Question

Are peptide hormones secreted on demand or continuously

Answer 1

Peptide hormone

Hormone are normally present in plasma and interstitial tissue about 10^-7M to 10^-10M concentration. Feedback mechanisms have evolved to regulate the production of peptide hormones. The response of the peptide hormone denoted by the receptors located on plasma membrane. Activation of these receptors by hormones (the first messenger) leads to the intracellular production of a second messenger, such as cAMP, which is responsible for initiating the intracellular biological response. Releasing hormones are synthesized in neural cell bodies of the hypothalamus and secreted at the axon terminals into the portal hypophyseal circulation, which directly bathes the anterior pituitary. These peptides initiate a cascade of biochemical reactions that culminate in hormone-regulated, whole-body biological end points. The secretion of hypothalamic, pituitary, and target tissue hormones is under tight regulatory control by a series of feedback and feed-forward loops. This complexity can be demonstrated using the growth hormone (GH) regulatory system as an example. The stimulatory substance growth hormone releasing hormone (GHRH) and the inhibitory substance somatostatin (SS) both products of the hypothalamus, control pituitary GH secretion. Somatostatin is also called growth hormone-inhibiting hormone (GHIH). Under the influence of GHRH, growth hormone is released into the systemic circulation, causing the target tissue to secrete insulin-like growth factor-1, IGF-1. Growth hormone also has other more direct metabolic effects; it is both hyperglycemic and lipolytic. Systemic IGF-1 also has hypothalamic and pituitary regulatory targets. The negative feedback loops cause down-regulation of GH secretion directly at the pituitary. The longer positive feedback loop, involving IGF-1 regulation at the hypothalamus, stimulates the secretion of GHIH, which in turn inhibits the secretion of growth hormone by the pituitary.

It would suggest that a threshold level of the hormone produce continuously, but the synthesis of hormone change low or high according to demand of immediate response.