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.

Voltage-clamp Studies of the Electrical Currents of the Ascaris Pharyngeal Muscle

L. Byerly, M. Masuda

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.