ausblenden:
Schlagwörter:
Pancreas; Active Transport; Ionophores; Membrane Enzymes; Secretion
Zusammenfassung:
The isolated cat pancreas was perfused with a Krebs-Henseleit-solution containing different concentrations of Amphotericin, Amiloride, Gramicidin, Dinitrophenol, Valinomycin, and Nigericin. In order to separate functionally the effects of these substances on the mitochondrial and the plasma membrane, we tested their influence on the secretory flow rate under aerobic and anaerobic conditions.
A Na+−K+- and a HCO3−-ATPase have been found in membrane fractions of pancreatic homogenates. Since there is a possibility that these enzymes are involved in active ion transport, we tested the effect of all substances on the level of activity of both Na+K+-ATPase and HCO3−-ATPase. The following were found: (1.) The secretory flow rate under anaerobic conditions was about 10% of that observed under aerobic conditions. Omission of glucose from the perfusate decreased flow rate to nearly zero. α-ketoglutarate and fumarate did not increase secretion in the absence of glucose which indicates that secretion was maintained by energy from anaerobic glycolysis. (2.) Amphotericin which increases the permeability of other membranes unspecifically, yielded an inhibition of pancreatic secretion under both aerobic and anaerobic conditions, presumably by an increase of the permeability of the luminal cell border. Amphotericin induced no effect on the Na+−K+-nor on the HCO3−-ATPase. (3.) Amiloride (2–4×10−3M) which inhibits sodium influx into epithelial cells, decreased pancreatic secretion by up to 50% in both aerobic and anaerobic conditions. Amiloride also inhibited the Na+−K+-ATPase activity in vitro but was without effect on the HCO3−-ATPase. In small concentrations however Amiloride stimulated the Na+−K+-ATPase activity. (4.) Gramicidin which creates rather unspecific channels for ions through membranes did not affect pancreatic secreatic secretion or ATPase activities. (5.) Dinitrophenol decreased secretion by about 30–60% under aerobic conditions, which might be due to the known uncoupling effect on oxidative phosphorylation. DNP stimulated in vitro Na+−K+-ATPase activity, which, however, was not reflected in an increase of secretory flow rate under anaerobic conditions. (6.) The ionophores Valinomycin and Nigericin, which increase membrane permeability to K+ and H+ ions in a variety of tissues, increased secretion up to 70% under anaerobic conditions but caused a decrease of 30%–90% under aerobic conditions. These findings suggest that both the plasma- and mitochondrial membrane must have been affected. Both ionophores stimulated the HCO3−-ATPase activity in vitro.
All tested substances except Gramicidin had an effect on pancreatic electrolyte secretion. Concerning the aerobic experiments, the inhibitory effect of Dinitrophenol and the ionophores might be due to uncoupling of oxidative phosphorylation.
The results of the anaerobic experiments are interpreted as being due to changes of cell membrane permeability.
As the effects of Nigericin, Valinomycin and Amiloride in the anaerobic experiments correspond to the effects of these substances on the ATPase activity, we suggest that these enzymes might be involved in the active transport processes of NaHCO3−-secretion.
