The Chronically Reserpinized Rat as a Model for Cystic Fibrosis: Abnormal Cl⁻ Transport as the Basis for Reduced Salivary Fluid Secretion

Abstract

ABSTRACT: Saliva secretion induced by 10⁻⁶ M acetylcholine was reduced 74% in isolated, perfused submandibular glands of control rats when the gland was perfused with solutions containing either furosemide (10⁻³ M) or sulfate (instead of chloride) as the major anion. In regular (Cl-containing) perfusates without furosemide, saliva secretion was reduced 74 % in isolated glands of rats treated with seven intraperitoneal doses of reserpine (0.5 mg/kg body weight). In the latter, addition of furosemide or replacement of perfusate Cl⁻ with SO⁼₄ caused a further 35% drop in saliva volumes. Salivary Cl⁻ concentrations were lower in saliva from the treated animals and were reduced further by furosemide, which also reduced the Cl⁻ concentrations of control saliva. In submandibular acini isolated from control glands, acute exposure to ³⁶Cl (1 μCi/ ml) resulted in a rapid uptake of tracer so that a constant content of isotope (approximately 9.5 nM/mg protein) was attained in 4–5 min and maintained for 30 min. This basal uptake reached 8.4 nM/mg protein in acini isolated from glands of reserpine-treated rats and attainment of a steady state of tracer content was delayed and required 8–10 min. Exposure to acetylcholine reduced uptake and steady state tracer content by 35% in control acini, but had no effect in acini of reserpine-treated rats. Acetylcholine caused a rapid decrease (42% in 1 min) in ³⁶CI content of control acini which were preloaded with tracer for 12 min, but only a 23% decrease in acini of reserpine-treated rats. Exposure to furosemide at zero time caused a 50% reduction in ³⁶Cl uptake in control acini and a 41% reduction in acini of the treated animals. Addition of the diuretic to acini preloaded with tracer caused a slower reduction in ³⁶Cl content of control acini (48% in 7 min) and a similarly slow and smaller reduction (26%) in acini of reserpine-treated rats. These findings indicate that: 1) Salivary fluid secretion requires the transepithelial movement of CI⁻ in salivary acinar cells. This involves both entry of Cl⁻, which occurs in part by a furosemide-sensitive transport system, and efflux of CI. 2) Both steps of this mechanism are abnormal in salivary glands of reserpine-treated animals. Cl⁻ uptake is delayed and shows a somewhat reduced sensitivity to furosemide, while Cl⁻ efflux is inhibited. The resulting decrease in overall transepithelial Cl⁻ transport can explain, therefore, the reduced saliva secretion observed in reserpine-treated animals. 3) Since the reserpine-treated rat has been used as an animal model for cystic fibrosis, similar abnormalities in Cl⁻ transport could exist in salivary cells of patients with this disease and, similarly, explain the reported disturbances in saliva secretion.

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