Correlation between function and proto-oncogene expression in isolated working rat hearts under various overload conditions.

Abstract

In isolated perfused working rat hearts we have studied the effects of norepinephrine (NE) and of pressure as well as volume overload alone, and in combination on heart function and expression of the proto-oncogenes c-fos and c-myc. This preparation was functionally stable over the observation period of 120 min. NE (3x10(-8) M) induced an immediate and sustained increase in aortic and coronary flow as well as in cardiac output. Volume overload was established by increasing left atrial filling pressure (preload) from 8-16 cm H2O and resulted in a marked increase in aortic flow and cardiac output which subsided somewhat over time. Pressure overload was created by elevation of afterload from 80-100 cm H2O and induced a decrease in aortic flow and a slight increase in coronary flow. Using specific cDNA clones, the mRNAs of c-fos and c-myc were measured by Northern blots and quantified with densitometry. In all three conditions, c-fos mRNA was increased first, after 30 min. It was more pronounced and of longer duration in the NE-stimulated hearts. The increase in c-myc-mRNA occurred after 60 or 90 min. After 120 min, all signals were normal, although heart function was still altered in a manner specific for the respective intervention. Combination of NE with preload and afterload elevation as well as combination of preload and afterload elevation led first to an increase in cardiac output which was followed by a decline to various degrees. In all these combinations, the c-fos mRNA signal appeared earlier, after 15 min, and persisted for a longer period of time compared with the effect of a single stimulus. The c-myc mRNA was increased later, after 30 or 60 min. This increase persisted throughout the entire observation period, showing a further progressive rise. These results show that stimuli which cause cardiac hypertrophy in vivo induce a transient and sequential increase in proto-oncogene expression in the isolated working rat heart. Combination of two hypertrophy-inducing stimuli elicit an earlier, more pronounced and longer-lasting expression of the proto-oncogenes c-fos and c-myc, while functional parameters deteriorate.