Effects of cyclic nucleotide dependent protein kinases on the endoplasmic reticulum Ca2+ pump of bovine pulmonary artery.

This paper describes the stimulation by cyclic nucleotide dependent protein kinases on the Ca2+ uptake by isolated endoplasmic reticulum (ER) vesicles from the bovine main pulmonary artery. This ER fraction has previously been shown to be highly enriched in phospholamban, a protein kinase substrate that has been well characterized in cardiac sarcoplasmic reticulum (SR), where its phosphorylation is accompanied by an increased rate of Ca2+ uptake. As previously observed for the phosphorylation of phospholamban, the stimulation of the rate of Ca uptake was as high with cGMP dependent protein kinase as with cAMP dependent protein kinase. The effect of phosphorylation of the ER membranes from smooth muscle on the Ca2+ uptake was smaller than that seen in cardiac SR, and it was only observed if albumin was included during the isolation of the membranes. This relatively small effect is probably not due to a lower ratio of phospholamban to Ca2(+)-transport enzyme in the ER membranes as compared to cardiac SR. Several alternative explanations are discussed.