Developmental Regulation of Phenylethanolamine N‐Methyl Transferase in Avian Sympathetic Nerves

The presence of the catecholamine synthetic enzyme, phenylethanolamine N‐methyl transferase (PNMT), has been detected in the expansor secundariorum, a smooth muscle of the avian wing. The concentration of the enzyme was estimated over a 10‐week time course from 17 days incubation to 9 weeks posthatch and found to increase rapidly up until hatch in parallel with dopamine β‐hydroxylase activity, but then to fall precipitously to very low levels. The time course of the initial increase in activity corresponds to the presence of ingrowing sympathetic nerve fibres, and denervation of the expansor results in loss of >80% of the PNMT activity. It is concluded that during the period of innervation the growing nerves contain the enzyme PNMT and therefore have the capacity to synthesize adrenaline, but that shortly after innervation is complete the capacity to synthesize adrenaline is lost. Several alternate mechanisms are proposed to explain the observations.