Ionic conductive properties of rabbit proximal straight tubule basolateral membrane

The ionic conductive properties of the nonperfused rabbit proximal straight tubule (S2) basolateral membrane were assessed by microelectrode techniques. The response of the basolateral membrane electrical potential difference, V_bl, to rapid changes in the peritubular bath concentration of K, HCO₃, Na, and Cl were monitored with microelectrodes. The control steady-state V_bl averaged -41 mV (cell negative). An increase in peritubular bathing medium K concentration from 5 to 40 mM resulted in an instantaneous and sustained depolarization of +14.6 mV (27% of ΔE_K). Addition of barium (2 mM) depolarized the V_bl by +15.8 mV and abolished the V_bl response to the high-K medium. In other studies, reduction of peritubular bicarbonate at constant pH from 25 to 2.5 mM instantaneously and transiently depolarized V_bl by +15.8 mV (26% of ΔE_HCO3). In these same tubules reduction of peritubular Na from 126 to 2.2 mM resulted in an instantaneous and paradoxical depolarization of V_bl of +21.5 mV. Both depolarization transients resulting from reduction of Na and HCO₃ were simultaneously inhibited by the addition of 4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid (SITS; 0.5 mM), consistent with the presence of a SITS-sensitive Na-HCO₃-coupled conductive pathway. In the absence of the bicarbonate buffer, reduction of Na resulted in a small sustained hyperpolarization of -5.8 mV (5% of ΔE_Na). Reduction of peritubular Cl from 120 to 4 mM resulted in an instantaneous and sustained depolarization of V_bl of +5.3 mV (6% of E_Cl) and was not affected by the addition of bumetanide (0.1 mM). It is concluded that the basolateral membrane of the nonperfused proximal straight tubule is characterized by a major barium-sensitive K conductance and a SITS-sensitive Na-coupled HCO₃ conductance that carries net negative charge. These pathways are paralleled by relatively minor, but important, Na-conductive and Cl-conductive pathways.