Desensitization in the innervated and in the chronically denervated soleus muscle of the mouse

Force and membrane potential in acetylcholine and potassium contractures of denervated mouse muscles

Depolarization and contracture force (P) provoked by acetylcholine (ACh) and by K ions were studied in bundles dissected from mouse soleus muscles that had been denervated for 4-7 days. Cl-free solutions were used. The muscle fibres were depolarized by solutions containing 150 mM K or 10 microM ACh to nearly zero mV resulting in maximum P (Pmax). Threshold P was produced when the membrane was depolarized to more than about -60 mV by both agents. 50% Pmax was produced by [K] causing the membrane to depolarize to -42 mV, whereas a potential more positive than -20 mV was required for 50% Pmax to be produced when ACh was used. The rate of depolarization was always higher for ACh than for K. Pretreatment by 0.05 microM ACh (about threshold for P) did not affect the P-[K] relation appreciably showing that ACh did not "stabilize" the membrane. Nearly equal P was provoked by successive applications of just suprathreshold agent concentrations when the order of application was ACh----K but not with the reverse order. Hypertonicity (by addition of 300 mM sucrose to all solutions) caused PACh to decrease and PK to increase. It was concluded that the ACh receptors are located in the surface membrane of the muscle fibres, not in the T-system membranes.

Desensitization in denervated mouse muscles

In denervated mouse soleus (DSOL) muscle preparations washed in Na methanesulfonate solutions containing 30 mmol X 1(-1) K+, bath-applied ACh (55 mumol X 1(-1) ) caused the resting potential to decrease from about -36 to -3.2 mV within 2-4 s; the potential remained stable or became slightly more negative when ACh was applied for 1- 2 min. Two components of the membrane current change (ACh current) were found in DSOL when using a point voltage clamp, an initial current component declining to 1/2 the initial amplitude in 11-15 s (desensitization) and a steady late current comprising 16-47% of the maximum ACh current. Membrane conductance (in microseconds X cm-2) was 0.35 in the absence of ACh, 20.3 at the peak of the initial current, and 1.57 during late current. The late current saturated at 0.55 to 5.5 mumol X 1(-1) ACh, whereas the initial current required 55 or more mumol X 1(-1) ACh to saturate. The null (reversal) potential was 6-13 mV more positive for the late current than for the initial current. The late current was masked when Na+ was replaced by Tris+, sucrose, or K+. An initial and a late current could also be distinguished in non-denervated endplates. The late current was more sensitive to ACh than the initial current, but the null potential was more negative than that for the initial current in endplates. In denervated membrane, the half time of desensitization was increased when Mg2+ was replaced by Ca2+ but the changes were on the average less than 15% the control values. It was concluded that desensitizing and non-desensitizing receptors may exist in extrajunctional membranes of denervated muscles and in endplates, the two being attached to different ionic channels.

The effects of glycerol detubulation, of manganese ions, of dantrolene and of nitrate ions on the responses of isolated chronically denervated soleus muscles of the mouse to acetylcholine

1 In the isolated chronically denervated soleus muscle of the mouse acetylcholine produced a biphasic contraction. 2 In muscles detubulated by glycerol treatment, the first phase of the acetylcholine contraction was almost abolished, but the second was only slightly reduced or was unaffected. 3 Manganese ions (10 mM) reduced the first phase of the acetylcholine response, but enhanced the second. 4 Dantrolene sodium (5.94 x 10(-5) M) reduced the first phase of the acetylcholine response, but not the second. 5 Nitrate ions (118 mM) augmented the first phase but not the second. 6 It was concluded that the first phase requires the excitation-contraction-coupling sequence: membrane depolarization-T-tubules-Ca2+ release from sarcoplasmic reticulum. In contrast, the second phase appears to occur independently of these processes.

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.