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

Abstract

(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 ( ƒR a ) averaged 0.55; while the concomitant transepithelial parameters, short circuiting current ( I sc) and electrical conductance ( G t), average 68.6 μA/cm 2 and 0.98 mS/cm 2, respectively. (2) The relation between these parameters and the transepithelial flow of Na + (or I 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 ƒR a = 0.92 . I sc and G t (equivalent to the shunt conductance under these conditions) averaged 5 μA/cm 2 and 0.35 mS/cm 2, 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.