Sodium/calcium selectivity of cloned calcium T-type channels

Abstract

We studied the peculiarities of permeability with respect to the main extracellular cations, Na⁺ and Ca²⁺, of cloned low-threshold calcium channels (LTCCs) of three subtypes, Ca_v3.1 (α1G), Ca_v3.2 (α 1H), and Ca_v3.3 (α1I), functionally expressed in Xenopus oocytes. In a calcium-free solution containing 100 mM Na⁺ and 5 mM calcium-chelating EGTA buffer (to eliminate residual concentrations of Ca²⁺) we observed considerable integral currents possessing the kinetics of inactivation typical of LTCCs and characterized by reversion potentials of −10 ± 1, −12 ± 1, and −18 ± 2 mV, respectively, for Ca_v3.1, Ca_v3.2, and Ca_v3.3 channels. The presence of Ca²⁺ in the extracellular solution exerted an ambiguous effect on the examined currents. On the one hand, Ca²⁺ effectively blocked the current of monovalent cations through cloned LTCCs (K _d = 2, 10, and 18 µM for currents through channels Ca_v3.1, Ca_v3.2, and Ca_v3.3, respectively). On the other hand, at the concentration of 1 to 100 mM, Ca²⁺ itself functioned as a carrier of the inward current. Despite the fact that the calcium current reached the level of saturation in the presence of 5 mM Ca²⁺ in the external solution, extracellular Na⁺ influenced the permeability of these channels even in the presence of 10 mM Ca²⁺. The Ca_v3.3 channels were more permeable with respect to Na⁺ (P _Ca/P _Na ∼ 21) than Ca_v3.1 and Ca_v3.2 (P _Ca/P _Na ∼ 66). As a whole, our data indicate that cloned LTCCs form multi-ion Ca²⁺-selective pores, as these ions possess a high affinity for certain binding sites. Monovalent cations present together with Ca²⁺ in the external solution modulate the calcium permeability of these channels. Among the above-mentioned subtypes, Ca_v3.3 channels show the minimum selectivity with respect to Ca²⁺ and are most permeable for monovalent cations.