerebellar granule neurons in primary culture have s hown that CaC potently blocks the L-type component of the Ca2+ current (K0.5 = 0.2 nM). Then CaC, in the nanomolar range, appears to be a hi ghly potent blocker of an L-subtype of neuronal Ca2+ channels.