Ion currents in Drosophila flight muscles

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Abstract
The dorsal longitudinal flight muscles of D. melanogaster contain 3 voltage-activated ion currents, 2 distinct K+ currents and a Ca2+ current. The currents can be isolated from each other by exploiting the developmental properties of the system and genetic tools, as well as conventional pharmacology. The fast transient K+ current (IA) is the 1st channel to appear in the developing muscle membrane. It can be studied in isolation between 60 and 70 h of pupal development. The channels can be observed to carry both outward and inward currents depending on the external potassium concentration. IA is blocked by both tetraethylammonium ion (TEA) and 3- or 4-aminopyridine. The inactivation and recovery properties of IA are responsible for a facilitating effect on membrane excitability. The delayed outward current (IK) develops after maturation of the IA system. IK can be isolated from IA by use of a mutation that removes IA from the membrane current response and can be studied before the development of Ca2+ channels. IK shows no inactivation. The channels are more sensitive to blockage by TEA than IA channels, but are not substantially blocked by 3- or 4-aminopyridine. The Ca2+ current (ICa) is the last of the major currents to develop and must be isolated pharmacologically with K+-blocking agents. ICa shows inactivation when Ca2+ is present but not when Ba2+ is the sole current carrier. When Ca2+ is the current carrier, the addition of Na+ or Li+ retards the inactivation of the net inward current. When the membrane voltage is not clamped, Ba2+ alone, or Ca2+ with Na+ (or Li+), produces a plateau response of extended duration. The synaptic current (IJ) evoked by motoneuron stimulation is the fastest and largest of the current systems. It has a reversal potential of approximately -5 mV, indicating roughly equal permeabilities of Na+ and K+. During a nerve-driven muscle spike, IJ is the major inward current, causing a very rapid depolarization away from resting potential. An exceptionally large synaptic current is necessary to rapidly discharge the high membrane capacitance (0.03 .mu.F[faraday]/cell) in these large (0.05 .times. 0.1 .times. 0.8 mm) isopotential cells.