1. The M-like current I(K(M, ng)) in differentiated NG108-15 mouse neuroblastoma x rat glioma hybrid cells has been studied using tight-seal, whole-cell patch-clamp recording. 2. When calculated from steady-state current-voltage curves, the conductance underlying I(K)(M, ng)) showed a Boltzmann dependence on voltage with half-activation voltage V(o) = -44 mV (in 3 mm [K+]) and slope factor (a) = 8.1 mV/e-fold increase in conductance. In 12 mm [K+] V(o) = - 38 mV and a = 6.9 mV. The deactivation reciprocal time constant accelerated with hyperpolarization with slope factor 17 mV/e-fold voltage change. 3. The reversal potential for deactivation tail currents varied with external [K+] as if P(Na)/P(K) were 0.005. 4. Steady-state current was increased on removing external Ca2+. In the presence of external Ca2+, reactivation of I(K(M, ng)) after a hyperpolarizing step was delayed. This delay was preceded by an inward Ca2+ current, and coincided with an increase in intracellular [Ca2+] as measured with Indo-1 fluorescence. Elevation of intracellular [Ca2+] with caffeine also reduced I(K(M, ng)). 5. I(K(M, ng)) was inhibited by external divalent cations in decreasing order of potency (mm IC50 in parentheses): Zn2+ (0.011) > Cu2+ (0.018) > Cd2+ (0.070) > Ni2+ (0.44) > Ba2+ (0.47) > Fe2+ (0.69) > Mn2+ (0.86) > Co2+ (0.92) > Ca2+ (5.6) > Mg2+ (16) > Sr2+ (33). This was not secondary to inhibition of I(Ca). since: (i) inhibition persisted in Ca2+-free solution; (ii) La3+ did not inhibit I(K(M,ng)) at concentrations which inhibited I(Ca), and (iii) organic Ca2+ channel blockers were ineffective. Inhibition comprised both depression of the maximum conductance and a positive shift of the activation curve. Addition of Ca2+ (10-mu-M free [Ca2+]) or Ba2+ (1 mM total [Ba2+]) to the pipette solution did not significantly change I(K(M, ng)). 6. I(K(M,ng)) was reduced by 9-amino-1,2,3,4-tetrahydroacridine (IC50 8-mu-M) and quinine (30-mu-M) but was insensitive to tetraethylammonium (IC50 > 30 mm), 4-aminopyridine (> 10 mm), apamin (> 3-mu-M) or dendrotoxin (> 100 nm). 7. I(K(M, ng)) was inhibited by bradykinin (1-10-mu-M) or angiotensin II (1-10-mu-M), but not by the following other receptor agonists: acetylcholine (10 mM), muscarine (10-mu-M), noradrenaline (100-mu-M), adrenaline (100-mu-M), dopamine (100-mu-M), histamine (100-mu-M), 5-hydroxytryptamine (10-mu-M), Met-enkephalin (1-mu-M), glycine (100-mu-M), gamma-aminobutyric acid (100-mu-M) or baclofen (500-mu-M). 8. Inhibition of I(K(M, ng)) with Ba2+ did not change the zero-current potential but suppressed outward rectification of the current-voltage curve and facilitated repetitive action potential discharges produced by depolarizing current injections. This latter action was not imitated by tetraethylammonium, apamin or 4-aminopyridine. 9. It is concluded that I(K(M, ng)) resembles the M-current recorded in sympathetic, ganglia in (i) kinetic behaviour, (ii) inhibition by transient rises in intracellular [Ca2+], and (iii) functional effects. It differs from the ganglionic M-current in certain pharmacological properties (sensitivity to divalent cations, organic K+ channel blockers and receptor agonists).
