Adenylate cyclase: Its probable localization in sarcoplasmic reticulum as well as sarcolemma of the canine heart ☆

Abstract Cardiac microsomes prepared by two different methods were compared in terms of Ca-binding and Ca-uptake, believed to represent markers for fragmented sarcoplasmic reticulum, and the (Na + + K + )-activated ATPase activity considered to be a marker for the plasma membrane. Microsomes prepared in dilute buffer (H 2 O-microsomes) contain two to four times the activity of sarcoplasmic reticulum markers when compared to microsomes prepared in 10% sucrose (suc-microsomes). Conversely, the plasma membrane marker was present in greater amounts in the suc-microsomes. Basal, epinephrine-stimulated and NaF-stimulated adenylate cyclase activities were slightly higher in H 2 O-microsomes, as was the degree of stimulation by the β-adrenergic agonist. These findings provide evidence that both β-receptor and adenylate cyclase activities may be present in the heart's sarcoplasmic reticulum, as well as in the sarcolemma.