Calcium currents of frog and insect skeletal muscle fibres measured during voltage clamp

Both vertebrate and invertebrate skeletal muscle fibres have Ca2+ permeability mechanisms which are turned on by depolarization of the surface membrane. In frog muscle, Ca currents are extremely slow and will be scarcely activated during the action potential that normally elicits a twitch. This Ca permeability cannot therefore play any substantial, direct role in excitation--contraction coupling. In insect (Carausius morosus) muscle, Ca currents activate within milliseconds of depolarization, even at low temperature, and may well play at least a triggering role in excitation--contraction coupling. These Ca currents show saturation with increasing [Ca]0, while the instantaneous current--voltage relation rectifies inwards, as expected from a very low [Ca]i. The Ca channel is permeable to Sr2+ and Ba2+. Inactivation of Ca currents under a maintained depolarization depends on Ca2+ carrying inward current, however, rather than on the depolarization itself.