EXPRESSION OF ISOFORMS OF INTERNAL CA2+ PUMP IN CARDIAC, SMOOTH-MUSCLE AND NONMUSCLE TISSUES

Smooth muscle and several non-muscle tissues contain mRNA for an alternative splice of the mRNA for the cardiac sarcoplasmic reticulum (SR) Ca,Mg-ATPase. Based on amino acid composition deduced from cDNA sequences the cardiac isoform (I(c)) is 110 kDa while the smooth muscle and the non-muscle isoform (I(s)) is 115 kDa. This prediction in their molecular masses was tested at the protein level in rabbit stomach, aorta, uterus and vas deferens smooth muscles; stomach mucosa, brain, liver, kidney and heart. The major species of the acylphosphates formed in the presence of Ca2+ and electrophoresed in acid SDS-acrylamide gels were 5 kDa smaller for the heart (I(c)) than those for all the other tissues (I(s)). The size difference was also confirmed in Western blots using a monoclonal antibody which binds to both I(s) and I(c). Thus consistent with the mRNA splices for the internal Ca2+ pumps previously reported to be present in these tissues, rabbit heart expresses predominantly the Ca2+ pump protein I(c) and the various smooth muscle, mucosa, brain, liver and kidney express mainly the isoform I(s).