Ca++-binding properties of cznine aortic microsomes: lack of effect of c-AMP

Ca++-binding to a microsomal fraction canine aorta has been studied and compared to Ca++-binding to canine cardiac microsomes. The aortic microsomes bound up to 60 nmoles Ca++/mg protein with a first-order binding rate constant of 0.025 sec-1. The binding rate of 5 sec was about half that of cardiac microsomes, 8 versus 15 nmoles/mg. The Ca++-binding to the aortic microsomes had a pH optimum of 7.4, was inhibited by monovalent cations and showed ATP preference when compared to other nucleotides. The binding was not stimulated by the presence of oxalate. c-AMP stimulated incorporation of 32P into the preparation, but had no effect on Ca++-binding.