The effect of cAMP on the cell membrane potential and intracellular ion activities in proximal tubule ofRana esculenta

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Abstract
Experiments were performed in proximal tubule of the isolated perfused frog kidney to evaluate peritubular cell membrane potentials (PDpt), and the intracellular ion activities of sodium (Nai ü), chloride (Cli −) and potassium (Ki ü) under control conditions and following peritubular application of dibutyryl-cyclic AMP (cAMP, 2·10−4mol·l−1). Conventional and ion-sensitive microelectrodes were applied to record continuously cAMP-induced changes of these parameters in individual proximal tubule cells. Within a few minutes a significant hyperpolarisation of PDpt (Δ=2.0±0.2 mV) occurs simultaneously with a decrease of Nai ü (Δ=2.5±0.5 mmol·l−1). Ki ü increases (Δ=3.6±0.9 mmol·l−1) and Cli − decreases (0.4±0.07 mmol·l−1) slightly, but significantly. With both ions the alterations of the chemical gradient is significantly smaller than the potential shift. PDte is not significantly altered by cAMP. The cAMP-induced hyperpolarisation of PDpt can be observed in presenceand absence of luminal glucose. However, omission of Naü from the luminal perfusate abolishes the hyperpolarising effect of cAMP on PDpt. The results suggest that cAMP reduces sodium entry from the lumen into the cell, thus hyperpolarising the cell membrane and decreasing Nai ü. Persistance of sensitivity of PDpt to cAMP after omission of glucose indicates that other Naü coupled transport processes and/or passive Naü conductance are affected by cAMP. the changes of Ki ü and Cli − are secondary, following the change of PDpt.