Epinephrine and norepinephrine are catecholamines which, when released from presynaptic nerve endings, function as neurotransmitters (see here). When released from adrenal medulla in response to low blood glucose levels, epinephrine interacts with second-messenger systems in many tissues, with varied effects. In muscle, epinephrine activates adenylate cyclase, with concomitant activation of glycogenolysis and inhibition of glycogen synthesis.

Triacylglycerol breakdown in adipose tissue is also stimulated by epinephrine, providing fuel for the muscle tissue. In consequence, glucose uptake into muscle is diminished, contributing to an increase in blood glucose levels.

Epinephrine also inhibits insulin secretion and stimulates glucagon secretion. These effects tend to increase glucose production and release by the liver. The net result is to increase blood glucose levels.

Unlike glucagon, the catecholamines have short-lived metabolic effects. Epinephrine action on skeletal and heart muscle cells is a crucial part of the "fight or flight" response.