Proximity of Na+–Ca2+-exchanger and sarco/endoplasmic reticulum Ca2+ pump in pig coronary artery smooth muscle: fluorescence microscopy

Pig coronary artery smooth muscle expresses the Na+–Ca2+-exchanger NCX1 and the sarco/endoplasmic reticulum (SER) Ca2+ pump SERCA2. NCX has been proposed to play a role in refilling the SER Ca2+ pool. Caveolae may also direct Ca2+ traffic during cell signaling. Here, we use immunofluorescence microscopy to determine if there is proximity between NCX1, SERCA2, and the caveolar protein caveolin-1. Stacks of images of cell surface domains were analyzed. Image stacks for one protein were analyzed for overlap with another protein, with and without randomization or image shifting. Within the resolution of light microscopy, there is significant overlap in the distributions of NCX1, SERCA2, and caveolin-1 but the three proteins are not always co-localized. The proximity between NCX1, SERCA2 is consistent with the assertion that NCX may supply Ca2+ for refilling the SER but this relationship is only partial. Similarly, caveolae may direct traffic in some Ca2+ signaling pathways but not others.