Hormone (amines and peptide or protein hormones) are water-soluble and act on the surface of target cells via second messengers; steroid hormones, being lipid-soluble, move through the plasma membranes of target cells (both cytoplasmic and nuclear) to act within their nuclei. Hormone secretion may occur in many tissues. Endocrine glands are the cardinal example, but specialized cells in various other organs also secrete hormones. Hormone secretion occurs in response to specific biochemical signals from a wide range of regulatory systems. For instance, serum calcium concentration affects parathyroid hormone synthesis; blood sugar (serum glucose concentration) affects insulin synthesis; and because the outputs
Glycogen synthase expression of the muscle isozyme and not the liver isozyme. However, muscle-specific glycogen synthase activation may lead to excessive accumulation of glycogen, leading to damage in the heart and central nervous system following ischemic insults. The reaction is highly regulated by allosteric effectors such as glucose-6-phosphate (activator) and by phosphorylation reactions (deactivating). Glucose-6-phosphate allosteric activating action allows glycogen synthase to operate as a glucose-6-phosphate sensor. The inactivating phosphorylation is triggered by the hormone glucagon, which is secreted by the pancreas in response to decreased blood glucose levels. The enzyme also cleaves the ester bond between the C1 position of glucose