Ca2+ homeostasis and fast-type sarcoplasmic reticulum Ca(2+)-ATPase expression in L6 muscle cells. Role of thyroid hormone.
The effect of thyroid hormone (L-tri-iodothyronine; T3) on the cytosolic free Ca2+ concentration ([Ca2+]i) in L6 myotubes was studied at rest and during activation to explore the possible mediating role of [Ca2+]i in the T3-induced net synthesis of fast-type sarcoplasmic reticulum (SR) Ca(2+)-ATPase. The mean [Ca2+]i at rest was approx. 115 nM in myoblasts, control myotubes and T3-treated myotubes. Therefore it is unlikely that the T3-induced elevation of Ca(2+)-ATPase levels is mediated by [Ca2+]i changes. To investigate the influence of the 4-fold higher Ca(2+)-ATPase levels in T3-treated myotubes (compared with controls) on [Ca2+]i, interventions with caffeine (10 mM) and a high extracellular K+ concentration ([K+]o) (30 mM) were applied which initially mobilize Ca2+ predominantly from the SR. The results showed a lower (caffeine) or not significantly different (high [K+]o) increase in [Ca2+]i in T3-treated myotubes compared with controls. No rise in [Ca2+]i was found in myoblasts with caffeine or high [K+]o. The role of [Ca2+]i in the regulation of Ca(2+)-ATPase levels was investigated by varying [Ca2+]i through exposure of cells to different concentrations of extracellular Ca2+ (0.2-1.8 mM) and ionomycin (0.1-0.25 microM). At subnormal [Ca2+]i (55 nM) the T3-induced net synthesis of Ca(2+)-ATPase was virtually abolished, and at supranormal [Ca2+]i (195 nM) it was greatly depressed. Intermediate stimulation of net Ca(2+)-ATPase synthesis was found at [Ca2+]i of 95 and 165 nM, with an optimum at approx. 125 nM. Similar but less pronounced effects were found for the basal Ca(2+)-ATPase levels. In contracting primary rat myotubes, Ca(2+)-ATPase levels were significantly lower than in tetrodotoxin-arrested myotubes. The same results were obtained in the presence of T3. Since the mean [Ca2+]i in contracting cells is higher than in resting cells, these data agree with those obtained in the L6 cells with ionomycin. A major conclusion of this study is the existence of a [Ca2+]i optimum, near resting levels, for the expression of the fast-type Ca(2+)-ATPase in the L6 muscle cell line.
Muller
A
A
Laboratory for Physiology, Free University, Amsterdam, The Netherlands.
van Hardeveld
C
C
Simonides
W S
WS
van Rijn
J
J
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002460:Cell Line; D004789:Enzyme Activation / Q000187:drug effects; D015759:Ionomycin / Q000494:pharmacology; D007700:Kinetics; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D009132:Muscles / Q000201:enzymology*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013779:Tetrodotoxin / Q000494:pharmacology; D014284:Triiodothyronine / Q000494:pharmacology
D014284:Triiodothyronine; D002110:Caffeine; D013779:Tetrodotoxin; D015759:Ionomycin; D000252:Calcium-Transporting ATPases; D011188:Potassium; D002118:Calcium