NORADRENALINE-EVOKED CATION CONDUCTANCE RECORDED WITH THE NYSTATIN WHOLE-CELL METHOD IN RABBIT PORTAL-VEIN CELLS

1. Noradrenaline-evoked currents were studied with the perforated patch technique (nystatin in the patch pipette) in freshly dispersed rabbit portal vein cells. 2. With potassium-containing solutions noradrenaline produced an inward current at -50 mV and an outward current (I(K)(Ca)) at 0 mV. In potassium-free conditions noradrenaline elicited two distinct membrane currents which could be differentiated according to their sensitivity to the presence of caffeine in the bathing solution. One current was blocked whereas the second response was unaffected by caffeine. 3. The reversal potential (E(r) = -2.0 mV) of the caffeine-sensitive current was altered when external chloride was replaced with more permeant anions but E(r) was not changed when external sodium was replaced by Tris. Therefore the caffeine-sensitive current appears to be a calcium-activated chloride current (I(Cl)(Ca)). 4. E(r) (+ 6.0 mV) of the caffeine-insensitive current was not altered by anion substitution but was changed when external sodium was replaced by Tris and barium ions. Thus the caffeine-insensitive conductance is a non-selective cation current (I(cat)). 5. When external NaCl was replaced by BaCl2, E(r) was shifted to more positive potentials which suggests that the cation conductance is more permeable to barium than to sodium. I(cat) was not affected by 10(-6) M-nifedipine. 6. The steady-state current-voltage relationship was linear between -50 and +50 mV for I(Cl)(Ca) but the cation conductance mechanism displayed pronounced inward rectification and little outward current flowed across the membrane at positive potentials. 7. Caffeine (which releases calcium from internal stores) and the calcium ionophore ionomycin, which are expected to increase intracellular calcium concentration, evoked I(Cl)(Ca) but not I(cat). Thus I(Cl)(Ca) but not I(cat) can be activated directly by an increase in intracellular calcium concentration. 8. When calcium was removed from the bathing solution the amplitude of I(cat) was not altered at early times in Ca2+-free conditions but was abolished after 10 min. I(cat) was readily activated by noradrenaline in the presence of ionomycin, which inactivated I(Cl)(Ca). A permissive role for calcium in the generation of I(cat) is suggested. 9. The times between the application of noradrenaline and the onset of the chloride and cationic conductances were similar (0.75 and 0.9 s respectively). In contrast the rise time (3.9 s) and half-decay time (11 s) of I(cat) were much longer than the corresponding values of I(Cl)(Ca) (respectively 1.9 and 2 s). In high-barium solution noradrenaline sometimes evoked a response that lasted for up to 10 min. 10. I(Cl)(Ca) and I(cat) were blocked by the alpha-1-adrenoceptor antagonist prazosin but were unaffected by the alpha-2-adrenoceptor antagonist yohimbine. Thus both conductance mechanisms are activated by the same pharmacological receptor subtype.