Expression of phenylethanolamine n-methyltransferase in the embryonic rat heart.

Phenylethanolamine N-methyltransferase (PNMT), the final enzyme in the pathway for epinephrine biosynthesis, serves as a marker for tissues and cells producing epinephrine. The present study examines the developmental expression of PNMT in the rat embryo. A transient burst in PNMT mRNA expression begins on embryonic day 9.5 (E9.5), peaks between E10.0 and E11.0, and declines to barely detectable levels by E13.0. Regional localization of PNMT mRNA and enzyme activity demonstrates that PNMT is concentrated in the heart. PNMT has not previously been reported to be expressed at these early stages of development, and its presence in the developing heart suggests that this embryonic tissue may produce epinephrine. Because this catecholamine is known to increase cardiac output and promote the growth of cardiomyocytes, local production of epinephrine by the heart could play an important role in the development of cardiac structure and function.