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

摘要

class="enumerated" style="list-style-type:decimal">The facilitatory effects of external Ca²⁺, Sr²⁺ and Ba²⁺ (Cao²⁺, Sro²⁺ and Bao²⁺) on the noradrenaline-evoked non-selective cation current (Icat) were compared in rabbit portal vein smooth muscle cells using patch pipette techniques.All divalent cations tested potentiated the amplitude of Icat and the potency sequence was Cao²⁺ > Sro²⁺ > Bao²⁺. Cao²⁺ and Sro²⁺ increased the amplitude of Icat by about eight times whereas Bao²⁺ produced only a threefold facilitation.The current-voltage relationship of Icat was not changed by Cao²⁺, Sro²⁺ or Bao²⁺.From noise analysis the single channel conductance (γ) was approximately 10 pS in divalent cation-free solution but was about 20 pS with Ca_o²⁺, Sr_o²⁺ and Ba_o²⁺.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_o²⁺ and Sr_o²⁺ 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_o²⁺ produced minimal effects.The data show that Ca_o²⁺, Sr_o²⁺ and Ba_o²⁺ 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_o²⁺ and Sr_o²⁺ alter the kinetic behaviour of the single channels whereas Ba_o²⁺ has little effect on the equilibrium between the channel states.