Cardiac hypertrophy in the Dahl rat is associated with increased tyrosine phosphorylation of several cytosolic proteins, including a 120 kDa protein.

Because of the well established role that tyrosine phosphorylation (tyr phos) plays in growth factor signalling and regulating cell growth, we hypothesized that cardiac hypertrophy might be associated with altered tyr phos of certain cellular proteins in the heart. Furthermore, we hypothesized that angiotensin II (ang II), a putative growth factor for cardiac cells, might be useful as a probe to highlight any differences in intracellular signalling between normal and hypertrophied hearts. The heart and, for comparison, skeletal muscle, from Dahl S rats, which are predisposed to cardiac hypertrophy, and Dahl R rats, which are not, were examined. Antiphosphotyrosine immunoprecipitation and immunoblotting of heart cell extracts revealed the presence of a constitutively tyr phos 120 kDa cytosolic protein. Hearts from Dahl R rats on a high salt diet displayed a smaller amount of constitutive tyr phos of this protein. In the hearts of both Dahl R and S rats maintained on low salt diets there was little evidence of constitutive tyr phos of this protein. Ang II induced tyr phos of this protein in Dahl S rats on a low salt diet and Dahl R rats on a high salt diet, both of which show mild cardiac hypertrophy. In contrast, the markedly hypertrophied ventricle showed a minimal response to Ang II. Thus the severity of cardiac hypertrophy correlated directly with the tyr phos level of this protein. In an attempt to identify this protein, immunoblotting was carried out with antibodies to the signal transducing proteins rasGAP, JAK2 iNOS, p125FAK, and the Src substrate, pp120, but all proved negative. Ang II also stimulated an increase in tyr phos of proteins with apparent molecular masses of 42, 55, and 69 to 85 kDa in hearts from Dahl S rats on high salt diet. By comparison, there was no 120 kDa tyr phos protein in skeletal muscle even in response to Ang II. Silver stained sodium dodecyl sulfate gels demonstrated that this 120 kDa tyr phos protein is present in substantial amounts in the ventricles of rats fed high salt diets. Thus cardiac hypertrophy is characterized by an abundant 120 kDa cytosolic tyr phos protein, which is apparent with Ang II stimulation in milder degrees of cardiac hypertrophy, and is most likely an as yet uncharacterized protein.