Troponin T- and Troponin I-like Proteins in Bovine Vascular Smooth Muscle

We have tested the hypothesis whether proteins with biochemical and immunochemical properties similar to those of troponin T (TnT) and troponin I (TnI) are expressed in bovine vascular smooth muscle (SM). Three monoclonal anti-TnT antibodies (TT-1, TT-2, and RV-C2) specific for the two isoforms of TnT present in the bovine cardiac muscle and two monoclonal antibodies (TI-1 and TI-5) reacting with cardiac TnI were used in this study. Anti-TnT antibodies were found to be unreactive with 1) skeletal and nonmuscle isoforms of glyceralde-hyde-3-phosphate dehydrogenase, a glycolytic enzyme that shares some structural homologies with skeletal TnT, and 2) calponin, a TnT-like calmodulin/tropomyosin binding protein with some antigenic properties in common with TnT. When tested on SM extracts from aorta or coronary arteries by Western blotting, the anti-TnT antibodies were able to react exclusively with one or two polypeptides whose electrophoretic mobility corresponds to the cardiac TnT subunits. Similarly, anti-TnI antibodies specifically recognized a component in the aortic or coronary SM extracts with electrophoretic properties identical to the cardiac TnI. Immunoflu-orescence analysis performed on the vascular SM cells of bovine aorta, coronary arteries, and intramural branches of coronary vessels confirmed the existence of cardiac troponin immuno-reactivity in these tissues. In addition, differences in the distribution of cardiac TnT- and TnI-like proteins were evidenced in nonvascular and vascular SM cells. This study shows for the first time that polypeptides with some structural properties in common with cardiac TnT and TnI can be found in the vascular SM system.