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

1 The facilitatory effect of external calcium ions (Ca2+o) 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. 2 Micromolar concentrations of Ca2+o 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. 3 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 Ca2+oγ was approximately 10 pS. 4 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 Ca2+o and was about 45 ms in micromolar concentrations of Ca2+o (1–100 μm). In contrast, the time constant of the higher frequency component (τ2) was similar in < 10 nm Ca2+o and 100 μm Ca2+o (between 1 and 2 ms). 5 The lower frequency Lorentzian component was responsible for about half the total current variance in < 10 nm Ca2+o whereas in micromolar concentrations of Ca2+o it was responsible for most of the measured current variance. 6 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 Ca2+o, but this relaxation was not observed in < 10 nm Ca2+o. 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 (τrelax,‐30 mV) remained unchanged (about 60 ms between 2 and 100 μm Ca2+o). 7 The data suggest that Ca2+o produces marked changes in the kinetics and single channel conductance of cation channels, which may account for the facilitatory effect of micromolar concentrations of Ca2+o on the peak amplitude of Icat.