Voltage-dependent mechanisms for raising intracellular free calcium concentration: calcium channels

Single-channel recording by means of the patch-clamp technique provides a method for observing the kinetic properties of individual ion channels. Three distinct types of voltage-dependent calcium channels have so far been characterized: T-type channels are responsible for a rapidly inactivating, transient Ca2+ current; L-type channel openings produce a long-lasting Ca2+ current. N-type channels have the kinetic features of neither T- nor L-type channels. In addition to differences in their gating and conductance properties the three types of channels can also be distinguished by their different sensitivities to pharmacological intervention. The respective distribution of the various types of Ca2+ channels may vary from tissue to tissue. For example, more L-type Ca2+ channels may open during excitation in cardiac and smooth muscle cells than in nerve cells, where the other channel types may prevail. This would explain the different sensitivities of these tissues to dihydropyridines. In view of their functional significance it will be of considerable interest to explore in greater detail the respective densities and sensitivities to neurotransmitters and drugs of various Ca2+ channels in different tissues.