“In the assay medium containing 3 mM ATP, 10 uM GTP and nucleotide regenerating system, the replacement of Mg2+ by … Mn2+ caused a progressive loss of PGI2 as well as TSH-stimulated adenylate cyclase activities, while high concentrations of Mg2+ (12 or 18 mM) … activity stimulated by either PGI2 ….”