Urea, in nondenaturing concentrations, inhibited Ca2+ uptake by sarcoplasmic reticulum vesicles with no concomitant effect on ATP hydrolysis. This inhibition was antagonized by 5 mm oxalate and 20 mm orthophosphate. At concentrations of 0.2 to 1.0 m, urea induced an increase in the Ca2+ efflux from preloaded vesicles diluted in a medium at pH 7.0 containing 2 mm ethylene glycol bis(β-aminoethyl ether)N,N′-tetraacetic acid, 0.1 mm orthophosphate, and 0.1 mm MgCl2. The urea-induced efflux was arrested by ligands of the (Ca2+-Mg2+) ATPase, namely, K+, Mg2+, Ca2+, and ADP, and by ruthenium red and the polyamines spermine, spermidine, and putrescine. In the case of polyamines a dissociation between the effect on the efflux and the net Ca2+ uptake was observed, as only the efflux could be blocked by the drugs. Glycine betaine, trimethylamine-N-oxide, and sucrose antagonized the effects of urea on both the net Ca2+ uptake and the rate of Ca2+ efflux.
ASJC Scopus subject areas
- Molecular Biology