EFFECTS OF EXTERNAL AND INTERNAL K+ IONS ON MAGNESIUM BLOCK OF INWARDLY RECTIFYING K+ CHANNELS IN GUINEA-PIG HEART-CELLS

1. Block of the inwardly rectifying K+ channel by intracellular Mg2+ was studied in guinea-pig ventricular cells at varying external or internal K+ concentrations. Sucrose or glucose was mainly used as a substitute for K+. 2. The current-voltage (I-V) relation for the single channel, in the absence of internal Mg2+, was almost linear in 30 mM-external K+ and 150 mM-internal K+ (30 mM [K+]o) and in 45 mM-internal K+ and 150 mM-external K+ (45 mM [K+]i) as well as in 150 mM-external and internal K+ (the control condition). The channel conductance was 31.7 +/- 1.7 pS (mean +/- S.D., n = 36) in the control, 23.1 +/- 1.2 pS (n = 8) in 30 mM [K+]o and 29.7 +/- 1.3 pS (n = 16) in 45 mM [K+]i, respectively. 3. Mg2+ on the cytoplasmic side blocked the outward currents without affecting the inward currents. Outward mean open-channel currents were measured at different Mg2+ concentrations (0-100-mu-M) and voltages. The current-voltage relation rectified inwardly in the presence of internal Mg2+ in a voltage- and concentration-dependent manner. 4. Outward mean open-channel currents were normalized to that measured in the absence of Mg2+. The normalized current-voltage relation in 45 mM [K+]i was almost superimposable on that obtained in the control at the same Mg2+ concentration, while that in 30 mM [K+]o was shifted in the negative direction by some 30 mV. 5. The normalized current-Mg2+ concentration curve was fitted by a one-to-one binding curve at each K+ condition and voltage. In a semilogarithmic plot of dissociation constant versus membrane potential, data points for 45 mM [K+]i were located on the same line as the control, whereas data points for 30 mM [K+]o were shifted in the negative direction by about 30 mV. The dissociation constant at 0 mV is 37-mu-M in the control and 45 mM [K+]i and 8.8-mu-M in 30 mM [K+]o. The voltage dependence of dissociation constants gives a value for the fractional electrical distance of the Mg2+ binding site of 0.57. 6. Subconductance levels with one-third and two-thirds of the unitary amplitude were seen with low internal Mg2+ at 45 mM [K+]i or 30 mM [K+]o as well as in the control condition. Blocking and unblocking rates were calculated on the assumption that the channel is composed of three identical conducting units and each unit is blocked by Mg2+ independently. An increase in the blocking rate was more evident than a decrease in the unblocking rate with reduction of external K+. 7. The results are considered in terms of a three-barrier, two-site ionic permeation model. Dependence of the dissociation constant at 0 mV on the external K+ implies that the depth of the energy well for Mg2+ is affected by external K+. Such an assumption is needed to reproduce the shift of the block comparable to the change in the zero-current potential when external K+ is changed.