Cyclic AMP regulates an inward rectifying sodium‐potassium current in dissociated bull‐frog sympathetic neurones.

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Abstract
Bull-frog sympathetic neurones in primary culture were voltage clamped in the whole-cell configuration. The pipette solution contained ATP (5 mM). 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 .mu.M), apamin (30 nM) and 4-aminopyridine (1 mM). IH was selectively blocked by caesium (10-300 .mu.M). 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 2.8s at -90 mV and 22.degree. C. The Q10 between 16 and 26.degree. C was 4.3. A non-hydrolysable ATP analogue in the pipette solution did not support IH activation. Intracellular ''loading'' of GTP-.gamma.-S (30-500 .mu.M) led to a progressive activation of IH. Forskolin (10 .mu.M) 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. Essentially the same results as with forskolin were obtained by intracellular ''loading'' with cyclic AMP (3-10 .mu.M) 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). The protein kinase inhibitor H-8 (1-10 .mu.m) decreased the peak amplitude of IH. Phorbol 12-myristate 13-acetate (10 .mu.M), a protein kinase C activator, was without effect. It is concluded that voltage-dependent cation current can be regulated by the basal activity of adenylate cyclase, presumably through protein kinase A, in vertebrate sympathetic neurones.