Electrical characteristics of the apical and basal-lateral membranes in the turtle bladder epithelial cell layer

(1) The active transport of Na⁺ across the turtle bladder epithelial cell layer consists of a passive entry step through a Na⁺-selective path in the apical membrane and an active extrusion step through Na⁺ pump-containing path in the basal-lateral membrane together with some back-leakage through the paracellular spaces and tight junctions between the epithelial cells. This hypothesis has now been verified qualitatively and to some extent, quantitatively by the use of an intracellularly-located microelectrode in conjunction with a conventional assembly of extracellularly-located macroelectrodes mainly in short-circuited bladders bathed by Na⁺-rich Ringer media. Under these conditions, the intracellular potential (V_sc) averaged 38.4 mV with the cell electronegative; the fractional resistance of the apical membrane ($\text{?R}{}_{\mathrm{<mtext>a</mtext>}}$) averaged 0.55; while the concomitant transepithelial parameters, short circuiting current (I_sc) and electrical conductance (G_t), average 68.6 μA/cm² and 0.98 mS/cm², respectively. (2) The relation between these parameters and the transepithelial flow of Na⁺ (orI_sc) is evoked by blocking Na⁺ entry into the cell (by the mucosal addition of amiloride or removal of mucosal Na⁺). Amiloride-induced blockade of the Na⁺ entry step results in a rapid hyperpolarization of the cell interior during which V_sc = —79.1 mV and $\text{?R}{}_{\mathrm{<mtext>a</mtext>}}\text{= 0.92}$. I_sc and G_t (equivalent to the shunt conductance under these conditions) averaged 5 μA/cm² and 0.35 mS/cm², respectively. The entire process is reversible on re-admission of Na⁺ entry into the cell. (3) A slow depolarization of the cell interior in the period of blocked transapical Na⁺ entry is opposite to that expected from an electroneutral Na⁺-K⁺ exchanging pump; but instead is the predictable response of an electrogenic Na⁺ pump in parallel with a passive K⁺-selective conductance in the basal-lateral membrane. (4) The electrogenicity concept is substantiated after pretreatment of the bladder with serosal ouabain, which changes the response of V_sc to amiloride (from the aforementioned biphasic response) to a step-function response, attributable mainly to the development of a slowly dissipating K⁺ diffusion potential across the basallateral membrane. (5) Under open-circuit conditions, the electronegativity of cell to mucosa (V_a) is a linear inverse function of the electropositivity of serosa to mucosa (V_t). For V_t ? 100 mV, V_a is positive; and for V_t between ?30 and 90 mV, V_a is negative.