Contrasting biophysical and pharmacological properties of T-type and R-type calcium channels.
In contrast to other kinds of voltage-gated Ca2+ channels, the underlying molecular basis of T-type and R-type channels is not well-understood. To facilitate comparisons with cloned Ca2+ channel subunits, we have carried out a systematic analysis of the properties of T-type currents in undifferentiated NG108-15 cells and R-type currents in cerebellar granule neurons. Marked differences were found in their biophysical and pharmacological features under identical recording conditions. T-type channels became activated at potentials approximately 25 mV more negative than R-type channels; however, T-type channels required potentials approximately 15 mV less negative than R-type channels to be available. Accordingly, T-type channels display a much larger overlap between the curves describing inactivation and activation, making them more suitable for generating sustained Ca2+ entry in support of secretion or pacemaker activity. In contrast, R-type channels are not equipped to provide a steady current, but are very capable of supplying transient surges of Ca2+ influx. In response to a series of increasingly strong depolarizations T-type and R-type Ca2+ channels gave rise to very different kinetic patterns. T-type current records crossed each other in a characteristic pattern not found for R-type currents. These biophysical distinctions were independent of absolute membrane potential and were, therefore, complementary to the conventional categorization of T- and R-type Ca2+ channels as low- and high-voltage activated. R-type channels deactivated approximately eight-fold more quickly than T-type channels, with clear consequences for the generation of divalent cation influx during simulated action potentials. Pharmacological comparisons revealed additional contrasts. R-type current was responsive to block by omega-Aga IIIA but not nimodipine, while the opposite was true for T-type current. Both channel types were potently inhibited by the non-dihydropyridine compound mibefradil. In all respects examined, R-type currents were similar to currents derived from expression of the alpha1E subunit whereas T-type currents were not.
Randall
A D
AD
Division of Neurobiology, MRC Laboratory of Molecular Biology, Cambridge, U.K. ar1@mrc-lmb.cam.ac.uk
Tsien
R W
RW
D000818:Animals; D001562:Benzimidazoles / Q000494:pharmacology*; D055592:Biophysical Phenomena; D001703:Biophysics; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects* / Q000502:physiology*; D002478:Cells, Cultured; D002531:Cerebellum / Q000187:drug effects; D020748:Mibefradil; D051381:Rats; D013237:Stereoisomerism; D013764:Tetrahydronaphthalenes / Q000494:pharmacology*
D001562:Benzimidazoles; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D013764:Tetrahydronaphthalenes; D020748:Mibefradil