Effect of imidazolines on Na+ transport and intracellular pH in renal proximal tubule cells.

Recently, we characterized an imidazoline-guanidinium receptive site (IGRS) in the renal proximal tubule of rabbit kidney. Although recognized by a series of imidazoline and guanidinium alpha-2 adrenergic compounds, IGRS is insensitive to catecholamines and can be physically separated from alpha-2 adrenergic receptors after solubilization. In the present study, we investigated the effect of imidazoline derivatives on 22Na+ uptake and intracellular pH in isolated cells from rabbit renal proximal tubule. After 5 min of preincubation, idazoxan inhibited the total 22Na+ influx (-30%) in a dose-dependent manner, with a maximum effect at 10(-5) M. The effect of idazoxan was not competitive as shown by the decrease of the maximal velocity of 22Na+ entry (control: 3.80 +/- 0.42; idazoxan 10(-5) M: 3.23 +/- 0.33 nmol/30 s per mg protein, P less than 0.01). A series of imidazoline derivatives inhibited 22Na+ entry with an order of potency similar to that previously found for inhibition of [3H]idazoxan binding to IGRS (cirazoline greater than idazoxan greater than UK 14304 greater than rilmenidine much greater than cimetidine). The inhibition of 22Na+ uptake by these compounds does not appear to be related to interaction with alpha-adrenergic receptors since it was observed in the presence of saturating concentrations of the adrenergic antagonists rauwolscine (alpha-2) or prazosin (alpha-1). When tested on the regulation of intracellular pH by fluorimetric techniques, 10(-5) M cirazoline or idazoxan inhibited by 20% the velocity of the sodium-dependent H+ efflux in acidified cells (P less than 0.02). The concomitant inhibition of 22Na+ entry and of cell realkalinization suggests that imidazoline derivatives inhibit Na+/H(+)-exchanger. This effect could be mediated via the renal IGRS and intracellular second messengers that are not yet known.