INDUCTION OF CELL-CYCLE INHIBITOR P21 IN RAT VENTRICULAR MYOCYTES DURING EARLY POSTNATAL TRANSITION FROM HYPERPLASIA TO HYPERTROPHY

To examine a possible involvement of p21 protein, an inhibitor of cycl in-dependent kinases (CDKs), in the transition from hyperplastic to hy pertrophic growth of rat ventricular myocytes during the first postnat al week, we analysed day-by-day changes in the number of p21 positive cells using specific antibodies against this protein. Paraffin-embedde d sections of the left ventricular myocardium were examined by means o f immunoperoxidase technique and hematoxylin-eosin counterstaining. Wh ile during the first three postnatal days, the positive reaction for p 21 was detected only in a small fraction of myocytes (12-20 %), a sudd en increase in positivity occurred on day 4 (54 %) and continued till day 6 when the fraction of cells expressing p21 reached 87 %. Our resu lts show that the induction of CDK inhibitor p21 in rat ventricular my ocytes is developmentally regulated. Moreover, the fact that the sudde n increase in p21 positivity occurred at the same stage when the myocy te proliferation rapidly ceases, suggests that this protein is likely to be involved in mediating this key event of cardiac development.