INVOLVEMENT OF PROTEIN-KINASES IN THE UP-REGULATION OF ACETYLCHOLINE-RELEASE AT END-PLATES OF ALPHA-BUNGAROTOXIN-TREATED RATS

1. ACh release from motor nerve endings in diaphragms of rats treated chronically with alpha-bungarotoxin (alpha-BuTX) is upregulated at the level of the individual endplate. Involvement of protein kinases in t his mechanism of synaptic adaptation was investigated. 2. Miniature en dplate potentials (MEPPs) and endplate potentials (EPPs) were recorded after mu-conotoxin treatment, which prevents muscle action potentials . The quantal content at endplates was calculated 'directly', i.e. by dividing the EPP amplitude by the MEPP amplitude. 3. Incubation of mus cles from control and alpha-BuTX-treated rats with K-7, a protein kina se C (PKC) inhibitor, reduced MEPP amplitudes but had no clear effect on quantal contents. Polymyxin B, another PKC inhibitor, had a similar effect on muscles from alpha-BuTX-treated rats. 4. Incubation of musc les from alpha-BuTX-treated rats with K252a, a broad-spectrum protein kinase inhibitor of, amongst others, PKC, Ca2+-calmodulin-dependent pr otein kinase II (CaMKII) and neurotrophin receptor tyrosine kinases, r esulted in a 30% decrease of the quantal content. However, K252a did n ot change the quantal content of controls. Incubations with the closel y related compound K252b, which has an exclusively extracellular actio n, had a similar effect. 5. KN62, a specific inhibitor of CaMKII, decr eased the mean quantal content of muscles from alpha-BuTX-treated rats by 18%. 6. Tyrphostin 51, a selective tyrosine kinase inhibitor, had no effect on quantal contents of muscles from alpha-BuTX-treated and c ontrol rats. However, it increased the frequency and amplitude of MEPP s in muscles from alpha-BuTX-treated rats, leaving those of controls u nchanged. 7. The extent of reduction of quantal content, caused by K25 2a, K252b and KN62, varied between endplates of individual muscles fro m alpha-BuTX-treated rats; quantal contents at endplates with small ME PPs were more sensitive than those at endplates with large MEPPs. 8. I t is concluded that PKC does not play a role in the mechanism of upreg ulation of ACh release at endplates of alpha-BuTX-treated rats. Instea d, CaMKII and some tyrosine kinases in the presynaptic membrane, as we ll as in the cytoplasm, might be involved.