Effect of amiloride on electrolyte transport parameters of the main duct of the rabbit mandibular salivary gland

We have studied the response of the rabbit mandibular main duct perfused in vitro to luminally administered amiloride. The half-maximal inhibitory concentrations (K_I) when the duct was bathed in Cl solutions were: for net Na⁺ transport, 3×10^–6 mol l^–1; for transepithelial potential difference, 6×10^–6 mol l^–1; and for transepithelial conductance, 3×10^–7 mol l^–1. Substitution of the impermeant SO₄^2– anion for Cl^– changed the K_I for conductance to 3×10^–6 mol l^–1. Within Cl^–-containing media, the time course of the amiloride effect on potential difference showed an early rapid fall of 10 mV with a half-time 2 s, followed by a slower depolarization of 9 mV, and the conductance change followed the slower component of the potential change. In SO₄^2–-containing media, the potential difference and conductance changes followed time courses similar to one another. Finally, experiments on the effect of serosal applications of ouabain revealed that, although, in general, ouabain reduced resistance, it caused an increase in resistance in those ducts where the initial resistance was low. We conclude that: i) luminal Na⁺ transport occurs via amiloride-sensitive, conductive Na⁺ channels; ii) the Cl^– conductance is the major determinant of transepithelial conductance; iii) the first phase of the potential response is due to blocking of the Na⁺ conductive channels, whilst the slow phase reflects secondary inhibition of an electrogenic Na⁺ pump; and iv) duct resistance changes are secondary to alterations in intracellular Cl^– concentration.