Evidence for the presence of enkephalins in the heart

Extracts of guinea pig hearts were subjected to high performance liquid chromatography (HPLC) and the eluted fractions monitored by radioimmunoassays (RIA) for their content of leucine5-enkephalin (Leu-ENK) and methionine5 enkephalin (Met-ENK). Distinct peaks of both Leu-ENK and Met-ENK immunoreactivity were found corresponding to the position of synthetic Leu-ENK and Met-ENK respectively. The ratio of Leu-ENK to Met-ENK content was about 1:4. Chemical sympathectomy with 5-hydroxydopamine (6-OH-DA) produced a dramatic fall in noradrenaline content of the heart by more than 99%, whereas the concentration in Leu-ENK was reduced by only 70%. The Leu-ENK content of the adrenal glands was not affected by this treatment. These observations point to an enkephalinergic innervation of the heart which appears to be mainly of sympathetic origin. The results suggest the participation of enkephalins in cardiac reflex mechanisms.