Nonsynchronous Accumulation of Alpha-skeletal Actin and Beta-myosin Heavy Chain MRNAs During Early Stages of Pressure-overload--induced Cardiac Hypertrophy Demonstrated by in Situ Hybridization

The development of cardiac hypertrophy secondary to pressure overload is accompanied by isoformic changes of contractile proteins such as myosin and actin. 35S-Labeled complementary RNA (cRNA) probes and in situ hybridization procedures were used for analysis of the regional distribution of newly formed transcripts from alpha-skeletal actin (alpha-sk-actin) and beta-myosin heavy chain (beta-MHC) genes during the early stages of pressure overload. The study was performed in 25-day-old rats submitted to a thoracic aortic stenosis and killed after surgery at times ranging from 4 hours to 3 days. Neither alpha-sk-actin nor beta-MHC messenger RNA (mRNA) was detected in the hearts of normal and sham-operated animals. However, alpha-sk-actin mRNA accumulated throughout the entire left ventricle as early as 4 hours after aortic stenosis, and by 12 hours was also detected in the left atrium. In contrast, beta-MHC mRNA was hardly detectable before day 1, and by days 2-3 was mainly restricted to the inner part of the left ventricle and around the coronary arteries. The absence of spatial and temporal coordination in the accumulation of alpha-sk-actin and beta-MHC mRNAs indicates that different signals and/or regulatory mechanisms are implicated in the induction of the two genes in response to hemodynamic overload.