EXCITATION-CONTRACTION COUPLING OF CULTURED HUMAN SKELETAL-MUSCLE CELLS AND THE RELATION BETWEEN BASAL CYTOSOL CA2+ AND EXCITABILITY

Cultured human skeletal muscle cells are frequently used as a model to study muscle pathology, in which Ca2+ homeostasis might be affected. However, their excitation-contraction (E-C) coupling has been poorly i nvestigated. In order to elucidate E-C coupling of cultured muscle cel ls, we activated the acetylcholine receptors, voltage-dependent Na+ ch annels, dihydropyridine receptors or ryanodine receptors both in the p resence and absence of external Ca2+, as well as after specific inhibi tion, and measured the effects on the cytosolic Ca2+ concentration ([C a2+](i)) using Fura-2. Furthermore, we examined the excitability of th ese cells during iterative high (125 mM) K+ stimulation with various r epolarisation intervals. The resting [Ca2+](i) in muscle cells of cont rols is about 130 nM. Acetylcholine, veratridine, high K+ and caffeine elicit dose-dependent Ca2+ transients, which are independent of extra cellular Ca2+ and can be inhibited by alpha-bungarotoxin, tetrodotoxin , nifedipine or ryanodine. During repetitive K+ stimulation, the excit ability of the muscle cells depends on the repolarisation interval bet ween successive stimulations. Upon shortening the repolarisation time the Ca2+ transients become smaller and slower. Thereby, the basal [Ca2 +](i) rises, the Ca2+ response amplitude declines and both the half-in crease and half-decay time increase. However, if the basal [Ca2+](i) e quals the resting [Ca2+](i), the initial Ca2+ response can be recovere d. The intracellular pH of 7.23, measured by BCECF, is unaffected by r epeated K+ stimulation, whatever the repolarisation interval was. In c onclusion, cultured human skeletal muscle cells possess a 'skeletal mu scle type' of E-C coupling and their excitability at iterative stimula tion is set by their basal [Ca2+](i).