The mechanism of NaCl transport across the epithelium of intact MDCK cysts grown in a collagen gel matrix was investigated. Doublebarreled microelectrodes were used to measure basolateral membrane PD (Vbl), transepithelial PD (Vt), and intracellular (Cli) and intralumenal (Clcy) Cl-activities in cysts under different conditions. In a control Ringer's solution (RS), Cli (60 ± 1 mM) and Clcy (107 ± 2 mM) exceeded the values corresponding to electrochemical equilibrium across the basolateral membrane and epithelium, respectively. Cli was reduced by superfusing the cysts with a low Cl- RS (Cli, 20 ± 3 mM), a low Na+ RS (Cli, 40 ± 4 mM), or by adding amiloride to the control RS (Cli, 46 ± 1 mM). Cli was unaffected by removal of either K+ or HCO3- from the RS or by adding furosemide or SITS to the control RS. Vb1, in the control RS was -50 ± 2 mV and was affected only by removal from the RS of K+ (Vbl, -31 ± 3 mV) or HCO3- (Vbl, -29 ± 4 mV) or by the addition of SITS to the control RS (Vbl, -59 ± 5 mV). Vt in control RS was -2 ± 0.2 mV (lumen negative), and was increased by reducing bath Na+ (Vt, -37 ± 2 mV) but not by reducing bath Cl-. These data indicate that Cl- is secreted in a basolateral to apical direction by the cyst epithelium. Basolateral Cl- transport probably occurs mainly by an electroneutral Cl-/HCO3- exchanger. Transepithelial Na+ transport seems to occur via a paracellular route which appears to be cation selective. These experiments also support the existence, in the basolateral membrane, of a Na+/K+ ATPase, a Na+/H+ exchanger, and possibly a Na+/HCO3-/ CO32- transporter.
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