Figure 5.

Representative experiment (of a group of 10 tissues) of the action of apical Ca²⁺ on the TJ response to withdrawal of basolateral Ca²⁺. The urinary bladders were short-circuited and bathed on the apical side by 75 mM KCl and on the basolateral side by NaCl Ringer's solution. (A) The presence of 10 mM Ca²⁺ in the apical bathing fluid (+Ca_ap = 10) markedly reduces the rate of TER decrease in response to Ca²⁺ removal from the basolateral solution (−Ca_bl). Subsequent removal of apical Ca²⁺ (−Ca_ap) substantially increases the rate of TER decrease. TER fully recovers upon addition of 1 mM Ca²⁺ to the basolateral solution. (B) In this bladder, the presence of 10 mM Ca²⁺ in the apical solution (+Ca_ap = 10) completely abolished the decrease of TER that normally follows the removal of Ca²⁺ from the basolateral solution (−Ca_bl). The subsequent withdrawal of apical Ca²⁺ (−Ca_ap) then triggers a drop of TER that starts with an extremely short delay after the Ca²⁺ removal. In the absence of basolateral Ca²⁺, the introduction of 10 mM Ca²⁺ into the apical solution (+Ca_ap = 10) leads to an almost complete recovery of TER. A full recovery of TER is attained when 1 mM Ca²⁺ is added to the basolateral fluid. The vertical deflections of SCC, caused by pulses of ±1 mV in the clamping potential, are proportional to the overall tissue electrical conductance (G), where G = 1/TER.