Fig. 5.

Evidence for an inward rectifier, Kir.A, Current–response (average of 9 trials) to voltage steps from a holding potential of −70 mV to various test membrane potentials between −120 and −30 mV. B, The current at the end of the 3 sec voltage step plotted versus the test membrane potential in (1) normal leech saline ([K⁺]_o = 4 mm) (average of 8 cells), (2) 0 Na⁺, 0 Ca²⁺, 1.8 mm Co²⁺saline (average of 7 cells), (3) saline with [K⁺]_o elevated to 15 (average of 6 cells), and (4) saline with [K⁺]_oelevated to 45 mm (average of 5 cells). C, Measured current, I_M, in normal saline ([K⁺]_o = 4 mm) minus the non-K⁺ leak current,I_L =G_L(V −E_L). The value of the equilibrium leak potential, E_L, was assumed to be −50 mV, and the non-K⁺ leak conductance was estimated for each cell from the measured current,I_M,EK, at the assumedE_K value of −80 mV:G_L =I_M,EK/(E_K− E_L). The insetshows the measured current and the estimated non-K⁺leak current for one representative cell. D, Potassium chord conductance, G = (I_M −I_L)/(V −E_K). The continuous curve is a least-squares fit of the Boltzmann equation to the data points. The fitting parameters are G_max = 40 nS,V_1/2 = −76 mV, and γ = 12 mV. For comparison the average non-K⁺ leak conductance,G_L = 28 ± 2 nS, is indicated by the horizontal arrow. The inset shows the potassium chord conductance for [K⁺]_o = 15 and 45 mm, estimated as described in the text. For comparison the chord conductance for [K⁺]_o = 4 mm (i.e., the data shown in the graph) is replotted in the inset.