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BIMM118
Sources of Ca ++ :
Calcium as Second Messenger
• Extracellular compartment: (predominantly in nerve, cardiac and smooth muscle cells)
Three types of plasma-membrane localized calcium channels:
– Voltage operated calcium channels:
Action potental depolarizes plasma membrane, which results in the opening of “voltage”
dependent calcium channels (channels can be opened by increase in extracellular K + ).
Each channel protein has four homologous domains, each containing six membrane spanning
α-helices (the fourth one functions as the “voltage” sensor.