Figure 5.

Kinetics of the activation by sodium of the amiloride-sensitive current. The current sensitive to 50 μM amiloride at −100 mV, I_(am), was measured in solutions containing 0, 1, 3, 10, 20, and 100 mM Na⁺. All current values were normalized to the value of the amiloride-sensitive current in 100 mM Na, which was 3,630 ± 1,070 nA (n = 12) in the αβγ group and 167 ± 73 nA (n = 8) in the αβγ2 group. In the αβγ group the sodium concentration/I_(am) relationship was well described by simple Michaelis–Menten kinetics (upper equation) with a K_d of 11.3 mM. In the αβγ2 group the sodium concentration/I_(am) relationship was obviously different, with a significantly lower value in 100 mM Na⁺ than in 20 mM Na⁺. The curve fitted to the αβγ2 data points (lower equation) represents a double effect of Na⁺ concentration on the amiloride-sensitive current; first, an activation effect with a K_d of 4.8 mM and second, an inhibitory effect with a K_I of 154 mM.