L-Nitroarginine (LNA, 10 μM; Ab) selectively abolished the slow hyperpolarization compared with controls (Aa) in response to a high-frequency train of stimuli (supramaximal voltage, 50 μs, 50 Hz, 1 s). In another preparation where recordings were made in the presence of 10 μM guanethidine (Ba), again 10 μM LNA selectively abolished the slow hyperpolarization (Bb) and the effect of LNA was partially reversed by L-arginine (1 mM; Bc). The scale bars on the right refer to all traces. Vm refers to resting membrane potential.