On the kinetics of the potassium channel activated by acetylcholine in the S-A node of the rabbit heart

Abstract

Voltage-clamp experiments were conducted on small specimens of rabbit sinoatrial node. In the same preparation the dose-response curve of the potassium current induced by application of different concentrations of acetylcholine (ACh), the time constant of relaxation and the current fluctuations were measured. From these measurements the apparent dissociation constant and the rate constants for the opening and closing of the ACh-activated potassium channel were estimated. In the presence of neostigmine a measurable response was recorded at around 10⁻⁸ M ACh, the saturation was reached at 10⁻⁴ M, and the half saturation was attained at around 10⁻⁶ M. The time constant of relaxation at −35 mV decreased from 100 ms at 10⁻⁸ M to 45 ms at 10⁻⁴ M ACh. The variance of the fluctuations of the ACh-activated current increased with increasing ACh concentration to a peak value of around 10⁻⁵ M. From the above 3 kinds of measurements, opening and closing rate constants of about 12 s⁻¹ and 10 s⁻¹, respectively, and a dissociation constant of 1.7 μM were calculated. The Katz-Miledi model was considered to be appropriate to describe the reaction of ACh with the muscarinic receptor in the S-A node. The current on ionophoretic application of ACh was computed using the rate constants and taking into account diffusion in the S-A node in which the density of receptors is low. The computed response had a similar time course to the recorded current.