ABSTRACT
The transductional events responsible for receptor-operated signaling across the cell membrane involve complex mechanisms that have a remarkable spatial and temporal organization. Binding of the extracellular signaling agent to the specific receptor promotes the generation of early signals that are followed by a sequence of multiple biochemical responses that eventually converge into common final pathways leading to late events that may include DNA synthesis and cell division. 1,2 Binding of the ligand to its receptor on the cell membrane triggers the operation of intracellular signals consisting in the production and/or translocation of second messengers such as cyclic nucleotides, phosphoinositide metabolites, and Ca2+. These alterations may cause the activation of enzymes, including protein kinases and protein phosphatases, which alters phosphorylation/dephosphorylation processes and leads to structural and/or functional changes associated with the specific cellular response. These effects may require only a few seconds to develop and may reach a maximum within one or a few minutes after exposure to the signal molecule.
