Selectivity signatures of three isoforms of recombinant T-type Ca channels

Voltage-gated Ca²⁺ channels (VGCCs) are recognized for their superb ability for the preferred passage of Ca²⁺ over any other more abundant cation present in the physiological saline. Most of our knowledge about the mechanisms of selective Ca²⁺ permeation through VGCCs was derived from the studies on native and recombinant L-type representatives. However, the specifics of the selectivity and permeation of known recombinant T-type Ca²⁺-channel α1 subunits, Ca_v3.1, Ca_v3.2 and Ca_v3.3, are still poorly defined. In the present study we provide comparative analysis of the selectivity and permeation Ca_v3.1, Ca_v3.2, and Ca_v3.3 functionally expressed in Xenopus oocytes. Our data show that all Ca_v3 channels select Ca²⁺ over Na⁺ by affinity. Ca_v3.1 and Ca_v3.2 discriminate Ca²⁺, Sr²⁺ and Ba²⁺ based on the ion's effects on the open channel probability, whilst Ca_v3.3 discriminates based on the ion's intrapore binding affinity. All Ca_v3s were characterized by much smaller difference in the K_D values for Na⁺ current blockade by Ca²⁺ (K_D1 ∼ 6 μM) and for Ca²⁺ current saturation (K_D2 ∼ 2 mM) as compared to L-type channels. This enabled them to carry notable mixed Na⁺/Ca²⁺ current at close to physiological Ca²⁺ concentrations, which was the strongest for Ca_v3.3, smaller for Ca_v3.2 and the smallest for Ca_v3.1. In addition to intrapore Ca²⁺ binding site(s) Ca_v3.2, but not Ca_v3.1 and Ca_v3.3, is likely to possess an extracellular Ca²⁺ binding site that controls channel permeation. Our results provide novel functional tests for identifying subunits responsible for T-type Ca²⁺ current in native cells.