In a separate but related system, exocrine tissues secrete their products into ducts and then to the outside of the body or to the intestinal tract.
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The G-GTP complex binds adenylate cyclase, activating the enzyme.
The activation of adenylate cyclase leads to c AMP production in the cytosol and to the activation of PKA, followed by regulatory phosphorylation of numerous enzymes. For more information on G-proteins and GPCRs visit the Signal Transduction page.
Tissues capable of responding to endocrines have 2 properties in common: they posses a receptor having very high affinity for hormone, and the receptor is coupled to a process that regulates metabolism of the target cells.
Receptors for most amino acid-derived hormones and all peptide hormones are located on the plasma membrane.
Subsequent to hormone binding, a signal is transduced to the interior of the cell, where second messengers and phosphorylated proteins generate appropriate metabolic responses.
The main second messengers are c AMP, Ca), and diacylglycerol (DAG).
Activation of these receptors by hormones (the first messenger) leads to the intracellular production of a second messenger, such as c AMP, which is responsible for initiating the intracellular biological response.
Steroid and thyroid hormones are hydrophobic and diffuse from their binding proteins in the plasma, across the plasma membrane to intracellularly localized receptors.
Hormones are normally present in the plasma and interstitial tissue at concentrations in the range of 10M.