Noise and relaxation studies of acetylcholine induced currents in the presence of procaine.

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Abstract
The effects of procaine on the excitatory effects of acetylcholine (ACh) on Aplysia neurons were studied by analyzing the current relaxations induced by voltage steps. Procaine was applied by perfusion and ACh was applied iontophoretically under 2 sets of experimental conditions designed to produce very different ACh concentrations at the membrane. When ACh was applied at high concentration, the current relaxation following a hyperpolarizing step showed 2 components at procaine concentrations of about 2 .times. 10-5 M; a fast increase was followed by a slow decrease. At high procaine concentrations (> 10-4 M), the relaxation had only 1 component, corresponding to an exponential current decrease. When ACh was applied at low concentrations, the current relaxation usually showed 2 components. At low procaine concentrations a fast increase was followed by a slower one; at higher procaine concentrations, a fast decrease preceded the slow increase. Procaine binds preferentially to the activated receptor-channel complex and converts it into a non-conducting complex. The association constant and Kd were calculated to be (at 12.degree. C, -80 mV) 1.3 .times. 106 M-1 s-1 and 10 s-1.