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Human Salivary Main Duct; Transepithelial Electrical Potential Difference; Salivary Secretion; Resorption of Sodium; Secretion of Potassium / Speicheldrüsengang des Menschen; Transepitheliale elektrische Potentialdifferenz; Speichelsekretion; Natriumresorption; Kaliumsekretion
Abstract:
The electrical potential difference (PD) across the main duct epithelium of the salivary glands was measured in human volunteers. In the resting gland the PD was 38±3 mV, lumen negative. After stimulation of secretion by pilocarpine the PD increased to about 100 mV (lumen negative) and returned to the resting level when secretion ceased. The same increase of PD was observed, when saliva was collected during stimulation and infused back into the duct during the resting state. From this it was concluded that pilocarpine had no direct action on the duct epithelium and that the increase of PD was caused by the changes that are known to occur in salivary electrolyte concentrations during stimulation.
This conclusion was tested by perfusion of the duct with different test solutions, so that the influence of single cations and anions on the PD could be studied. With sulfate solutions it was found that the luminal surface of the epithelium behaved like a Na-electrode; a tenfold change of Na-concentration developed nearly 61 mV while K and choline did not affect the PD. Thus the luminal cellwall appears to be selectively permeable to Na. When the duct was perfused with chloride solutions the PD was found to follow a typical time course with the initial transient values yielding a slope of 61 mV and the steady state values a slope of 35 mV for a tenfold change of Na-concentration. This observation can be explained when chloride acts as a shunt ion and when the chloride concentration within the epithelium, which determines the chloride conductance, follows the luminal chloride concentration with a time delay.
From the Na und K concentrations of saliva found previously during stop flow experiments and from the present PD measurements it was concluded that the human salivary main ducts, like those of the rat, actively reabsorb Na from the saliva and probably also actively secrete K into the saliva. The localization of the single active and passive transport steps with respect to the luminal and contraluminal cell side is discussed on the basis of Ussing's model for Na transport across frog skin, in favour of which new evidence can be put forward.
