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 (Ca-o(2+)) on the alpha(1)-adrenoceptor-activated nonselective 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 Ca-o(2+) 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 mu M. 3. From noise analysis the estimated single channel conductance (gamma) was approximately 23 pS with [Ca2+](o) between 3 and 100 mu M, whereas in < 10 nM or 1 mu M Ca-o(2+) gamma 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 (tau(1)) was about 11 ms in < 10 nM Ca-o(2+) and was about 45 ms in micromolar concentrations of Ca-o(2+) (1-100 mu M). In contrast, the time constant of the higher frequency component (tau(2)) was similar in < 10 nM Ca-o(2+) and 100 mu M Ca-o(2+) (between 1 and 2 ms). 5. The lower frequency Lorentzian component was responsible for about half the total current variance in ( 10 nM Ca-o(2+) whereas in micromolar concentrations of Ca-o(2+) 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,I--30mV) leading to an increase in the steady-state amplitude of I-cat in micromolar concentrations of Ca-o(2+), but this relaxation was not observed in < 10 nM Ca-o(2+). The relative amplitude of I-relax,I--30mV increased in an [Ca2+](o)-dependent manner (EC50 was 2 mu M) although the time constant of this relaxation (tau(relax,-30mV)) remained unchanged (about 60 ms between 2 and 100 mu M Ca-o(2+)). 7. The data suggest that Ca-o(2+) produces marked changes in the kinetics and single channel conductance of cation channels, which may account for the facilitatory effect of micromolar concentrations of Ca-o(2+) on the peak amplitude of I-cat.