Cholesterol has a low solubility, but is clustered in the hydrophobic middle of micelles. If these precipitate, then gallstones form. Oral ingestion of bile acids can be therefore one approach to breaking up gallstones, in addition to surgery, interventional radiology, or extraorporeal shock-wave lithotripsy. Trypsin then activates other zymogens by cleaving off polypeptide sequences. Some of the trypsin in the pancreas is active, but inhibited by pancreatic trypsin inhibitor.
Endocrine glands lack ducts and so secrete their hormones directly into the blood.
A particular target tissue may respond to different hormones or combination of hormones. 2 One can often have a priming effect on the target cells. More receptors can be formed on target cells, or up-regulation. In contrast, one can have desensitization of target cells in response to prolonged exposure to polypeptide hormones, from downregulation of the number of receptor proteins. To avoid this, many polypeptide and glycoprotein hormones are secreted in a pulsatile fashion.