Calcium action potentials in innervated and denervated rat muscle fibres.

We studied the generation of calcium action potentials (Ca APs) in innervated and denervated fibres of the extensor digitorum longus of the rat in a tetraethylammonium (TEA) sulphate solution plus 3-4 diaminopiridine (3-4 DAP). The main results are the following: (1) more than 90% of the innervated fibres were capable of developing well-sustained Ca APs that were blocked by Cd or nifedipine; (2) the incidence of Ca APs obtained from the denervated fibres was substantially lower than in the control preparations; (3) no relation was found between the appearance of Ca APs in the denervated fibres and the resting membrane potential (Vm), stimulus duration (500-2000 ms) or holding potential (-80, -100 mV); (4) The percentage of denervated fibres that exhibited Ca APs was increased significantly with the following procedures. First, by raising the external Ca concentration to 14 mM; second, by depleting the intracellular K concentration by overnight exposure of the muscles to a free K-Cs solution; (c) and third, by incubating the muscles in 500 nM apamin, a venom that inhibits the K conductance activated by Ca. Several factors may be involved in the lower incidence of Ca APs obtained in denervated fibres: (1) a diminished Ca current due to a reduction in the driving force as a result of an increment in the intracellular Ca concentration; (2) a persistence of a shunting K conductance that is not inhibited by TEA and 3-4 DAP; (3) a shift in the voltage dependence of the activation and inactivation parameters of the Ca current or the appearance of a new type of Ca channel with a different kinetics.