A current hypothesis suggests that the mechanism of salivary production involves the formulation of a precursor solution followed by reabsorption of sodium and water but not potassium. Since this is similar to kidney physiology, clearance techniques were used to test the hypothesis. At parotid flow rates below about 0.2 cc/min. the concentration of Na varies directly and that of K indirectly with the flow. At higher rates the K concentration is independent of the flow and closely approximates that of the plasma. Potassium clearance, then, is independent of flow only at very low flows. The concentration of Na in parotid saliva varies with that in the plasma. Extrapolation of the curve cuts the abscissa at a point indicating that if the plasma Na concentration were reduced below approximately 100 mEq/l. no Na would appear in the saliva. The relationship between plasma concentration and the saliva Na/plasma Na ratio is an ascending curve. As the plasma K concentration increases so does the concentration in the parotid saliva. Extrapolation of this curve approaches the origin. The saliva K/plasma K ratio is independent of the plasma K concentration. When K is infused salivary flow rate increases. The infusion of Na decreases flow. These results are consistent with the selective reabsorption theory at low flow rates but there must be another mechanism to explain the findings at higher flows. It is suggested that the influence of Na and K infusion on salivation may be, at least partially, indirect.
The Influence of Low Salivary Flow Rates on the Absorption of a Sublingual Fentanyl Citrate Formulation for Breakthrough Cancer Pain