Comparison of the effects of divalent cations on the noradrenaline-evoked cation current in rabbit portal vein smooth muscle cells

1. The facilitatory effects of external Ca2+, Sr2+ and Ba2+ (Ca-o(2+), Sr-o(2+) and Ba-o(2+)) on the noradrenaline-evoked non-selective cation current (I-cat) were compared in rabbit portal vein smooth muscle cells using patch pipette techniques. 2. All divalent cations tested potentiated the amplitude of I-cat and the potency sequence was Ca-o(2+) > Sr-o(2+) > Ba-o(2+). Ca-o(2+) and Sr-o(2+) increased the amplitude of I-cat by about eight times whereas Ba-o(2+) produced only a threefold facilitation. 3. The current-voltage relationship of I-cat was not changed by Ca-o(2+), Sr-o(2+) or Ba-o(2+). 4. From noise analysis the single channel conductance (gamma) was approximately 10 pS in divalent cation-free solution but was about 20 pS with Ca-o(2+), Sr-o(2+) and Ba-o(2+). 5. From noise and voltage-jump experiments it was apparent that at least three kinetically resolvable channel states are associated with I-cat in divalent cation-free solution. Ca-o(2+) and Sr-o(2+) produced marked changes in the characteristics of the power spectrum and relaxations of I-cat in response to voltage steps, consistent with a shift in the equilibrium between the channel states, whereas Ba-o(2+) produced minimal effects. 6. The data show that Ca-o(2+), Sr-o(2+) and Ba-o(2+) increase the amplitude of I-cat, which results in part from an increase in the single channel conductance. In addition the results suggest that Ca-o(2+) and Sr-o(2+) alter the kinetic behaviour of the single channels whereas Ba-o(2+) has little effect on the equilibrium between the channel states.