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 (ty r phos) plays in growth factor signalling and regulating cell growth, we hypothesized that cardiac hypertrophy might be associated with alte red tyr phos of certain cellular proteins in the heart. Furthermore, w e hypothesized that angiotensin II (ang II), a putative growth factor for cardiac cells, might be useful as a probe to highlight any differe nces in intracellular signalling between normal and hypertrophied hear ts. The heart and, for comparison, skeletal muscle, from Dahl S rats, which are predisposed to cardiac hypertrophy, and Dahl R rats, which a re not, were examined. Antiphosphotyrosine immunoprecipitation and imm unoblotting of heart cell extracts revealed the presence of a constitu tively tyr phos 120 kDa cytosolic protein. Hearts from Dahl R rats on a high salt diet displayed a smaller amount of constitutive tyr phos o f 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 t his 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 sh ow mild cardiac hypertrophy. In contrast, the markedly hypertrophied v entricle showed a minimal response to Ang II. Thus the severity of car diac hypertrophy correlated directly with the tyr phos level of this p rotein. In an attempt to identify this protein, immunoblotting was car ried out with antibodies to the signal transducing proteins rasGAP, JA K2 iNOS, p125(FAK), and the Src substrate, pp120, but all proved negat ive. Ang II also stimulated an increase in tyr phos of proteins with a pparent molecular masses of 42, 55, and 69 to 85 kDa in hearts from Da hl S rats on high salt diet. By comparison, there was no 120 kDa tyr p hos protein in skeletal muscle even in response to Ang II. Silver stai ned sodium dodecyl sulfate gels demonstrated that this 120 kDa tyr pho s protein is present in substantial amounts in the ventricles of rats fed high salt diets. Thus cardiac hypertrophy is characterized by an a bundant 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.