What are Hormones? What is a hormone? What are the types of hormones?
What are endocrine organs?
The endocrine organs are ductless glands which secrete hormones that have actions at remote sites. They have an important regulatory influence on cellular metabolism. Along with the nervous system, the endocrine system has both the adaptive and the vegetative functions, which often overlap and are closely related. The neurohypophysis and adrenal medulla which are in fact parts of the nervous system are functionally endocrine glands as they release vasopressin and adrenaline respectively. Similarly the peptides with opiate activity and certain gut hormones like gastrin and cholecystokinin are also present in the nervous system.
What are Hormones?
What is a hormone? The word hormone is derived from a Greek verb meaning “to set in motion” or “to spur on”. It is a substance which is released in one tissue and through the circulation it reaches the target tissue where it elicits a particular response.
What are Prohormones?
Prohormone is a biological inactive precursor of the hormone which on cleavage releases the active hormone.
What are the types of hormones?
There are two broad categories of hormones-peptide and steroid. The peptide hormones include:
- Complex polypeptides: e.g. LH, HCG.
- Intermediate sized peptides: e.g Insulin, glucagon.
- Small peptides: e.g. TRH.
- Dipeptides: e.g. T4 and T3 (Amine hormones)
- Derivatives of single amino acid (Amino hormones) e.g. catecholamines, serotonin and histamine. Steroid hormones are of two types:
- Those with intact steroid nucleus: e.g. adrenal and gonadal steroid.
- Those in which the beta ring of the steroid is open e.g. vitamin D and its metabolites.
How are Hormones produced?
The synthesis of various hormones is genetically determined. The peptide hormone is synthesized within the cisterni of the endoplasmic reticulum and transported to the golgi complex. It is then released into the extracellular fluid, either by exocytosis, pinocytosis or diffusion e.g. Insulin is secreted as pre-pro-insulin (Mol Wt. 11,500) and converted to proinsulin, (Mol Wt. 9,000) and then insulin, (Mol Wt. 6,000) by cleavage.
The steroid hormone is synthesized from cholesterol, whereas amine hormones are synthesized from tyrosine.
How are hormones Stored AND Released?
Most of the hormones synthesized are released and stored to a limited extent. Though the thyroid gland does not store thyroxine in large quantities, about 2 weeks supply is stored in thyro- globulin.
The release of most of the hormones stimulate the synthesis of fresh hormone. In addition, in case of ACTH and Cortisol, the release is predominantly pulsatile.
How are hormones transported?
The polypeptide hormones like insulin circulate as free hormones. When they are injected, antibodies to the hormone are formed and they combine with the hormone to form antigen-antibody complex which acts as a reservoir from which the hormone can be released.
The steroid and thyroid hormones circulate in blood bound to specific plasma proteins with high affinity so that only a small amount of the hormone is free. It is this free hormone which plays a role in the physiologic regulatory mechanisms. The free hormone is also responsible for the endocrine status of the person. The bound fraction probably serves as a reservoir which buffers the plasma against rapid fluctuations in hormonal release. It also decreases the excretion of the hormone.
This is a brief answer to the question, What are Hormones.