Embryonic epinephrine synthesis in the rat heart before innervation - Association with pacemaking and conduction tissue development

Epinephrine is a potent neurotransmitter and hormone that can influence car diac performance beginning shortly after the first myocardial contractions occur in developing vertebrate embryos, fn the present study, we provide ev idence that the heart itself may produce epinephrine during embryonic devel opment. Using antibodies that selectively recognize the catecholamine biosy nthetic enzymes, tyrosine hydroxylase, dopamine beta -hydroxylase, and phen ylethanolamine N-methyltransferase, we used coimmunofluorescent staining te chniques to identify cardiac cells that have the capability of producing ca techolamines. Initially, cells expressing catecholamine biosynthetic enzyme s were found interspersed throughout the myocardium, but by embryonic day 1 1.5 (E11.5), they became preferentially localized to the dorsal venous valv e and atrioventricular canal regions. As development proceeded, catecholami ne biosynthetic enzyme expression decreased in these regions but became qui te strong along the crest of the interventricular septum by E16.5. This exp ression pattern was also transient, decreasing in the ventricular septum by E19.5. These data are consistent with a transient and progressive associat ion of catecholamine-producing cells within regions of the heart that becom e the sinoatrial node, atrioventricular node, and bundle of His. This is th e first evidence demonstrating that intrinsic cardiac adrenergic cells may be preferentially associated with early pacemaking and conduction tissue de velopment.