Stress, the onset of physical activity and low blood glucose levels activate the release of epinephrine to maintain the normal levels within the body such as blood pressure, heart rate, cardiac output, glucose levels, oxygen and carbon dioxide levels. [See] How is this done?.
Firstly in times of stress such as physical activity, the sympathetic nervous system is stimulated by changes in the body that is detected by baroreceptors which detect changes in pressure, cardiac mechanoreceptors, peripheral chemoreceptors which detect oxygen and carbon dioxide levels, aortic and carotid arterial mechanoreceptors. [McA07] With these detected changes, the sympathetic nervous system sends the neural stimuli down the neurons to the preganglionic sympathetic fibres of the adrenal medulla which activates the release of acetylcholine. The acetylcholine then stimulates the secretion of epinephrine from the adrenal medulla into the blood stream. [Bea07] Epinephrine continues to circulate around the body via the blood until it meets its target tissue. Once it has reached its target tissue it has to binds to β-adrenergic protein receptors that are found on the outside of the cell because epinephrine cannot cross the hydrophobic plasma membrane of the target cell due to it being a hydrophilic water soluble molecule and the plasma membrane consisting of a biphospholipid layer. Once bound to the receptor the G-proteins are activated. G-proteins consist of 3 subunits (alpha, beta and gamma). One of these G-protein subunit separates from the other two. The separated subunit carries guanosine diphosphate (GDP) which is replaced by guanosine triphosphate (GTP) when the subunit is activated. The activated G-protein subunit then diffuses within the plasma membrane until it encounters adenylate cyclase which is a membrane enzyme that is inactive until it interacts with the G-protein subunit. Once activated, adenylate cyclase catalyses the formation of cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP).