FIGURE 4.

NE-mediated hyperpolarization is mediated by activation of a background K⁺ channel. A, replacing K⁺ in the intracellular solution with the same concentration of Cs⁺ blocked NE-induced increases in outward holding currents (n = 8). B, voltage-current relationship induced by a ramp protocol from -140 mV to -70 mV at a speed of 0.1 mV/ms prior to and during the application of NE (100 μm). Subtraction of the current prior to from the current during the application of NE generated a net current. The traces were averages from six cells. Note that the reversal potential was -97.7 ± 1.6 mV (n = 6), close to the calculated K⁺ reversal potential (-92.2 mV). C, NE-induced increases in outward holding currents were insensitive to extracellular application of the classic K⁺ channel blockers (TEA, 10 mm, n = 5, p = 0.4; Cs⁺, 3 mm, n = 6, p = 0.12; 4-aminopyridine (4-AP), 2 mm, n = 5, p = 0.1; tertiapin, 50 nm, n = 5, p = 0.2, versus application of NE alone) but significantly reduced when the extracellular K⁺ concentration was increased to 10 mm (n = 6, p < 0.001). D, inclusion of Ba²⁺ (3 mm) in the extracellular solution induced an inward holding current per se but prevented NE-induced increases in outward holding currents (n = 6).