Microperfusion investigation of chloride fluxes across the epithelium of the main excretory duct of the rat submaxillary gland

Summary Nett chloride fluxes between interstitium and the lumen of the rat submaxillary main duct were studied by microperfusion at constant rates comparable to unstimulated salivary flow rates. The perfusion fluid contained inulin-¹⁴C, and steady-state concentrations of sodium (2.5 mEq/litre) and potassium (125–135 mEq/litre); the anion content was either all chloride, all bicarbonate or a chloridebicarbonate mixture.During all-chloride perfusion there was nett chlorideefflux from lumen to interstitium which was greatest at high perfusion rates and fell towards zero as perfusion rate decreased (and contact time increased). During all-bicarbonate perfusion there was nett chlorideinflux, greatest at faster rates and falling towards zero as perfusion rate declined. Perfusion with chloride-bicarbonate mixture producted a small nett chloride influx at higher perfusion rates which fell almost to zero below 213 nanolitres/minute (contact time about 4 minutes). The steady-state luminal chloride concentration was 78.5 mEq/litre ± 1.5 (S.E.), and the mean interstitial chloride concentration was 118.0±1.1 mEq/litre.In conjunction with the previously determined steady-state trans-epithelial potential difference (–11 mV, lumen negative), these data show that chloride is passively distributed across the duct epithelium. Since preliminary data suggest that bicarbonate may be transported into the lumen against an electrochemical gradient, it seems probable that the bicarbonate system (HCO₃^–, H⁺, and OH^–) is coupled in some way to an active transport mechanism.A preliminary report of this paper has already been published [15].