492 ELECTRICAL SENSES 



through the shunt resistance. This allows a simple and quite direct 

 measurement of the shunt resistance. It turns out that the shunt resistance is 

 low compared to the series sum of lumenal leakage and basal membrane 

 resistances of the receptor cells; at least 90% of the current flows across the 

 shunt resistance. (In the short-circuited ampulla the receptor cells are active 

 and more of an applied current flows through them, as described later.) 



The voltage— current relations show that the resistance of the epithelium 

 decreases when the Ca current is activated and decreases further when the 

 late outward current turns on. Because of the shunt resistance the change in 

 the receptor cell resistance is much greater than the change in the overall 

 epithelial resistance. The decrease is about twentyfold for Ca alone and 

 thirtyfold for both conductances. If all the residual resistance is in the basal 

 faces of the receptor cells, then the resting resistance of the lumenal 

 membranes must be 30 times as great as the resistance of the basal 

 membranes. The importance of this observation is that most of a voltage 

 applied across the inactive epithelium is developed across the lumenal 

 membranes. It also allows one to obtain a good approximation of the resting 

 resistance of the lumenal membrane from total resting and shunt resistances. 



There remain three uncharacterized resistances and two slopes in the 

 voltage— current relation. The third independent variable required for 

 solution of the circuit comes from measurement of three potentials: the Ca 

 equilibrium potential, the reversal potential for when both Ca and late 

 current are activated together, and the reversal potential of the late current 

 alone after repolarization of the epithelium when the Ca conductance has 

 returned to norma 1 but the late conductance persists. From these three 

 numbers, neglecting rnjM an( ^ r BAS ( see Figure 6), one obtains the ratio of 

 the Ca and late resistances, which with the slopes during Ca and late currents 

 allows calculation of the other values. 



The details of the calculation are given in Clusin and Bennett (19776) and 

 will not be repeated here. The important result is that the basal membrane 

 resistance, while small compared to the resting lumenal membrane resistance, 

 is considerably larger than the resistance of the active membrane (Table 1). 

 Thus, in their negative slope regions the membrane potentials cannot be 

 clamped. This property appears essential to the operation of the ampulla as 

 an electroreceptor (see below) but prevents accurate measurement of the 

 kinetics of the Ca and late conductances. 



The potentials across the cell membranes are important quantities that 

 cannot be measured by transepithelial measurements. Equilibrium potentials 

 are measured only as changes from the zero or resting potential across the 

 epithelium. As noted, the isolated epithelium shows a lumen positive resting 

 potential of 10 to 30 mV. This value is largely determined by the potential 

 of the shunt pathway most of which would appear to be a relative 

 hyperpolarization of lumenal as compared to basal membranes of the 

 supporting cells. If these membranes are similarly selective to the membranes 

 of the receptor cells, the transepithelial potential would represent the true 

 difference in resting potential in both kinds of cells and no currents would 

 flow through the receptor cells and shunt pathway in the isolated ampulla. 



