AJP - Renal Physiology, Vol 253, Issue 4 606-F612, Copyright © 1987 by American Physiological Society
Characteristics of H+ current transients induced by adverse H+ gradient pulses in toad bladder
A. C. Nero, J. H. Schwartz and M. R. Furtado
Department of Medicine, School of Medicine, Sao Paulo, Brazil.
Acidification in the toad bladder occurs as a result of electrogenic H+
secretion (JH). When a pH gradient is applied in a stepwise fashion in the
absence of exogenous CO2, JH decreases linearly with the mucosal (M)
solution pH and is null when pHm is approximately 4.5. When pHm is returned
to initial values (7.4) in a stepwise fashion, JH increases linearly with
pHm. However, on this return, higher values of JH are initially obtained.
To investigate this hysteresis, hemibladders mounted in chambers were used
to measure the change in the H+ current before and after acid pulses were
applied to the mucosal solution. In the absence of exogenous CO2, the
application of graded acid pulses to mucosa for 1, 2, 4, and 8 min resulted
in a graded decrease in JH. The restoration of pHm to 7.4 was followed by
an immediate transient overshoot of reversed short-circuit current (Irsc),
which was related to the time of exposure and the magnitude of the acid
pulse. The longer the acid pulse or the larger the pulse, the greater the
Irsc overshoot. The addition of protonophores, dinitrophenol, or
salicylate, into the mucosal solution enhanced this overshoot. Similar Irsc
overshoots could be obtained with the application of pulses of adverse
electrical gradients. Introduction of exogenous CO2 into the system (3%)
completely inhibited the overshoot in JH after an acid pulse. In
conclusion, when pHm is decreased JH is reduced and the cell pH presumably
decreases because of continued exit of alkali at the serosal side of the
cell and entry of H+ from the mucosal solution. The decrease in cell pH
then triggers the pump to produce a sharp overshoot in JH when pHm returns
to 7.4.
