title: Reversible inhibition of sarcoplasmic reticulum Ca-ATPase by altered neuromuscular activity in rabbit fast-twitch muscle.
abstract: A 50% decrease in both the initial rate and the total capacity of Ca2+ uptake by the sarcoplasmic reticulum (SR) occurred 2 days after the onset of chronic (10 Hz) nerve stimulation in rabbit fast-twitch muscle. Prolonged stimulation (up to 28 days) did not lead to further decreases. This reduction, which was detected in muscle homogenates using a Ca2+-sensitive electrode, was reversible after 6 days cessation of stimulation and was not accompanied by changes in the immunochemically (ELISA) determined tissue level or isozyme characteristics of the SR Ca2+-ATPase protein. However, as measured in isolated SR, it correlated with a reduced specific activity of the Ca2+-ATPase. Kinetic analyses demonstrated that affinities of the SR Ca2+-ATPase towards Ca2+ and ATP were unaltered. Positive cooperativity for Ca2+ binding (h = 1.5) was maintained. However, a 50% decrease in Ca2+-dependent phosphoprotein formation indicated the presence of inactive forms of Ca2+-ATPase in stimulated muscle. The reduced phosphorylation of the enzyme was accompanied by an approximately 50% lowered binding of fluorescein isothiocyanate, a competitor at the ATP-binding site. In view of the unaltered affinity for ATP, this finding suggests that active Ca2+-ATPase molecules coexist in stimulated muscle with inactive enzyme molecules, the latter displaying altered properties at the nucleotide-binding site.
mesh_terms: D000255:Adenosine Triphosphate / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004558:Electric Stimulation; D016650:Fluorescein-5-isothiocyanate; D005452:Fluoresceins; D007120:Immunochemistry; D008297:Male; D009124:Muscle Proteins / Q000378:metabolism; D009469:Neuromuscular Junction / Q000502:physiology*; D010750:Phosphoproteins / Q000096:biosynthesis; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013861:Thiocyanates
chemical_list: D005452:Fluoresceins; D009124:Muscle Proteins; D010750:Phosphoproteins; D013861:Thiocyanates; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D016650:Fluorescein-5-isothiocyanate; D002118:Calcium