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 calciu m currents (I(Ca)) trigger contractile activation in physiological sal ine (5 mM Ca) but can be studied in the absence of contractile activat ion in a low Ca saline (less-than-or-equal-to 2.5 mM). Barium (Ba) ion s (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 v ery voltage dependent, with steady-state conductance increasing e-fold in approximately 4 mV. Half-activation occurs at -25 mV. Neither I(Ca ) nor I(Ba) show rapid inactivation, but a slow, voltage-dependent ina ctivation eliminates I(Ca) at voltages positive to -40 mV. Kinetically , scorpion channels are more similar to L-type Ca channels in vertebra te 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 app lied tetraethylammonium and 3,4 diaminopyridine.