Functional diversity of P-type and R-type calcium channels in rat cerebellar neurons.
By combining single-channel and whole-cell patch-clamp recordings, we have established the sensitivity to omega-agatoxin IVA and omega-conotoxin MVIIC (SNX-230) of G1, G2, and G3, the three novel non-L-, non-N-type Ca2+ channels characterized previously in rat cerebellar granule cells. G1 channels were blocked irreversibly by both omega-conotoxin MVIIC and low doses of omega-agatoxin IVA (saturation at 50 nM). Thus, according to pharmacological criteria, G1 channels must be classified as P-type Ca2+ channels. Being slowly inactivating during depolarizing pulses and completely inactivated at voltages in which steady-state inactivation of P-type channels in Purkinje cells is negligible, G1 represents a novel P subtype. Neither G2 nor G3 was blocked irreversibly by omega-conotoxin MVIIC, and therefore both are R-type Ca2+ channels. G2 and G3 have some biophysical properties similar to those of low-voltage-activated (LVA) Ca2+ channels (e.g., voltage range for steady-state inactivation, V 1/2 = -90 mV), some properties similar to those of high-voltage-activated (HVA) Ca2+ channels (e.g., high sensitivity to Cd2+ block), and other properties intermediate between those of LVA and HVA Ca2+ channels, with LVA properties prevailing in G2 and HVA properties prevailing in G3. The R-type whole-cell current was inhibited by Ni2+ with a biphasic dose-response curve (IC50: 4 and 153 microM), suggesting that G2 and G3 may have a different sensitivity to Ni2+ block. Our results uncover functional diversity of both native P-type and R-type Ca2+ channels and show that R subtypes with distinct biophysical properties are coexpressed in rat cerebellar granule cells.
Tottene
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Department of Biomedical Sciences, University of Padova, Italy.
Moretti
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Pietrobon
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D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000145:classification / Q000187:drug effects / Q000502:physiology*; D002478:Cells, Cultured / Q000187:drug effects; D002531:Cerebellum / Q000187:drug effects / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D018408:Patch-Clamp Techniques; D010455:Peptides / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D013997:Time Factors; D020960:omega-Conotoxins
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D010455:Peptides; D020960:omega-Conotoxins; C075851:omega-conotoxin-MVIIC