CYCLIC-AMP REGULATES AN INWARD RECTIFYING SODIUM-POTASSIUM CURRENT IN DISSOCIATED BULLFROG SYMPATHETIC NEURONS

1. Bull‐frog sympathetic neurones in primary culture were voltage clamped in the whole‐cell configuration. The pipette solution contained ATP (5 mM). 2. A hyperpolarization‐activated sodium‐potassium current (H‐current: IH) was separated from other membrane currents in a nominally calcium‐free solution containing cobalt (2 mM), magnesium (4 mM), barium (2 mM), tetraethylammonium (20 mM), tetrodotoxin (3 microM), apamin (30 nM) and 4‐aminopyridine (1 mM). IH was selectively blocked by caesium (10‐300 microM). 3. The steady‐state activation of IH occurred between ‐60 and ‐130 mV. The H‐conductance was 4.1‐6.6 nS at the half‐activation voltage of ‐90 mV. With the concentrations of potassium and sodium ions in the superfusate at 20 and 70 mM, respectively, the reversal potential of IH was about ‐20 mV. IH was activated with a time constant of 2.8 s at ‐90 mV and 22 degrees C. The Q10 between 16 and 26 degrees C was 4.3. 4. A non‐hydrolysable ATP analogue in the pipette solution did not support IH activation. Intracellular ‘loading’ of GTP‐gamma‐S (30‐500 microM) led to a progressive activation of IH. 5. Forskolin (10 microM) increased the maximum conductance of IH by 70%. This was associated with a depolarizing shift in the half‐activation voltage (5‐10 mV) and in the voltage dependence of the activation/deactivation time constant of IH. 6. Essentially the same results as with forskolin were obtained by intracellular ‘loading’ with cyclic AMP (3‐10 microM) or bath application of 8‐bromo cyclic AMP (0.1‐1 mM), dibutyryl cyclic AMP (1 mM) and 3‐isobutyl‐1‐methylxanthine (0.1‐1 mM). 7. The protein kinase inhibitor H‐8 (1‐10 microM) decreased the peak amplitude of IH. Phorbol 12‐myristate 13‐acetate (10 microM), a protein kinase C activator, was without effect. 8. It is concluded that a voltage‐dependent cation current can be regulated by the basal activity of adenylate cyclase, presumably through protein kinase A, in vertebrate sympathetic neurones. © 1990 The Physiological Society