Control of sarcoplasmic/endoplasmic-reticulum Ca2+ pump expression in cardiac and smooth muscle

Cardiac muscle expresses sarcoplasmic/endoplasmic-reticulum Ca2+ pump isofo rm SERCA2a; stomach smooth muscle expresses SERCA2b. In 2-day-old rabbits, cardiac muscle contained levels of SERCA2 protein that were 100-200-fold th ose in the stomach smooth muscle. In nuclear run-on assays, the rate of SER CA2 gene transcription in heart nuclei was not significantly higher than in the stomach smooth-muscle nuclei. However, the SERCA2 mRNA levels (mean +/ - S.E.M.) were (29 +/- 4)-fold higher in the heart. In both tissues the SER CA2 mRNA was associated with polyribosomes, In a sucrose-density-gradient s edimentation velocity experiment on polyribosomes, there was no difference in the sedimentation pattern of SERCA2 mRNA between the two tissues, sugges ting that the translation efficiency of SERCA2 RNA in the two tissues is qu ite similar. Thus the main difference in the control of SERCA2 expression i n the two tissues is posttranscriptional and pretranslational.