Two Ion Currents Activated by Acetylcholine in the ARC Muscle of Aplysia

1. This work continues our examination of the electrophysiology and contractions of single fibers dissociated from a widely studied molluscan muscle, the accessory radula closer (ARC) muscle of Aplysia californica , aimed at understanding its excitation-contraction mechanisms and their modulation.

2. Extensive previous work has characterized a number of basal ion currents present in the fibers and effects of transmitters and peptide cotransmitters that modulate ARC-muscle contractions in vivo. Here we use current clamp, voltage clamp, and contraction measurements to examine the actions of acetylcholine (ACh), the transmitter that induces the contractions.

3. As in the whole ARC muscle, ACh depolarizes unclamped fibers maximally to about —25 mV where, no matter how much ACh is applied, the depolarization saturates.

4. The underlying ACh-activated current is in fact the sum of two quite distinct components, I _ACh,cat and I _ACh,Cl .

5. I _ACh,cat is itself a mixed current carried by cations (physiologically mainly by Na ⁺ , but to a significant degree also by Ca ²⁺ ), reverses near +20 mV, rectifies inwardly, exhibits prominent voltage-dependent kinetics of activation with hyperpolarization, and is selectively blocked by hexamethonium.

6. In contrast, I _ACh,Cl is carried by Cl ^- , reverses near —60 mV, exhibits little rectification or voltage-dependent kinetics, is activated selectively by suberyldicholine, and is blocked by α -bungarotoxin.

7. Both currents activate fast when ACh is applied, desensitize relatively slowly in its presence, then deactivate fast. Both currents are activated at similar ACh concentrations, half-maximally at approximately 10 µM. Both currents also are activated by carbachol and propionylcholine and blocked by d-tubocurarine, bicuculline and paraoxon. Picrotoxin and atropine block I _ACh,cat better, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), and anthracene 9-carboxylic acid I _ACh,Cl better.

8. The two currents are virtually identical to ACh-activated cationic (Na) and Cl currents that are ubiquitous in molluscan neurons. As has been proposed for the neuronal currents, I _ACh,cat resembles vertebrate neuronal nicotinic ACh-receptor (nAChR) currents, whereas I _ACh,Cl resembles vertebrate skeletal muscle nAChR currents.

9. Functionally, we believe that I _ACh,cat serves primarily to depolarize the ARC muscle to open voltage-activated L-type Ca channels, allow Ca ²⁺ influx, and initiate contraction. Physiologically significant Ca ²⁺ may also enter through the ACh,cat channels themselves.

10. By superimposing on I _ACh,cat , I _ACh,Cl brings the reversal potential of the combined current to around —25 mV and thereby sets a relatively negative upper limit to the ACh-induced depolarization. We propose that this is its physiological role. By limiting the depolarization, I _ACh,Cl limits the degree of activation of the Ca current and Ca ²⁺ influx, and so prevents excessive contraction. More importantly, it moderates the voltage during contraction to a range where small voltage changes can finely grade contraction amplitude in this nonspiking muscle.

11. Indeed, in contraction experiments on the single fibers, there is an inverse correlation between the I _ACh,Cl / I _ACh,cat ratio and the magnitude of the ACh-induced depolarization and contraction. Furthermore, increased pharmacological activation of I _ACh,Cl depresses, and block of I _ACh,Cl enhances, both the depolarization and contraction.

12. Obligatory simultaneous coactivation of I _ACh,cat and I _ACh,Cl in the ARC muscle may be part of a peripheral control mechanism that automatically keeps the size of its contractions within behaviorally optimal limits.