Necessity of divalent cations for recovery from carbachol-induced nicotinic acetylcholine receptor inactivation at snake twitch fibre endplates

Necessity of protein kinase C activity for maintenance of acetylcholine receptor function at snake twitch fibre endplates

1. The extent of recovery of endplate sensitivity following a 5 or 10 min exposure to carbachol was determined from measurements of miniature endplate current (m.e.p.c.) amplitudes in voltage-clamped snake twitch fibre endplates. M.e.p.c. amplitude recovery was dependent on the carbachol concentration (0.27-5.4 mM) and duration of application. Staurosporine pretreatment (0.5 microM for approximately 15 min) further decreased the extent of m.e.p.c. amplitude recovery. 2. The decrease in m.e.p.c. amplitude at control endplates exposed to high concentrations of agonist (5.4 mM carbachol for 10 min) was due to an apparent decrease in postsynaptic receptor density, not to a change in the conductance of the acetylcholine (ACh)-activated channels. 3. Pretreatment with either 1 microM lavendustin A or 50 microM KN-62 had no effect on m.e.p.c. amplitude recovery, whereas pretreatment with either 0.5 microM staurosporine, 50 microM sphingosine, or 0.5 microM calphostin C significantly reduced m.e.p.c. amplitude recovery following carbachol exposure. 4. Sphingosine and staurosporine produced a concentration-dependent decrease in the extent of m.e.p.c. amplitude recovery, but had no effect on m.e.p.c. characteristics in the absence of carbachol. In addition, this decrease in m.e.p.c. amplitude was not due to the presence of a subpopulation of small amplitude m.e.p.cs. 5. Prolonged treatment (18-20 h) of muscles with 200 nM phorbol 12-myristate 13-acetate (PMA), to down regulate protein kinase C, resulted in a significant reduction in m.e.p.c. amplitudes following exposure to carbachol. Conversely, treatment with 200 nM 4 alpha PMA, an inactive analogue, had no effect on m.e.p.c. amplitude recovery. 6. Only large amplitude ACh-activated channels (~50 pS) were recorded from fibres either in the presence of 50 micro M sphingosine or from fibres chronically exposed to PMA. However, following recovery from a 10 min exposure to 540 micro M carbachol, both small conductance (-25 pS) and large conductance ACh-activated channels were recorded in both sphingosine- and phorbol-treated preparations. The conductance of these two populations of channels was virtually identical to those seen in staurosporine treated fibres following carbachol exposure.7. We conclude that protein kinase C is required for full recovery of AChR sensitivity following carbachol-induced receptor inactivation. Exposure to high concentrations of agonist for prolonged periods appears to result in the inactivation of a subpopulation of receptors. These receptors must be replaced or reactivated by a process involving protein kinase C. When this phosphorylation step is inhibited, the AChRs remain in an activatable form, but with a reduced conductance.

Requirement of a colchicine-sensitive component of the cytoskeleton for acetylcholine receptor recovery

1. The effect of colchicine treatment on acetylcholine receptor function was examined in potassium depolarized, voltage-clamped snake twitch fibre endplates. Receptor function was assessed by analysis of miniature endplate currents (m.e.p.c.) as well as acetylcholine (ACh)-induced single channel currents. 2. Pretreatment of snake muscle fibres with colchicine (10 microM to 100 microM) for 16-18 h had no effect on m.e.p.c. amplitude or decay rates. At higher concentrations (1 mM), there was a slight decrease in the average m.e.p.c. amplitude. 3. Colchicine produced a concentration-dependent decrease in the extent of m.e.p.c. amplitude recovery following a 10 min exposure to 540 microM carbachol. Exposure of 100 microM colchicine-treated preparations to 0.5 microM staurosporine further reduced the extent of m.e.p.c. amplitude recovery following carbachol exposure. 4. The decrease in m.e.p.c. amplitude following carbachol exposure was not due to a shift in the m.e.p.c. reversal potential. In addition, the distribution of m.e.p.c. amplitudes remained unimodal in both control and colchicine (100 microM)-treated preparations following carbachol exposure. 5. In addition to the normal, large conductance (approximately 48 pS) ACh-activated channels, a population of small conductance (approximately 29 pS) channels was observed in colchicine-treated preparations following exposure to carbachol. In preparations treated with both colchicine and staurosporine and then exposed to carbachol, the conductance of these small channels was identical to that of colchicine or staurosporine alone. 6. We suggest that prolonged exposure of snake twitch fibre endplates to agonist results in the activation and desensitization of ACh receptors. Furthermore, we propose that for a subpopulation of the inactivated receptors, restoration of function requires both the integrity of a subsynaptic cytoskeletal component and phosphorylation by a staurosporine-sensitive protein kinase. One plausible mechanism is that some receptors become destabilized in the membrane and phosphorylation of a cytoskeletal component, whose distribution may depend on an intact microtubular system, is required to re-anchor these receptors. If this anchoring process is inhibited either by disruption of the cytoskeleton with colchicine, or inhibition of the kinase by staurosporine, these receptors remain activatable, but have a reduced conductance.