The extracellular receptor currents evoked by step displacements of the otolithic membrane of the isolated saccular macula of Rana esculenta were recorded under transepithelial voltage clamp conditions. With the aim to depolarize the hair cells and increase the fractional resistance of the apical membranes, the basal side of the preparation was bathed in saline with an increased K+ concentration (62 mM). This caused a shift in the non-linear receptor current-voltage relation along the voltage axis of -51 mV +/- 10 mV; (mean +/- SD; n = 32) and a reduction in the transepithelial resistance of 10%. Under these conditions the electrical properties of the macula are assumed to be controlled by the apical membranes. The effects of different concentrations of Ca2+ in the apical solution on the receptor current-voltage relation were examined. Change of the apical Ca2+ concentration (range 3 mM to 70 microM) varied the transepithelial voltage at which the receptor current was zero (Vrev). Fitting a modified constant field equation to the relation between the apical Ca2+ concentration and the change in Vrev gave an estimate of PCa/PK of the transduction channels of 212. Furthermore, a high relative permeability of the transduction channels for other divalent cations (Ba2+, Sr2+) was measured, whereas Mn2+ inhibited the receptor current. The receptor current was inhibited by amiloride (IC50 3.2 microM +/- 1.7 microM) and nifedipine (IC50 1.9 microM +/- 0.6 microM). Reduction of the apical Ca2+ concentration to 90 microM in standard apical solution reduced the size of the receptor current to 67% +/- 30% (n = 17) compared to control but did not affect the shape of the receptor current-voltage relation. Subsequent substitution of K+ by Na+ caused a further reduction of the receptor current to 32% +/- 29% (n = 9), changed the receptor current-voltage relation into a linear relation and diminished the adaptation of the receptor current. These results indicate that the mechano-electrical transduction channels of the frog saccular hair cells are highly selective to Ca2+ and that the conductance of the channels may be influenced by the apical monovalent cation species.