Heinrich Knauf

The action of aldosterone on Na+ and K+ transport in the rat submaxillary main duct

SummaryThe main excretory duct of the submaxillary gland of normal and adrenalectomized rats was perfused with bicarbonate Ringers solution and the following values were measured: the transepithelial electrical potential difference, the specific electrical resistance of the epithelium, and the transepithelial net fluxes for Na+ and K+. From the potential difference and the resistance, the short circuit current was calculated. Following adrenalectomy the short circuit current dropped to about one half, while the electrical resistance increased around twofold and the transepithelial potential difference remained constant. The reduction of short circuit current was accompanied by a 30% reduction of Na+ reabsorption whereas K+ secretion was only slightly diminished Acute substitution of aldosterone to adrenalectomized animals led to a restitution of the Na+ fluxes and showed a tendency to increase K+ secretion. Following the administration of Actinomycin D to normal animals, Na+ resorption declined as in adrenalectomized rats but K+ secretion remained essentially unchanged. From these observations it is concluded that the hypothetical aldosterone-induced proteins act only on Na-resorption and that they may act by both increasing the sodium permeability of the luminal cell membrane and stimulating active Na+ transport. The latter effect does not seem to consist of a non specific enhancement of the energy supply since it does not influence the active potassium secretion of the cell.

The isolated salivary duct as a model for electrolyte transport studies

SummaryAn approximately 1 cm length of the main excretory duct of the rabbit submaxillary gland is dissected free from the animal and transferred to a lucite perfusion chamber containing rabbit serum. The ends of the duct are mounted over a pair of capillaries so that the duct could be perfused through one capillary while samples are collected from the other. A special multibarrelled perfusion system allowed intraluminal current application and transepithelial voltage recording. Both sides of the duct epithelium were bathed with continuously flowing solutions, which could be rapidly exchanged.Net Na+ transport across the isolated duct is as great as net Na+ transport in vivo. Net Na+ reabsorption ranges from 380 to 600 neq/min·cm2, and net K+ secretion is 13 to 26 neq/min·cm2. Cation transport is maintained stable during a 3 h period, and thereafter slowly decreases. During perfusion with bicarbonate Ringers solution, the mean transepithelial electrical potential difference is 17±3 mV (lumen negative). During sodium sulfate perfusion the duct epithelium generates a p.d. of up to 185 mV. Using cable analysis, the specific electrical resistance of the duct epithelium was determined to be 11.2±1.6 Ωcm2. Since the duct epithelium shows a linear current-voltage relation, the short-circuit-current can be calculated from the open-circuit-p.d. and the specific wall resistance during perfusion with bicarbonate Ringers solution. A value of 1.5±0.3 mA/cm2 is obtained, which is primarily accounted for by active transport of sodium.

Die Kationenausscheidung der großen Speicheldrüsen des Menschen

SummaryHuman saliva was collected separately from the three major salivary glands by catheterization of the main ducts and the effect of secretion rate on salivary Na and K concentrations was studied.In the resting state, with flow rates of 0.1 to 0.4 ml/min, Na concentration of submandibular and parotid saliva was 2 meq/l and K concentration ∼20 meq/l. Similar data were obtained from the resting sublingual gland.Following stimulation with pilocarpine salivary Na concentrations rose in all glands in a typical nonlinear fashion, whereas K concentrations declined and reached constant values, which were significantly greater than those of plasma. This result confirms earlier observations in the submandibular and parotid gland of human beings and of various animals and demonstrates unequivocally that the sublingual saliva in man unlike that in cat and dog is poor in Na.By means of microanalytical methods it was possible to investigate changes in cation concentrations when the salivary flow rates were less than the normal resting values; experiments were also done when salivary flow was stopped completely.In the submandibular gland Na concentration remained at 1.4 meq/l, whereas K concentrations rose to values of 74 meq/l. K concentrations, however, did not reach a plateau value even after a contact time of 15 min. These results, together with measurements of transepithelial P.D. (reported in the following paper), indicate that Na resorption and probably also K secretion, are governed by active transport mechanisms in the duct epithelium. Thus the main duct epithelium exhibits the properties which are generally taken to be essential for the upper part of the glandular duct system, which forms final saliva from primary secretion.Similar data were obtained in the upper portion of the parotid duct. In the lower portion of this duct, however, cation concentrations followed a different pattern in showing a tendency to equilibrate with plasma. Such results would be expected if there were no mechanisms for active transport in the lower portion of the main parotid duct.In order to describe the salivary Na concentration as a function of flow rate a mathematical model was developed. It is based on the assumption that primary secretion is plasmalike and that the hypotonicity of final saliva results from active resorption of sodium in the duct system in accordance with Thaysens hypothesis. Furthermore the rate of sodium resorption is assumed to be constant and the duct wall is considered impermeable to water. Under conditions of high salivary flow rates there was a good agreement between predicted values and experimental data. This model permits the calculation of the amount of sodium that is actively reabsorbed in the salivary duct system.

The effects of carbachol on water and electrolyte fluxes and transepithelial electrical potential differences of the rabbit submaxillary main duct perfusedin vitro

SummaryThe effect of carbachol on transepithelial potential difference and transepithelial nett electrolyte transport has been studied in the rabbit submaxillary main duct perfusedin vivo andin vitro with bicarbonate saline. The two preparations function similarly, reabsorbing Na, Cl and water and secreting K. In control ducts nett Na reabsorption was 683±55 nmol · cm−2 · min−1 and K secretion was 31.2±2.4 nmol · cm−2 · min−1. Nett water reabsorption was 970±71 nl · cm−2 · min−1 and the hydraulic conductivity was (14.0±1.6)×10−6 ml · cm−2 · s−1 · atm−1. The mean transepithelial potential difference was 13.1±0.8 mV (lumen negative) and, assuming no active transport of Cl, the partial conductance of the duct to Cl was (12.7±2.6)×10−2 mho · cm−2. Carbachol,in vivo andin vitro, caused partial depolarization of the transepithelial potential difference and reduction of nett Na and Cl reabsorption. It was without effect on duct K and HCO3 transport.In vitro, the drug was effective in concentrations as low as 10−7 M and perhaps lower. Atropine was able completely to block the effects of carbachol present at twice the atropine concentration. The results are consistent with the hypothesis that carbachol acts in some way to reduce the sodium conductance of the luminal face of the duct epithelial cell, this response being secondary to an undefined primary action of carbachol on the interstitial face of the cell.

The presence of a HCO 3 − ATPase in glandula submandibularis of rabbit

SummaryAn ATPase stimulated by HCO3−-ions and inhibited by SCN-ions could be found in glandula submandibularis of rabbit with following properties:1.It is stimulated by a variety of oxybases according to a general base catalytic mechanism.2.It is inhibited by both SCN−-and OCN−-ions.3.It can be solubilized by Triton X 100 hereby yielding a marked increase in the activity response to HCO3−-and SCN−-ions.4.It can be distinguished from mitochondrial ATPase because of the different behaviour of this enzyme and mitochondrial marker such as succinic dehydrogenase using sucrose gradient centrifugations. This enzyme resembles closely an enzyme preparation which had been found in plasma membranes of gastric mucosa and pancreatic tissue and which is assumed to be involved in H+/HCO3− transport system of these organs. The presence of HCO3− ATPase in glandula submandibularis indicates that a similar H+/HCO3− transport-mechanism may be present in this organ like in gastric mucosa and pancreatic tissue.

Elektrische Untersuchungen am Hauptausführungsgang der Speicheldrüsen des Menschen

SummaryThe 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 Ussings model for Na transport across frog skin, in favour of which new evidence can be put forward.

The minimum requirements for the maintenance of active sodium transport across the isolated salivary duct epithelium of the rabbit

SummaryThe main excretory duct of the rabbit submaxillary gland was isolated and studied in a lucite perfusion chamber. Measurements of transepithelial electrical potential difference (p.d.) and specific electrical wall resistance (Rm) were performed and short-circuit-current (SCC) was determined under conditions of different test solutions placed at the blood side of the duct. 1.High molecular compounds such as albumin or dextran are required in the bathing medium to maintain transport function.2.Active transport of sodium from the lumen to the interstitium can be maintained only when there is both pyruvate and acetate present in a bicarbonate Ringers bathing medium. Isocitrate can substitute for pyruvate and acetate. SCC was found to be 1.7±0.2 mA/cm2 with the artificial bathing fluid as compared to 1.9±0.2 mA/cm2 with rabbit serum placed at the blood side of the duct.3.In the absence of bicarbonate Na+ transport is reduced to 55±10% of control.4.Potassium in a 4 mM concentration on the blood side proved to be indispensable for transport function. In the absence of interstitial potassium the Na+ conductance of the luminal cell membrane is reduced. Rubidium and cesium ions can partially substitute for potassium.