Worm Breeder's Gazette 3(2): 28
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
It has proven difficult to study the currents that generate the electrical activity in the somatic muscle cells of Ascaris due to the inability to control the voltage across the excitable membrane. Therefore, we have directed our attention to the pharyngeal muscle, where it is possible to directly measure the voltage and pass large currents across the excitable membrane. We have developed a system which allows us to do current-clamp and voltage-clamp experiments on an isolated segment of the pharyngeal membrane. We find that this membrane has no pacemaker activity. In the absence of nervous input the membrane potential is flat at a level near -40 mV. Two types of spontaneous postsynaptic potentials are frequently seen; one type has a reversal potential near -40 mV and the other has a reversal potential near -10 mV. When the membrane is at the resting level, this second type PSP triggers a positive-going action potential, which reaches a level between +30 and +50 mV. The membrane potential then falls to a plateau near O mV, where it remains until a negative-going PSP triggers a negative-going action potential that reaches about -50 mV (the potassium reversal potential). The membrane potential remains at the plateau level for periods ranging from 100 msec to several minutes. The positive-going action potential is produced by an inward current that appears to be carried by both Na+ and Ca++. This current is prolonged, showing little inactivation by 200 msec after a positive voltage step. Clamping the membrane to positive potentials elicits essentially no delayed-rectification K current, the current that normally repolarizes active membranes. However, stepping the membrane potential back to the resting level after a large positive pulse elicits a strong, transient outward K+ current; this is the current that produces the negative-going action potential. We have done a detailed analysis of this current and have shown that it is a voltage- inverted analogue of the Hodgkin-Huxley Na+ current. It is activated by negative steps in potential. It shows inactivation, being completely inactivated at the resting potential. Conditioning pulses to levels more positive than -10 mV are necessary to remove the inactivation from the channel. This is a new K+ conductance that has not been found in any other animal. This demonstration of unique mechanisms in nematode physiology should serve as a caution in trying to interpret the function of the nematode nervous system. The pharyngeal nervous system must not only generate the signal that triggers the pharynx to contract (open the lumen), but also the signal to relax (close the lumen). Since the relaxation of the pharynx is the 'power stroke' of this muscle, it is not surprising that the membrane has developed a special K+ current to produce a fast, separately-triggerable repolarization.