Membrane biophysics of calcium currents

Voltage-dependent Ca currents have now been identified in almost every excitable membrane. In invertebrate muscle and many egg cells Ca currents produce pure Ca spikes, while in nerve axons and vertebrate skeletal muscle Ca currents contribute little to the action potential. Nerve cell bodies and secretory cells have action potentials with both Na and Ca components. Since the measurement of the Ca reversal potential is practically impossible, the selectivity of Ca channels can only be determined by the current-carrying ability of various ions. The movement of ions through the Ca channel is described in terms of an affinity factor for an external binding site and a mobility factor for crossing the membrane. The biophysical study of Ca currents has been limited by the absence of preparations where control of membrane potential was satisfactory. Recently, spherical cells such as isolated ganglion cells and egg cells have allowed more satisfactory voltage clamp studies. However, the separation of the Ca current from the background currents is a much more difficult problem than was the isolation of the Na current. This difficulty is due to the multiple interrelations between the background current and the Ca current. In general alterations that change the Ca current also appear to change the background current.