Proto-oncogene expression in the isolated working rat heart: combination of pressure and volume overload with norepinephrine.

We have studied the effects of pressure and volume overload as well as of norepinephrine (NE) alone and in combination on the expression of the proto-oncogenes c-fos and c-myc in isolated perfused working rat hearts. Using specific cDNA clones, the mRNAs of c-fos and c-myc were measured by Northern blots and quantified with densitometry. Total RNA was isolated from hearts after stimulation with NE (3 x 10(-8) M), after increasing left atrial filling pressure (preload) from 8 to 16 cmH2O, and after elevation of afterload from 80 to 100 cmH2O for 15, 30, 60, 90, and 120 min. NE increased the expression of c-fos and c-myc after 30 and 60 min, respectively. This enhancement occurred at about the same time as that induced by elevation of pre- and afterload, but was more pronounced. Combination of NE with pre- and afterload elevation induced the c-fos mRNA signal to appear earlier and to persist for a longer period of time. Similar results were obtained in regard to the c-myc mRNA, however, the signal was more pronounced in intensity. Simultaneous pre- and afterload increase also led to an earlier expression of the proto-oncogenes and to a longer duration of the signals. These results indicate that the combination of two hypertrophy-inducing stimuli, which may cause a higher degree of cardiac hypertrophy in vivo, induce an earlier, more pronounced and longer-lasting expression of the protooncogenes c-fos and c-myc.