Control of Hormone Secretion The rate at which a hormone is secreted from an endocrine gland is dependent on the magnitude of the input and whether it is stimulatory or inhibitory in nature. The input in every case is a chemical one, be it an ion (e.g., Ca or a substrate (e.g., glucose) in the plasma, a neurotransmitter such as acetylcholine or norepinephrine, or another hormone. Most endocrine glands are under the direct influence of more than one type of input, which may either reinforce or in terfere with each other's effect. An example of this interaction is found in the control of insulin release from the pancreas. Figure 5.1 shows that the pan- creas, which produces insulin, responds to changes in plasma glucose and amino acids, to norepineph- rine released from sympathetic neurons as well as circulating epinephrine, to parasympathetic n which release acetylcholine, and to a variety of hor mones. Elevations of plasma glucose and amino acids