Voltage-dependent Calcium and Potassium Conductances in Striated Muscle Fibers from the Scorpion, Centruroides Sculpturatus

Ionic currents responsible for the action potential in scorpion muscle fibers were characterized using a three-intracellular microelectrode voltage clamp applied at the fiber ends (8-12 degrees C). Large calcium currents (ICa) trigger contractile activation in physiological saline (5 mM Ca) but can be studied in the absence of contractile activation in a low Ca saline (< or = 2.5 mM). Barium (Ba) ions (1.5-3 mM) support inward current but not contractile activation. Ca conductance kinetics are fast (time constant of 3 msec at 0 mV) and very voltage dependent, with steady-state conductance increasing e-fold in approximately 4 mV. Half-activation occurs at -25 mV. Neither ICa nor IBa show rapid inactivation, but a slow, voltage-dependent inactivation eliminates ICa at voltages positive to -40 mV. Kinetically, scorpion channels are more similar to L-type Ca channels in vertebrate cardiac muscle than to those in skeletal muscle. Outward K currents turn on more slowly and with a longer delay than do Ca currents, and K conductance rises less steeply with voltage (e-fold change in 10 mV; half-maximal level at 0 mV). K channels are blocked by externally applied tetraethylammonium and 3,4 diaminopyridine.