Contrasting biophysical and pharmacological properties of T-type and R-type calcium channels

• Published in: Neuropharmacology Vol. 36; no. 7; pp. 879 - 893
• Main Authors: Randall, A.D., Tsien, R.W.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.07.1997; Elsevier
• Subjects: Animals; Benzimidazoles - pharmacology; Biological and medical sciences; Biophysical Phenomena; Biophysics; Calcium Channel Blockers - pharmacology; Calcium Channels - drug effects; Calcium Channels - physiology; Cells, Cultured; Central nervous system; Cerebellum - drug effects; Electrophysiology; Fundamental and applied biological sciences. Psychology; Mibefradil; Rats; Stereoisomerism; Tetrahydronaphthalenes - pharmacology; Vertebrates: nervous system and sense organs; Cerebellum; Cell culture; Calcium ion; Animal; Central nervous system; Electrophysiology; Ionic channel; Ionic current; In vitro; Brain (vertebrata)
• ISSN: 0028-3908; 1873-7064
• DOI: 10.1016/S0028-3908(97)00086-5

In contrast to other kinds of voltage-gated Ca 2+ channels, the underlying molecular basis of T-type and R-type channels is not well-understood. To facilitate comparisons with cloned Ca 2+ 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 Ca 2+ 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 Ca 2+ influx. In response to a series of increasingly strong depolarizations T-type and R-type Ca 2+ 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 Ca 2+ channels as low- and highvoltage 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 ω-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 α 1 E subunit whereas T-type currents were not.