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

R. M. Helliwell and W. A. Large

1. The facilitatory effect of external calcium ions (Ca²⁺_o) on the ₁-adrenoceptor-activated non-selective cation current (I_cat) 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 I_cat at a holding potential (V_h) 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 () was approximately 23 pS with [Ca²⁺]_o between 3 and 100 µM, whereas in < 10 nM or 1 µM Ca²⁺_o was approximately 10 pS.

4. The spectral density function of I_cat 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 (₁) 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 (₂) 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 I_cat. Stepping back to -30 mV produced an exponential inward relaxation (I_{relax,-30 mV}) leading to an increase in the steady-state amplitude of I_cat in micromolar concentrations of Ca²⁺_o, but this relaxation was not observed in < 10 nM Ca²⁺_o. The relative amplitude of I_{relax,-30 mV} increased in an [Ca²⁺]_o-dependent manner (EC₅₀ was 2 µM) although the time constant of this relaxation (_{relax,-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 I_cat.

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