Electrical transients produced by the toad bladder in response to altered serosal composition at constant osmolality.

The effects of step-changes in the ionic composition of the serosal medium bathing the toad urinary bladder under voltage-clamped conditions have been studied. A decrease in the K+ concentration from 4 to 3 mmol/l in the serosal fluid increased transiently the transepithelial current which after 30 min returned to the initial value. The peak current was reached after 3 min. The current response of the bladder to the reverse step in K+ concentration, from 3 to 4 mmol/l was much smaller. Surprisingly, the partial replacement of Cl- with gluconate produced a transient increase in current. It is suggested that secondary active transport plays an important role in this phenomenon and leads to an increased apical Na+ conductance. The second phases of the biphasic responses to Na/K+ and Cl-/gluconate substitutions have been interpreted as osmotic effects. Since the exchange of solutions in these studies was isoosmotic but not necessarily isotonic, experiments were also performed with osmotic changes in the serosal fluid for the purpose of comparison.