AMPULLARY ELECTRORECEPTORS 497 



voltage clamp the transepithelial potential is constant; i.e., the difference 

 between the potential of lumenal and basal membranes is the clamped 

 potential, but within this constraint the potentials can change leading to 

 oscillatory currents. Many voltage-clamped receptors show damped oscilla- 

 tions at a frequency of about 20/s when stepped into their negative slope 

 region (Figure 5B). These oscillations are similar in time course to those in 

 the short-circuited receptor, and presumably the mechanisms are similar in 

 each case. The oscillations that occur in the negative slope region of the 

 clamped receptor do not involve any outward current through the receptor 

 cells as shown by subtraction of current through the leakage pathway. Thus, 

 the oscillations could not involve repolarization of the lumenal membranes 

 produced by activation of the late outward current of the lumenal mem- 

 branes. However, activation of an outward current in the basal membranes 

 cloud lead to repolarization of the lumenal membranes. 



The role of the basal faces in generating the oscillations is demonstrated 

 by perfusing these membranes with 2 mM TEA, 7 mM Co, or 5 mM 

 [ethylene bis(oxyethylenenitrilo)] tetraacetate (EGTA). The oscillations in 

 voltage clamp are blocked by each treatment and the epithelial response 

 becomes greatly simplified in the negative slope region. The responses to 

 larger stimuli are, as previously noted, essentially unaffected by these 

 treatments, which indicates that the active conductances in the basal 

 membranes are relatively small. Blocking by Co and EGTA could mean that 

 there is a Ca-activated outward current in the basal faces. However, the 

 effectiveness of 2 mM TEA in blocking the responses suggests that the basal 

 membranes have a voltage-sensitive K conductance, for Ca-activated K 

 conductances are generally relatively insensitive to TEA (Meech and Standen 

 1975). (The more rapid turnoff of the basal outward current as compared to 

 the lumenal outward current does not imply voltage sensitivity rather than Ca 

 activation, because it could result from more rapid removal of Ca from the 

 cytoplasm in this region of the cell.) Blocking by Co and EGTA may mean 

 that depolarization of the basal membranes adequate to activate a 

 voltage-sensitive outward (K) current requires Ca activation in these 

 membranes. 



To summarize, the oscillations are probably generated as follows: Ca 

 activation in the lumenal membranes caused by an appropriate excitatory 

 stimulus generates an inward current that depolarizes the basal faces and 

 allows both membrane potentials to go more positive regeneratively. Ca 

 activation in the basal faces may contribute to this potential change. Either 

 voltage-sensitive or Ca-activated outward current in the basal faces then 

 reduces the potential across these faces and consequently that -across the 

 lumenal faces as welL Ca activation in the lumenal membranes rapidly 

 declines when they are sufficiently repolarized. With some delay the outward 

 current-carrying conductance in the basal membranes declines; their 

 potential and that of the lumenal membranes goes more positive and Ca 

 activation again occurs regeneratively. And so it goes. 



The oscillations generated by relatively large stimuli show rather little 

 change in frequency with stimulus strength over a wide range of amplitudes 



