title: The enhancement of Ca2+ efflux from sarcoplasmic reticulum vesicles by urea.
abstract: 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(beta-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 (Ca(2+)-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.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000818:Animals; D001622:Betaine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D009132:Muscles / Q000378:metabolism; D010070:Oxalates / Q000494:pharmacology; D019815:Oxalic Acid; D010710:Phosphates / Q000494:pharmacology; D011073:Polyamines / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D011817:Rabbits; D012430:Ruthenium Red / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D014508:Urea / Q000037:antagonists & inhibitors / Q000494:pharmacology*
chemical_list: D010070:Oxalates; D010710:Phosphates; D011073:Polyamines; D012430:Ruthenium Red; D001622:Betaine; D000244:Adenosine Diphosphate; D014508:Urea; D019815:Oxalic Acid; D008274:Magnesium; D011188:Potassium; D002118:Calcium