somatostatin, a 14 amino-acid peptide, binds to G-protein receptors on cell surfaces and inhibits the release of numerous physiological compounds, including insulin, glucagon, gastrin, and secretin. This peptide also functions as a neurotransmitter and recently received notoriety when it was shown to have an inhibitory effect on tumor angiogenesis.
Other examples of regulatory peptides are
endothelin, a potent contractor of smooth muscle, and oxytocin, a reproductive hormone that is essential for uterine contractions during labor.
Endothelins are 21-amino acid vasoconstricting peptides produced primarily in the endothelium having a key role in vascular homeostasis.
