Facilitatory effect of Ca2+ on the noradrenaline-evoked cation current in rabbit portal vein smooth muscle cells

摘要

class="enumerated" style="list-style-type:decimal">The facilitatory effect of external calcium ions (Cao2+) on the α1 -adrenoceptor-activated non-selective cation current (Icat) was investigated in rabbit portal vein cells using noise and voltage-jump relaxation analysis of the whole-cell macroscopic current.Micromolar concentrations of Cao2+potentiated the peak amplitude of Icat at a holding potential (Vh) of −50 mV. The effective [Ca²⁺]o which produced a 50% potentiation (EC50) was 3 μm.From noise analysis the estimated single channel conductance (γ) was approximately 23 pS with [Ca²⁺]o between 3 and 100 μm, whereas in < 10 nm or 1 μm Cao2+γ was approximately 10 pS.The spectral density function of Icat at negative potentials could be described by the sum of two Lorentzians in every [Ca²⁺]o examined. The time constant of the lower frequency Lorentzian component (τ1) was about 11 ms in < 10 nm Cao2+ and was about 45 ms in micromolar concentrations of Cao2+ (1–100 μm). In contrast, the time constant of the higher frequency component (τ2) was similar in < 10 nm Cao2+ and 100 μm Cao2+ (between 1 and 2 ms).The lower frequency Lorentzian component was responsible for about half the total current variance in < 10 nm Cao2+ whereas in micromolar concentrations of Cao2+ it was responsible for most of the measured current variance.In voltage-jump experiments, on stepping the voltage from −50 to +50 mV the instantaneous current was followed by an exponential decline of Icat. Stepping back to −30 mV produced an exponential inward relaxation (Irelax,-30 mV) leading to an increase in the steady-state amplitude of Icat in micromolar concentrations of Cao2+, but this relaxation was not observed in < 10 nm Cao2+. The relative amplitude of Irelax,-30 mV increased in an [Ca²⁺]o -dependent manner (EC₅₀ was 2 μm) although the time constant of this relaxation (τ_{relax,-30 mV}) remained unchanged (about 60 ms between 2 and 100 μm Cao2+).The data suggest that Cao2+ produces marked changes in the kinetics and single channel conductance of cation channels, which may account for the facilitatory effect of micromolar concentrations of Cao2+ on the peak amplitude of I_cat.