A kinetic approach for an interpretation of the acetylcholine--d-tubocurarine interaction on chronically denervated skeletal muscle

On the appearance of acetylcholine receptors in denervated rat diaphragm, and its dependence on nerve stump length

Acetylcholine (ACh) sensitivity and extrajunctional receptor distribution of the rat diaphragm were closely monitored during the early period following denervation. Both contracture in response to 10 microgram/ml of ACh and extrajunctional binding of [125I]alpha-bungarotoxin ([125I]alpha-BTX)) were first detectable 30 h after cutting the phrenic nerve in the thorax. If the nerve were cut more proximally, leaving a 3.5 cm distal nerve stump, the same level of ACh contracture and [125I]alpha-BTX binding did not appear until 40 h after operation. This 10-h delay was far longer than the 3-h delay in transmission failure reportedly dependent on stump length. The earliest detectable extrajunctional [125I]alpha-BTX binding appeared throughout the entire muscle fiber, and was not localized to the endplate region as would be expected if degeneration in the nerve terminal induced new receptors. However, later significant increases in [125I]alpha-BTX binding at the endplate region could have resulted from such degeneration. All these results are consistent with neurotrophic regulation of muscle ACh receptors, working via a mechanism involving axonal transport.

The role of extracellular calcium in the contractions produced by acetylcholine in chronically denervated muscle

1 Acetylcholine-induced contractions of the isolated chronically denervated soleus muscle of the mouse consist of two phases, but both phases are equivalent to the contracture phase seen in vivo.2 Low [Ca(2+)](0) (0.5-1.5 mM) augmented peak tension, as well as the rate of relaxation, of the first phase, but inhibited the second phase. Ethyleneglycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) or La(3+) (2 mM) also inhibited the second phase, but not the first.3 It was concluded that the first phase requires Ca(2+) release from the sarcoplasmic reticulum, and is terminated by inactivation of the contractile process. The second phase is caused by the entry of activator Ca(2+) from the extracellular space.4 Increasing [Ca(2+)](o) to 5 or 10 mM after the addition of acetylcholine caused a contraction, starting after a delay of about 50 seconds. EGTA or La(3+) added during the second phase of the acetylcholine contraction caused relaxation after a much shorter lag time.5 It is concluded that most of the Ca(2+) entering from the extracellular fluid is taken up by the sarcoplasmic reticulum.6 The acetylcholine second phase was augmented in low (25 mM) [Na(+)](0). It is concluded that Na(+) and Ca(2+) compete for the acetylcholine controlled ionic channels.7 Isolated chronically denervated diaphragm muscles were less sensitive to acetylcholine and the contraction usually consisted of a first phase only.8 It is concluded that sequestration of Ca(2+) entering from the extracellular fluid is more complete in the diaphragm.

Further evidence that extrinsic acetylcholine acts preferentially on extrajunctional receptors in the chick biventer cervicis muscle

The specificity of action of extrinsic acetylcholine on extrajunctional and junctional receptors in the chick biventer cervicis muscle was studied by determining its ability to protect the responses evoked by acetylcholine and by tetanic nerve stimulation from the blockade by alpha-bungarotoxin, an irreversible binding agent of acetylcholine receptors. At concentrations of 50-100 mug/ml, acetylcholine caused a desensitization to extrinsic acetylcholine but not to nerve stimulation and protected only the contractile response to extrinsic acetylcholine from the toxin blockade whereas neither the response to tetanic nerve stimulation nor the endplate potentials were protected. For the protection of the latter, higher concentrations of acetylcholine were needed. In the presence of physostigmine, a concentration of acetylcholine as low as 10 mug/ml protected the endplate potentials from the toxin blockade. By contrast, d-tubocurarine protected the tetanic contraction and the endplate potentials induced by nerve stimulation at a concentration which produced the same protection of acetylcholine-induced contraction as that produced by 50-100 mug/ml acetylcholine. These results indicate that in contrast to d-tubocurarine, extrinsic acetylcholine at low concentrations acts preferentially on the extrajunctional receptors in the absence of an anticholinesterase.