Kinetics of Na transport in the rat submaxillary main duct perfusedin vitro

Summary The main duct of the rat submaxillary gland has been isolated and perfusedin vitro while incubated in a plasma bath. It reabsorbs Na and secretes K and HCO₃ at rates comparable to those reported for the ductin vivo and develops a transepithelial potential difference with magnitude dependent on the luminal Na concentration.The kinetics of ductal Na transport were studied by varying the Na concentration in the plasma bath step-wise from 149 to 320 mM and then the Na concentration in the luminal perfusate so as to produce zero nett flux of Na and water between lumen and bath. It was found that active Na transport by the duct showed saturation kinetics with a half-saturation constant of about 29 mM. The saturation curve could not be described in terms of a single Michaelis-Menten hyperbola although a more complex expression consisting of two such hyperbolic transport terms could easily be fitted to the data.Utilizing published values for unidirectional Na flux ratios it was calculated that the active Na transport rate when the luminal fluid had a plasma-like Na concentration was 4.37×10^–9 mol·cm^–2·s^–1 and the epithelial Na permeability coefficient was 5.27×10^–6cm·s^–1.Although the kinetics of K and HCO₃ secretion were not studied in detail it was observed incidentally that ductal secretion of these two ions still persisted even when their luminal concentrations were raised, respectively, to 197–225 mM and 80–120 mM.A preliminary report of this work was presented to a meeting of the Australian Physiological and Pharmacological Society and a Regional Meeting of the International Union of Physiological Sciences held at Sydney University in August 1972 [5].