Plasma Na+-iron concentration or pH as regulator of pancreatic HCO-3 secretion

Lack of effect of amiloride, furosemide, bumetanide and triamterene on pancreatic NaHCO3 secretion in pigs

This study examines the effects of the Na entry blockers amiloride, furosemide, bumetanide and triamterene on pancreatic NaHCO3 secretion. The purpose of the study was to elucidate whether Na flux through a Na-H-antiport, a Na,K,Cl-symport or a Na-K(H)-antiport respectively, may be rate limiting for pancreatic NaHCO3 secretion. The effect of each drug on secretion was tested in four groups of six anaesthetized pigs (20-25 kg body wt) during supramaximal, intravenous secretin infusion (3.0 CU kg-1 body wt. h-1), following laparotomy and cannulation of the pancreatic duct. Secretion was measured over a wide range of arterial pH values, both before and after drug administration in each animal. Pancreatic NaHCO3 secretion averaged 268 (219-308) mumol min-1 at physiological arterial pH 7.40 in the control period. Neither amiloride (90 mg kg-1 body wt), furosemide (10 mg kg-1 body wt), bumetanide (0.6 mg kg-1 body wt) nor triamterene (25 mg kg-1 body wt) affected the rate of secretin-dependent NaHCO3 secretion or the relation between NaHCO3 secretion and arterial pH. It is concluded that Na flux through neither a Na-H-antiport, a Na,K,Cl-symport nor a Na-K(H)-antiport appears to be rate limiting for pancreatic secretion of NaHCO3.

Hypernatremia inhibits NaHCO3 reabsorption and associated NaCl reabsorption in dogs

To examine the effect of selective rise of plasma NaCl concentration (hypernatremia) on NaHCO3 reabsorption and associated NaCl reabsorption remaining during continuous ethacrynic acid infusion, hypertonic NaCl solution was infused in three groups of anesthetized volume-expanded dogs. In six dogs examined at constant hematocrit and plasma pH, bicarbonate and water reabsorptions were inversely related to PNa and reduced by 37% and 39% respectively by raising PNa from 140 to 200 mM. Chloride reabsorption remained essentially constant until PNa exceeded 170 to 180 mM. At PNa 200 mM, sodium reabsorption was reduced by 22 +/- 6%. In six other dogs, mechanical variations of GFR showed that the inhibitory effects of hypernatremia (PNa 199 +/- 3 mM) were less pronounced at low GFR. After subsequent administration of acetazolamide (30 mg/kg body wt), only 20% of control bicarbonate reabsorption remained and glomerulo-tubular balance was completely abolished. Both hypernatremia and acetazolamide inhibited NaHCO3 and NaCl reabsorption in a molar ratio of about 1:2, as in normonatremic dogs. Finally, experiments in six dogs showed that the inhibitory effects of hypernatremia (PNa 213 +/- 4 mM) were not altered by varying PCO2 and plasma pH. We conclude that hypernatremia inhibits paracellular water and NaCl reabsorption in the proximal tubules by reducing the osmotic force caused by transcellular NaHCO3 reabsorption. A rise in PNa does not stimulate transcellular NaCl reabsorption during distal inhibition by ethacrynic acid.

Effects of digitoxin and hypokalaemia on pancreatic NaHCO3 secretion and pancreatic Na,K-ATPase activity

To study the role played by Na,K-ATPase in the pancreatic secretion of NaHCO3, experiments were performed in 20 anaesthetized, secretin-infused pigs (3.0 clinical units X kg b. wt. X h-I). The relationship between pancreatic NaHCO3 secretion and arterial pH was obtained before and during Na,K-ATPase inhibition by digitoxin and hypokalaemia. Na,K-ATPase activity in pancreatic tissue homogenate averaged 5.45 (5.02-6.68) mumol Pi X mg X protein X h-I. Retrograde injection of 0.5 ml 1.4 X 10(-4) mol X l-I digitoxin into pancreatic ducts reduced pancreatic Na,K-ATPase activity by 3I(I8-47)%, while intra-arterial injection of 0.2 mg X kg b. wt-I digitoxin reduced pancreatic Na,K-ATPase activity by 50(45-56)%. Digitoxin and hypokalaemia reduced the rate of pancreatic NaHCO3 and shifted the normal, proportional relationship between NaHCO3 secretion and arterial pH towards higher pH. Hypokalaemia reduced Na,K-ATPase activity and NaHCO3 secretion in proportion. These effects indicate that Na,K-ATPase helps to sustain the requisite electrochemical potential gradients for driving H+ ions, and hence HCO-3 ions, out of secretory cells.

Role of sodium, bicarbonate, and plasma osmolality in biliary secretion

To examine the effect of changes in biliary sodium and bicarbonate secretion on bile formation, experiments were performed on fasted, pentobarbital-anesthetized pigs. During continuous intravenous secretin infusion (2.7 CU X kg body wt-1 X h-1) sodium secretion was altered by increasing or reducing plasma sodium concentration. Bicarbonate secretion was altered by varying arterial plasma pH. At increased biliary sodium secretion, bile formation was depressed, but changes in bicarbonate secretion were accompanied by parallel alterations in bile formation. Bile acid secretion was increased during elevated plasma sodium concentration, whereas reduced plasma sodium concentration depressed bile acid secretion. To distinguish between the effect of changes in plasma osmolality and sodium concentration, bile formation was also studied during intravenous sucrose infusion at normal plasma sodium concentration. About 50% of the effect on bile formation of changing plasma sodium concentration is solely caused by the changes in plasma osmolality. During secretin stimulation bile formation is mainly determined by bicarbonate. Changes in plasma osmolality affect bile secretion through alterations in the net osmotic force across the hepatocellular membrane. Sodium has an impact on the bile-acid-dependent fraction, whereas bicarbonate is the mediator of the bile-acid-independent fraction of bile secretion.

Effect of intravenous infusion of hypertonic glucose solutions on pancreatic HCO3(-) secretion

To examine why intravenous infusion of hypertonic non-electrolyte solutions inhibit pancreatic HCO3(-) secretion, the relationship between pancreatic HCO3(-) secretion and plasma pH was examined before and following intravenous infusion of hypertonic glucose to 5 anesthetized, secretin infused (2.7 C.U./kg b.wt.h-1) pigs. Hyperglycemia (plasma glucose 103 +/- 6 mmol/l) did not significantly change plasma pH, Na+, K+, Cl- and HCO3(-) concentrations. Hyperglycemia reduced pancreatic water flux by 48 +/- 5% and raised pancreatic juice HCO3(-) concentration by 43 +/- 4 mmol/l. Concurrently, HCO3(-) secretion fell by 34 +/- 5%. Acidosis, produced through intravenous HCl infusion and CO2 addition to inspired air, reduced HCO3(-) secretion by 40 +/- 6 mumol/min and 30 +/- 5 mumol/min per 0.1 pH unit reduction in plasma pH before and during hyperglycemia, respectively, and abolished HCO3(-) secretion at an estimated plasma pH of 6.51 +/- 0.06 before and a pH of 6.63 +/- 0.05 during hyperglycemia. We conclude that hypertonic glucose infusions inhibit pancreatic water flux and cause an increase in pancreatic juice HCO3(-) concentration which may inhibit HCO3(-) secretion through an effect on acid-base balance in secretory cells.

Abolished relationship between pancreatic HCO-3 secretion and arterial pH during carbonic anhydrase inhibition

After acetazolamide administration, CO2 hydration in pancreatic cells would be slow and might become a rate-limiting factor to pancreatic HCO-3 secretion. Correspondingly, pancreatic HCO-3 secretion-normally pH dependent-would become slow and pH-independent. However, acetazolamide would not be expected to interfere with the capacity of the secretory mechanism to generate a proton potential gradient between pancreatic cells and interstitial fluid. These predictions were examined in 5 anesthetized, secretion infused (2.7 C. U./kg b.wt. h-1) pigs. Pancreatic juice was collected from a catheter in the pancreatic duct. Arterial pH was varied through i.v. HCl and NaHCO3 infusions and CO2 addition to inspired air. Before acetazolamide, HCO-3 secretion varied with plasma pH and averaged 298 +/- 30 mumol/min at control arterial pH. Acetazolamide (150 mg/kg, i.v.) reduced HCO3 secretion to 84 +/- 12 mumol/min and rendered secretion independent of arterial pH between pH 7.6 and pH 7.0. It is concluded that acetazolamide imposes a pH-independent transport maximum on pancreatic HCO-3 secretion, but does not reduce the capacity of the secretory mechanism to sustain a proton potential gradient between cells and interstitial fluid.