Comparison of the effects of fluoride on the calcium pumps of cardiac and fast skeletal muscle sarcoplasmic reticulum: evidence for tissue-specific qualitative difference in calcium-induced pump conformation.

Comparison of the effects of fluoride (NaF, 1-10 mM) on the catalytic and ion transport functions of the Ca(2+)-ATPase in sarcoplasmic reticulum (SR) vesicles isolated from rabbit cardiac and fast-twitch skeletal muscles revealed similarities as well as striking tissue-specific differences depending on the experimental conditions employed. Short preincubation (3 min at 37 degrees C) of cardiac or fast muscle SR with fluoride in the absence of Ca2+ and ATP prior to initiating enzyme turnover by simultaneous addition of Ca2+ and ATP to the assay medium resulted in a strong inhibitory effect of fluoride on ATP-energized (oxalate-facilitated) Ca2+ uptake and Ca(2+)-ATPase activity. On the other hand, when turnover was initiated by the addition of ATP to SR preincubated with fluoride in the presence of Ca2+ but in the absence of ATP, fluoride caused concentration-dependent stimulation of active Ca2+ uptake by fast muscle SR with no appreciable change in Ca(2+)-dependent phosphoenzyme (EP) formation (from ATP) or Ca(2+)-ATPase activity but inhibition of active Ca2+ uptake by cardiac SR with concomitant inhibition of EP formation and Ca(2+)-ATPase activity. Exposure of cardiac or fast muscle SR to fluoride in the presence of both Ca2+ and ATP resulted in concentration-dependent stimulatory effect of fluoride on Ca2+ uptake with no change in EP formation or Ca(2+)-ATPase activity, this effect diminished substantially at saturating oxalate concentration in the assay. Assessment of the effects of deferoxamine (1 mM) and exogenous aluminum (10 microM) did not indicate a requirement for aluminum in the inhibitory or stimulatory effect of fluoride. These results suggest that (a) the Ca2+ and ATP-deprived (E1/E2) but not the Ca2+ plus ATP-liganded (CaE1ATP) conformation of the SR Ca(2+)-ATPase is susceptible to inhibition by fluoride in both cardiac and fast muscle; (b) the Ca(2+)-bound conformation (CaE1) of the SR Ca(2+)-ATPase is susceptible to inhibition in cardiac muscle but is refractory to fluoride in fast muscle; and (c) the stimulatory effect of fluoride is largely secondary to its ability to mimic the action of oxalate in intravesicular Ca2+ trapping when the fluoride-resistant enzyme is turning over normally. Fluoride inhibited phosphorylation of the Ca(2+)-free enzyme by Pi in cardiac and fast muscle SR indicating that fluoride sensitivity of the phosphorylation site of the SR Ca(2+)-ATPase is similar in cardiac and fast muscle.(ABSTRACT TRUNCATED AT 400 WORDS)