Motoneuronal location and acetylcholine sensitivity of developing rat slow and fast twitch muscle fibres

The postnatal changes of the extrajunctional acetylcholine (ACh) sensitivity of fast twitch (extensor digitorum longus muscle, EDL) and slow twitch (soleus muscle, SOL) of the rat were investigated in vitro. As a measure of the ACh-sensitivity serves the threshold dose of ACh necessary to evoke a contracture. Immediately after birth rat muscles show a high extrajunctional ACh-sensitivity, but the SOL exhibits a somewhat higher sensitivity (threshold dose 0.5-5 microM ACh/l) than the EDL (threshold dose 5-20 microM ACh/l). These sensitivities are unchanged up to the 8th (EDL) or 10th (SOL) day of life, afterwards the ACh-sensitivity decreases. Values of adult EDL (unsensitivity against ACh in the bathing solution) and SOL (threshold dose 1-5 mM ACh/l) are reached around the 16th (EDL) and 20th (SOL) day after birth. Fast twitch muscles innervated by motoneurons of the cervical spinal cord (sternomastoideus muscle or biceps brachii muscle) show an accelerated decrease of ACh-sensitivity in comparison to fast twitch EDL innervated by motoneurons located in the lumbar spinal cord according to the cranio-caudal relation of the development of the spinal cord.