Methionine-enkephalin facilitates the cardiovascular response to epinephrine in the conscious dog

Methionine-enkephalin (ME) is present in high concentrations in the adrenal medulla; it is co-stored with catecholamines in chromaffin vesicles, and released together with catecholamines during adrenal stimulation. We have examined the interactions of intravenously administered bolus doses of ME and epinephrine (EPI) in the conscious dog. EPI, 1.0 microgram/kg, increased mean arterial pressure (MAP) from 104 +/- 6 to 130 +/- 11 mm Hg, while reflexly reducing heart rate(HR) from 103 +/- 13 to 83 +/- 13 beats/min (bpm). ME, 5.0 micrograms/kg, increased MAP from 106 +/- 7 to 122 +/- 7 mm Hg and increased HR from 111 +/- 12 to 139 +/- 14 bpm. EPI and ME administered together increased MAP in apparently additive fashion from 106 +/- 6 to 153 +/- 12 mm Hg, and also increased HR from 102 +/- 10 to 114 +/- 17 bpm. ME, 1.0 microgram/kg, exerted a similar effect. Thus, in these concentrations, ME exerts a co-operative influence upon the EPI cardiovascular response in the conscious, neurologically intact dog, probably by inhibiting baroreceptor reflexes. These findings suggest a possible role for ENK as an excitatory stress hormone.