The time course of the muscarinic response to ionophoretic acetylcholine application to the S-A node of the rabbit heart
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In the rabbit S-A node the factors which control the time course of hyperpolarizations (muscarinic response) induced by ionophoretic application of acetylcholine were studied.
The muscarinic response is characterized by an apparent delay of minimally 30 ms and a slow rise to the peak. The time to peak rises continually from about 200 ms to 1,000 ms when the amount of ACh released is increased.
For a given strong releasing pulse both peak time and delay time depend on the distance of the pipette tip to the surface of the preparation with a power of 2 for large distances (about >60 μm). For shorter pipette distances the peak time remains constant, and it was concluded that for this location the chemical reactions of agonist binding and channel opening are slow compared to the change of ACh concentration (non-equilibrium) and possibly saturation occurs.
Peak shape and total duration of the response are strongly influenced by the activity of the tissue cholinesterase: In the presence of neostigmine the same amount of ACh released produces a larger and longer response. Lowering the temperature from 37°C to 27°C prolongs the peak time by a factor of 1.7–2.1.
The hyperpolarizations could be reconstructed on the basis of the Katz-Miledi model including the rate constants of the opening and closing of the ion channel, and the diffusion of the agonist in the sinus node preparation with a low density of muscarinic receptors. The calculations suggest that the apparent delay is mainly due to the kinetics of the ion channel and that diffusion determines the slow time course of the response.
The model simulates changes of the external factors which strongly influence the shape of the response (e.g. strength of the releasing pulse, temperature and activity of the cholinesterase).
Key wordsS-A node Muscarinic response ACh-activated potassium channel Ionophoresis
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