The oslo CoMet study: randomised controlled study of open and laparoscopic liver resection for colorectal metastases

The 2013 Alpine Liver and Pancreatic Surgery meeting was held in Madonna di Campiglio, Italy. The meeting was organised by the Association of Upper Gastrointestinal Surgeons of Great Britain and Ireland. The following abstracts were selected for presentation at the meeting.

Single-centre experience of laparoscopic pancreatic surgery

BACKGROUND

Laparoscopic resection is regarded as safe and feasible in selected patients with benign pancreatic tumours. Few data exist on laparoscopic surgery for malignant lesions and larger neoplasms in unselected patients.

METHODS

The study included all patients admitted to Oslo University Hospital, Rikshospitalet, from March 1997 to March 2009 for surgery of lesions in the body and tail of the pancreas, and selected patients with lesions in the pancreatic head, who underwent surgery by a laparoscopic approach with curative intent.

RESULTS

A total of 166 patients had 170 operations, including 138 pancreatic resections, 18 explorations, nine resections of peripancreatic tissue and five other therapeutic procedures. Four patients had repeat procedures. There were 53 endocrine tumours (31.0 per cent), 28 pancreatic carcinomas (16.4 per cent), five cases of metastases (2.9 per cent), 48 cystic tumours (28.1 per cent) and 37 other lesions (21.6 per cent). The total morbidity rate was 16.5 per cent. Fistula was the most common complication (10.0 per cent). Three patients needed reoperation for complications. There were three hospital deaths (1.8 per cent). Median hospital stay following surgery was 4 days.

CONCLUSION

Laparoscopic resection of lesions in the body and tail of the pancreas in an unselected patient series was safe and feasible, and should be the method of choice for this patient group in specialized centres.

A comparative study of the short-term outcome following open and laparoscopic liver resection of colorectal metastases

BACKGROUND

Laparoscopic resection of liver tumors is feasible, but few studies have compared short-term outcome of the laparoscopic approach to that of a conventional technique.

METHODS

Eighteen tumor resections performed during 14 procedures (14 patients) by conventional surgery were compared to 21 similar resections performed laparoscopically during 15 procedures (13 patients). All patients had colorectal liver metastases.

RESULTS

No perioperative mortality occurred. Surgical time, peroperative bleeding and blood transfusion requirement were similar in the two groups. The resection margin was involved by tumor tissue in one specimen laparoscopically resected and in two specimens conventionally resected (p = 0.58). Patients operated laparoscopically remained in hospital for median 4 days, while patients operated conventionally stayed median 8.5 days (p <0.001). Patients operated laparoscopically required less opioid medication than patients having conventional surgery (median 1 vs 5 days; p = 0.001).

CONCLUSIONS

Short-term outcome of laparoscopic liver resection compares to that of conventional surgery, with the additional benefits derived from minimal invasive therapy.

Laparoscopic cholecystectomy: bile duct and vascular injuries: management and outcome

BACKGROUND

This is a retrospective study of 32 consecutive patients referred in the period 1992-2000 for management of serious bile duct injuries caused by elective laparoscopic cholecystectomy.

METHODS

The patients were referred on median 29 days (0 days to 34 months). Only 7 patients were referred immediately after discovery of the injury. At the local hospital, 25 patients underwent various procedures in attempts at repair. Ten of the patients were treated for bile duct strictures after previous repairs in other hospitals.

RESULTS

At referral, 23 patients (72%) had complete transection of the bile duct, while 9 had bile leakage injuries. Additional complications were occlusion of the right hepatic artery in 8 patients (24%) and occlusion of the mesenteric superior artery in 1 patient. Infectious complications were prominent in 21 patients (70%), 6 of whom had septicaemia. Operative management with hepaticojejunostomy Roux-Y was employed in 22 patients. Various non-operative strategies were chosen, including endoscopically or transhepatic stenting of the bile duct and embolization of the right hepatic artery. There was no difference in hospital stay between operative and non-operative procedures which on median was 16 days ( range 7-69 days). Three patients died: one had thrombosis of the superior mesenteric artery, while the other two died of complications to bile peritonitis. Median observation period is 5 years (5 months to 8 years). Two patients have cholangitis; both had injury to the right hepatic artery. The other patients all had normal ultrasonograms of the liver and normal/almost normal liver function tests.

CONCLUSIONS

Bile duct injuries continue to occur, are serious and may result in death. Injury to the right hepatic artery is present in many cases. Patients are referred late to a competent center, resulting in serious infection in 70%.

Magnetic-resonance-guided percutaneous cryoablation of hepatic tumours

OBJECTIVE

To study the feasibility of percutaneous cryoablation of hepatic tumours monitored by magnetic resonance imaging (MRI).

DESIGN

Prospective study

SETTING

University hospital, Norway

PATIENTS

Six patients with hepatic metastases from colorectal cancer.

INTERVENTIONS

Percutaneous cryoprobe positioning under general anaesthesia. Positioning and freezing monitored by near-real-time MRI using an open 0.5 Tesla MRI configuration system.

MAIN OUTCOME MEASURES

Safety and feasibility of the procedure. Measurement of volumes of cryolesions.

RESULTS

One patient developed a biliary leakage that had to be drained. Four patients developed pleural fluid. Two small tumours were adequately cryoablated. In the remaining 4 patients with large (>4 cm) tumours, an adequate cryolesion could not be formed. Cryolesion volumes larger than 105 cm3 were not produced even using 3-4 probes. MRI visualised the growing cryolesion well, but positioning of the cryoprobes was time-consuming.

CONCLUSION

MR guided cryoablation is clinically feasible and gives good visualisation of the procedure. Patients with small tumours (<3 cm) seem to be best suited to this percutaneous approach as cryolesion volumes claimed to be adequate for tumour destruction can be produced. Measurement of tumour volume preoperatively may help to select patients who will respond.

Veno-venous bypass in liver transplantation: heparin-coated perfusion circuits reduce the activation of humoral defense systems in an in vitro model

We studied the effects of bypass circuit surface heparinization on kallikrein-kinin, coagulation, fibrinolytic and complement activation in a closed model system for simulating veno-venous bypass (WBP) in orthotopic liver transplantation (OLT). The circuits were identical to those in routine use during clinical OLT in our institution. Fresh whole human blood diluted 1:2 with Ringer's acetate was circulated at a non-pulsatile flow (2 l/min) and at a constant temperature (37.5 degrees C) for 12 h. In 10 experiments, the entire inner surface of the circuits was coated with end-point attached heparin (HC). In the remaining 10, non-treated PVC tubing was used (NC). Components of the plasma kallikrein-kinin, coagulation, fibrinolytic and complement systems were analyzed using functional techniques (chromogenic peptide substrate assays) and enzyme immunoassays at baseline, 3 and 12 h. Significant activation of the initial (C3bc) and terminal (TCC) components of the complement system were found in both the NC and HC groups after 3 and 12 h: C3bc: NC: baseline = 4 (3.5-7.7), 3 h = 17.3* (12.5-27), 12h = 31* (17.7-63.6), HC: baseline = 4.9 (3.2-6.8), 3h = 9* (6-14.4), 12h = 13.7* (7.4-18.1). TCC: NC: baseline = 0.4 (0.2-0.6), 3h = 5*(0.8-11.9), 12 h: 13.1* (4.2-25.7). HC: baseline = 0.5 (0.1-0.6), 3 h = 0.6* (0.1-0.8), 12 h = 1.2* (0.3-2) AU/ml; median and range (*: p < 0.05). The C3bc and TCC concentrations were significantly higher in the NC group at 3 and 12 h, compared to the HC group: C3bc (NC vs. HC group): 3 h, p < 0.001; 12 h, p < 0.001. TCC (NC vs. HC group): 3h, p < 0.001; 12 h, p < 0.001. Significant increases in the values of thrombin-antithrombin complexes (p = 0.003), prothrombin fragment 1 + 2 (p = 0.006) and plasmin-alpha2-antiplasmin complexes (p = 0.016) were found in the non-coated group, but not in the heparin-coated group during the observation period, showing that the coagulation and fibrinolytic systems were activated in the non-coated circuits. We conclude that heparin-coating of the internal surface of the extracorporeal perfusion circuit used for WBP reduces activation of the plasma cascade systems in a closed venous system in vitro.

Plasma proteolytic activity in liver transplant rejection

In this study, we evaluated the role of proteolytic enzymes belonging to the coagulation, fibrinolytic, and plasma contact systems in the early postoperative phase after orthotopic liver transplantation (OLT). Twenty-nine patients were studied at the time of OLT and during the first 2 postoperative weeks. Blood samples were collected daily after OLT and analyzed for kallikrein-like activity (KK), functional kallikrein inhibition (KKI), plasmin-like activity (PL), and alpha2-antiplasmin (AP). In addition, prekallikrein (PKK), prothrombin (PTH), antithrombin III (AT III), plasminogen (PLG), prothrombin/antithrombin III complexes (TAT), prothrombin fragment 1 + 2 (F1 + 2), and plasmin/alpha2-antiplasmin complexes (PAP) were measured. Nineteen patients experienced biopsy-verified acute rejections (AR) and ten patients had uneventful courses and served as controls. Plasma analyses showed that the contact, coagulation, and fibrinolytic systems were activated during OLT. Following OLT, continuous thrombin and plasmin generation was observed, and these effects were more pronounced in the group having an uneventful course than in patients with AR. Factors that could possibly affect plasma proteolytic activity, such as blood product usage during and after OLT and cold ischemia time of the liver graft, did not differ between the groups, nor did the routine liver function tests, alanine aminotransferase (ALT) and aspartate aminotransferase (AST).

Percutaneous technique for venovenous bypass including a heat exchanger is safe and reliable in liver transplantation

We have introduced and evaluated several modifications of the conventional venovenous bypass (VVBP) in 29 adult patients undergoing liver transplantation (OLT). A percutaneous technique for insertion of a jugular venous return cannula and a femoral vein cannula was applied. The inferior mesenteric vein (IMV) was used for splanchnic decompression, which facilitated dissection of the recipient liver and allowed portal anastomosis to be performed without disconnecting the portal bypass. A heat exchanger was introduced into the bypass circuit to prevent heat loss. The percutaneous technique prevented complications related to dissection in the axilla and groin. Hemodynamic characteristics corresponded to those found using the traditional technique. Complications related to the VVBP were seen in only one patient in whom the femoral catheter was accidentally introduced into the femoral artery. We conclude that percutaneous cannulas, use of the IMV for splanchnic decompression and the introduction of a heat exchanger offer significant benefits and that they are safe and reliable.

[Liver transplantation in Norway. Results after 10 years and 114 transplantations]

A total of 114 liver transplantations were performed in 106 patients in Norway during 1984-1994. Survival after one year was 65% and after three years 57%. The most frequent causes of death were infections and rejections. The survival rate improved considerably during the period, and after 1990 the 1 year survival was 70%. Approximately 2/3 of the patients return to work or education. Very few patients die later than 12 months after the transplantation. The most frequent indications were primary biliary cirrhosis, metabolic liver disease, primary sclerosing cholangitis, autoimmune cirrhosis and fulminant liver failure. The number of liver transplantations (approximately 4 per million inhabitants) is lower in Norway than in the other Nordic countries. The number should be increased to 7-8 per million inhabitants.

Activation of the plasma contact system and hemodynamic changes after graft revascularization in liver transplantation

In this study, the relation between activation of the plasma contact system and hemodynamic changes during orthotopic liver transplantation was evaluated. Nineteen consecutive courses of OLT in 17 adult patients were investigated. Veno-venous bypass was used in all patients. Blood samples were drawn through all phases of the procedure, and analyzed for the following parameters using functional techniques (chromogenic peptide substrate assays): plasma kallikrein (KK), prekallikrein, functional plasma kallikrein inhibition, C1 inhibitor, and alpha 2-macroglobulin. Plasma high molecular weight kininogen (HK) degradation was evaluated using the immunoblotting technique. An abrupt rise in KK activities occurred within 1 min after portal reperfusion of the liver graft (7-16 U/L, P < 0.05). Simultaneously, proteolytic breakdown of HK was seen. The elevated KK activities were maintained the next 1 1/2 hr. Ten min after graft reperfusion, a significant increase in cardiac output compared with the anhepatic phase (7.2-12.4 L/min, P < 0.05) was found. At the same time, systemic vascular resistance fell significantly (817-408 dynes x sec/cm-5, P < 0.05). The increase in plasma KK activities accompanied by simultaneous degradation of HK seen immediately after reperfusion of the liver graft may be due to contact activation as recipient blood contacts with the underlying basement membrane of injured sinusoidal endothelium in the transplanted liver. We suggest that hemodynamic changes associated with the postreperfusion syndrome seen after revascularization of the liver in OLT could at least be caused in part by bradykinin release due to contact activation.

Risk factors associated with mortality and morbidity after elective splenectomy

OBJECTIVE

To assess the effect of certain risk factors on the outcome of elective splenectomy.

DESIGN

Retrospective cohort study.

SETTING

University hospital, Norway.

SUBJECTS

135 patients who underwent elective splenectomy from 1978 to 1992 for autoimmune diseases (n = 58), myeloproliferative disorders (n = 58), and other diagnoses (n = 19).

MAIN OUTCOME MEASURES

Morbidity and mortality, and the effect on outcome of age, malignancy, platelet count, treatment with steroids, and blood transfusion during the operative period.

RESULTS

Three patients died, one from each diagnostic group; all had been treated with steroids. The overall complication rate was 31% (42/135). Most of the complications (31, 74%) were infective (pneumonia n = 23, abscess or sepsis n = 8). Nine patients required reoperation (haemorrhage, n = 6, and wound dehiscence, n = 2, and bowel obstruction, n = 1). Neither treatment with steroids nor severe thrombocytopaenia were risk factors for any complication, but age (p = 0.05) and a diagnosis of myeloproliferative disease (p = 0.08) had an important though not significant role. Blood transfusion during the operative period was the only independent risk factor (p = 0.001), and further analysis showed that transfusion of blood had a dose dependent effect on the incidence of infective complications (chi-square for linear trend 14.21 p < 0.001).

CONCLUSION

To reduce the complication rate of elective splenectomy blood transfusion during the operative period should be avoided.

Radiology of the ileal J-pouch-anal anastomosis (IPAA)

Forty-four consecutive patients operated on with ileal J-pouch-anal anastomosis (IPAA) and diverting ileostomy were examined with barium contrast medium of the pouch before closure of the ileostomy. CT was performed in 4 of the patients. The anatomy of the ileal reservoir as well as complications were assessed. As normal postoperative anatomy we found a "blind loop" at the upper part of the reservoir in 29 patients and a contrast lucency at the anastomosis between the anal channel and the pouch in 5 patients. Complications were revealed at barium contrast medium examinations in 13 patients, including stenoses at the anastomosis between the pouch and the anal channel in 8 patients and fistulas in 5 patients. CT was especially valuable in the exact diagnosis and location of a pelvic abscess in 3 patients, none of which was shown at pouchography.

[Natural products can be hazardous to health]

Side effects of herbal and health food products have been infrequently reported such as hepatic damage after use of such products. Four such patients were treated in our department in the course of two years. In all four patients, the use of herbal remedies was the probable cause of serious hepatic damage, but both etiology and pathogenesis were difficult to establish. Two major areas of concern are inaccurate formulation and contaminated preparations. As long as no therapeutic effect can be demonstrated from this type of medicine, serious side effects are unacceptable. A critical attitude should be adopted towards these medicines and the use of them.

Hepatic and splanchnic metabolism of plasma proteolytic enzymes before, during, and after clamping of the hepatic artery and portal vein

Parameters of the kallikrein-kinin, fibrinolytic, and coagulation systems before (control), during, and after occlusion of the hepatic artery and the portal vein were studied in anesthetized pigs. Hepatic synthesis was observed for kallikrein, plasmin, and plasminogen. The other factors, prekallikrein, kallikrein inhibitor, alpha 2-antiplasmin, prothrombin, and antithrombin-III were all released by the liver and underwent partial hepatic inactivation. The splanchnic area showed a significant synthesis of antithrombin-III. After 90 min of hepatic arterial occlusion, the hepatic release of all factors declined, and synthesis of kallikrein, plasmin, and plasminogen fell an average of 54%. The inactivation of prekallikrein and alpha 2-antiplasmin was increased and the splanchnic synthesis of antithrombin-III fell. Portal vein blood flow fell by 38%. After 15 min of portal vein occlusion (combined occlusion of the hepatic artery and portal vein) the splanchnic area started to synthetize kallikrein and the hepatic inflow of kallikrein increased by more than 50%. Splanchnic metabolism of other factors was also altered, but after revascularization of the hepatic artery changes were mostly insignificant. Hepatic synthesis of kallikrein and plasmin remained depressed after reperfusion, and that of plasminogen fell to zero. After this short period of occlusion of the hepatic artery and portal vein, plasma concentrations of prekallikrein, plasmin, and alpha 2-antiplasmin were decreased, causing a state of fibrinolysis. A role for kallikrein/bradykinin in hepatic blood flow regulation is proposed and implications of these observations for liver transplantation are discussed.

Effect of phlorizin on hepatic bile production before and during glucose infusion

During intravenous infusion of glucose, bile secretion is reduced (cholestasis), indicating that hepatocellular metabolism of glucose could have harmful effects on the liver. Phlorizin has been identified as a compound capable of impeding glucose uptake of liver cells. To examine whether phlorizin had any effect on glucose-associated cholestasis, three groups of experiments were performed on anaesthetized pigs. In group I phlorizin (100 mg/kg body wt) during normoglycaemia stimulated bicarbonate-dependent bile secretion by 56 +/- 4%. After phlorizin, hyperglycaemia decreased both bile acid- and bicarbonate-dependent bile secretion by 37 +/- 4%. But after the glucose load normalization of plasma glucose concentration increased the bicarbonate-dependent fraction by 38 +/- 4%. In group II phlorizin (100 mg/kg body wt, infused intravenously) during hyperglycaemia stimulated bicarbonate-dependent bile secretion by 35 +/- 5%. In group III bile secretion was continuously stimulated by infusion of Na-taurocholate. Hyperglycaemia reduced bicarbonate-dependent bile secretion by 33 +/- 4%, but after phlorizin both bile acid- and bicarbonate-dependent bile secretion increased on average by 121 +/- 8%. The osmotic effect of hyperglycaemia cannot be blocked by phlorizin, but judged by the effect on bile secretion, phlorizin may decrease the cholestatic effect induced by metabolism of glucose. Phlorizin could be an interesting compound for use in solutions for organ preservation.

Effect of insulin on hepatic bile secretion during normoglycaemia and hyperglycaemia

The roles of hyperosmolality, hyperglycaemia, and insulin in glucose-induced reduction of bile secretion were examined in anaesthetized pigs. Compared with normoglycaemia, intravenous infusion of isotonic glucose reduced bile acid-dependent bile secretion at a plasma glucose concentration of 18 mmol/l, with 34 +/- 4%. Lowering of plasma glucose concentration to normoglycaemia after administration of insulin (10 U/kg body wt. low dose) increased bicarbonate-dependent bile secretion by 23 +/- 3%. Induction of hyperglycaemia (plasma glucose concentration, 16 mmol/l) and the combined infusion of isotonic glucose and the low insulin dose decreased bile secretion by 22 +/- 3%. During hyperglycaemia (plasma glucose concentration, 16 mmol/l) the combined infusion of isotonic glucose and a high dose of insulin (60 U/kg body wt) increased bile acid-dependent bile secretion by 26 +/- 3%. Hyperglycaemia reduces bile secretion without altering plasma osmolality. Endogenous production (or too rapid degradation) of insulin may be too small during intravenous glucose infusion to cope with the metabolic demands of hepatocellular glucose conversion. This may be overcome by administration of insulin in a large dose, which stimulates bile acid secretion.

Mechanism of ursodeoxycholic acid- and canrenoate-induced biliary bicarbonate secretion and the effect on glucose- and amino acid-induced cholestasis

The mechanism of ursodeoxycholic acid (UDCA)- and canrenoate-induced bicarbonate choleresis was studied before and during the administration of glucose or amino acids in anaesthetized pigs. Previous studies have shown that the canalicular secretion has, on a molar basis, a relationship among the secretion of chloride, bicarbonate, and bile acids of 0.9, 0.3, and 1, respectively. Ductular secretion is associated with the transport of 0.25 mol chloride per 1 mol bicarbonate. In control experiments UDCA was associated with a biliary secretion of about 1.3 mol chloride and 0.5 mol bicarbonate per 1 mol bile acid, and canrenoate caused the secretion of 1.2 mol chloride per 1 mol bicarbonate. Intravenous infusion of glucose or amino acids increased these relationships, and after administration of UDCA or canrenoate, these relationships were still increased by at least 70% on average when compared with the control experiments. A reduction in bile secretion after glucose or amino acid infusion is opposed by UDCA or canrenoate. The effect of UDCA or canrenoate on bile secretion is not disturbed by glucose or amino acids. Both substances stimulate canalicular bicarbonate secretion and could be of importance in improving cholestatic conditions.

Perivascular low attenuation zone at CT of liver transplants. A follow-up study

A follow-up study of 30 liver transplant patients confirmed that periportal and pericaval rims of low attenuation found at CT reflect hepatic lymph stasis. The perivascular rims are found coexistent with acute allograft rejection, but are not correlated with acute rejection, immunosuppressive agents (cyclosporin A), or intercurrent infection. Disappearance of the perivascular low attenuation zone in our material coincided with normalization of liver biochemistry probably associated with reestablishment of lymphatic drainage. Like previous reports on experimental hepatic lymph stasis, our study thus indicates that lymph stasis may have harmful effects on human liver allografts.

Dose-dependent effects of glycine, alanine, and glucose on hepatic bile secretion, oxygen consumption, and hemodynamics

To assess the effects of a small (0.5%) and a large dose (5%) of glycine and alanine and of hypertonic glucose on hepatic bile secretion, oxygen consumption, and hemodynamics, experiments were performed on anesthetized pigs. Only the large dose of amino acids exerted significant changes. Glycine, alanine, and glucose reduced bile acid-dependent bile secretion gradually, which was nearly halved from a control value of 0.32 +/- 0.04 ml/min. Oxygen consumption was thereby continuously stimulated during amino acid and glucose infusion and increased from 448 +/- 132 mumol/min before to 995 +/- 226 mumol/min after the infusion of glycine, alanine, and glucose. Hepatic arterial blood flow increased from 214 +/- 14 ml/min to 238 +/- 14 ml/min after glycine infusion, whereas portal venous blood flow decreased from 542 +/- 50 ml/min to 481 +/- 47 ml/min. Total hepatic blood flow remained unchanged. Alanine and glucose provoked no further changes in hepatic blood flow. Bile secretion is a sensitive marker of hepatic metabolism, whereas hepatic blood flow is not a dominant regulator of bile secretion. Stimulation of hepatic metabolism is not followed by changes in total hepatic blood flow.

Computed tomography after modified Whipple procedure with pancreatic duct occlusion

Eighty-two CT examinations performed on 28 patients who had undergone a modified Whipple procedure including pancreatic duct occlusion were reviewed. Reduction of the antero-posterior diameter of the body and tail of the pancreatic remnant was observed on consecutive scans in 8 patients (29%). Decreasing liver attenuation was seen in 4 patients (14%) postoperatively, and pseudocysts in the pancreatic remnant in 6 (21%). In 10 examinations performed because of suspected intraabdominal abscess postoperatively, abscess was diagnosed in 2 patients. In 62 routine follow-up CT examinations, significant positive findings were diagnosed in 5 patients: tumor recurrence or metastases in 4, and a large pseudocyst in one. CT is of value in the early postoperative phase to reveal postoperative complications and in the follow-up of patients with specific symptoms indicating tumor recurrence or metastases.

Amino acid- and glucose-induced cholestasis before and during secretin stimulation

To identify the mechanisms of reduced bile flow after hypertonic amino acid and glucose infusion, acute experiments were performed on anesthetized pigs. When secretin was not administered, amino acids or glucose reduced bile acid-dependent bile secretion to 65 +/- 3% of control. During secretin stimulation amino acids or glucose diminished bile acid-independent bile secretion to 78 +/- 2% of control. No changes in serum bilirubin, alanine aminotransferase, and aspartate aminotransferase were observed. Amino acids and glucose attack different mechanisms responsible for bile formation, but the result is that when secretin is not administered, biliary secretion of bile acids is reduced, and, accordingly, bile acid-dependent bile flow diminished. During secretin stimulation biliary NaHCO3 secretion is depressed, accounting for a fall in bile acid-independent bile flow. Amino acids exert no effect on bile acid secretion or, as a result, on bile acid-dependent bile flow after secretin infusion.

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.

Role of osmosis in biliary NaCl secretion and bile formation

To challenge the osmotic hypothesis of biliary NaCl secretion and bile formation, experiments were performed in anaesthetized pigs. An increase in plasma osmolality of 7 +/- 1 mosm/kg H2O induced by intravenous sucrose infusion decreased NaCl secretion, NaHCO3 secretion, and bile flow by 36 +/- 3%, 34 +/- 2%, and 34 +/- 3%, respectively. There was no change in the biliary concentration of NaCl and NaHCO3. When bile acids were infused intravenously, the secretion of 1 mmol bile acids caused an osmotic flow of 12.0 ml bile containing 0.92 mmol NaCl and 0.30 mmol NaHCO3 in an isotonic solution. Bile acids are therefore much stronger choleretic substances than NaHCO3. When the plasma sodium concentration was increased to 200 mM, bile flow increased by 31 +/- 5% and the secretion of bile acids, NaHCO3, and NaCl was increased by 63 +/- 3%, 96 +/- 4%, and 93 +/- 4%, respectively. These data are consistent with osmotic transport as the main mode of bile formation, but diffusion could be responsible for a small fraction. A raised plasma sodium concentration stimulates osmotic formation of bile by increasing both the bile acid-dependent and -independent secretion through stimulation of biliary bile acid and NaHCO3 secretion.

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.

Dependency of renal potassium excretion on Na,K-ATPase transport rate

Potassium secretion may depend on the transport rate of Na, K-ATPase in basolateral cell membranes of distal tubular cells. To examine this hypothesis experiments were performed in anaesthetized dogs during inhibition of proximal potassium reabsorption by acetazolamide or mannitol (fractional potassium excretion 1.2 - 1.4) or additional stimulation of potassium secretion by ethacrynic acid (fractional potassium excretion 2.1). Ouabain in a dose which inhibits 70-80% of the Na, K-ATPase activity reduced fractional potassium excretion to 0.8 - 0.9 by an effect on distal tubular secretion since potassium transport in the proximal tubules was not affected. Ouabain-sensitive potassium excretion varied in proportion to ouabain-sensitive sodium reabsorption during variation in glomerular filtration rate, even at urinary sodium concentrations exceeding 80 mmol X 1(-1). In experiments without ouabain, saline infusion raised potassium excretion and sodium reabsorption until maximal Na,K-ATPase transport rate was reached, as judged from heat production measurements, but not during further increments in urine flow. After inhibition of Na,K-ATPase activity by hypokalaemia, potassium excretion and cortical heat production remained constant over a wide range of urine flow and sodium excretion. We conclude that potassium secretion is dependent on intact Na,K-ATPase activity and is stimulated by sodium delivery to the distal nephron until maximal transport rate of the enzyme is reached.

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.

Mechanism of hepatic bicarbonate secretion and bile acid independent bile secretion

To examine hepatic bicarbonate transport and bile acid independent bile secretion, bile was sampled via a T-tube inserted into the common bile duct of anaesthetized pigs. Secretin was infused intravenously at a rate of 2.7 C.U./kg body weight h-1 (large dose) or 0.45 C.U./kg body weight h-1 (small dose). Hepatic water and electrolyte secretion were studied during systemic acid-base disturbances while secretin was continuously administered. Systemic acidosis reduced the rate of NaHCO3 secretion which fell in proportion to changes in plasma pH, by 9% and 2% per 0.1 pH unit for the large and small dose of secretin, respectively. Plasma pCO2 and bicarbonate concentration had little influence on NaHCO3 secretion. Consequently, plasma pH appeared to be the main determinant of hepatic NaHCO3 secretion during acid-base changes. Secretion of 1 mol NaHCO3 was accompanied by an isotonic solution containing water and 0.25 mol NaCl. After secretin infusion, 14C-erythritol clearance increased in proportion to bile flow. Bicarbonate secretion is determined by a gradient limited H+-pump at the contraluminal cell. During secretin stimulation bile acid independent bile secretion is osmotically driven by bile NaHCO3 flux.

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.

Filtered bicarbonate and plasma pH as determinants of renal bicarbonate reabsorption

To examine if bicarbonate reabsorption varies with filtered bicarbonate and plasma pH, we infused anesthetized dogs i.v. with sodium chloride and sodium bicarbonate to alter plasma bicarbonate concentration (PHCO3) without changing hematocrit. Examinations in five dogs over a wide range of glomerular filtration rates (GFR) during ethacrynic acid infusion showed that bicarbonate reabsorption at equal filtered load and equal plasma pH of 7.5 was not significantly changed by increasing PHCO3 from 30.2 +/- 0.4 to 55.2 +/- 0.6 mM and PCO2 from 33.8 +/- 0.7 to 74.1 +/- 2.1 mm Hg. Examinations during respiratory and metabolic alkalosis in five dogs at plasma pH of 7.8 showed that bicarbonate reabsorption at equal filtered load was not significantly different at a PCO2 of 20.2 +/- 0.8 and 36.8 +/- 0.8 mm Hg. Finally, in five dogs that did not receive ethacrynic acid, plasma pH was lowered by inducing respiratory acidosis at a PHCO3 of 30 mM and raised during progressive respiratory and metabolic alkalosis, Bicarbonate reabsorption was linearly related to plasma pH within the range 7.1 to 7.85 (r = 0.92). By altering plasma pH by 0.1 unit, bicarbonate reabsorption was altered by 10 +/- 1%. Thus, filtered bicarbonate rather than GFR and plasma pH rather than PCO2 are important acute regulators of bicarbonate reabsorption. This regulation may be achieved by determining pH and bicarbonate concentration in the luminal fluid along the proximal tubules.

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

Pancreatic HCO-3 secretion is caused by proton flux from pancreatic cells to interstitial fluid which, hypothetically, may be derived by a Na-pump or a proton pump. A Na-pump would reabsorb protons from pancreatic duct in proportion with plasma Na+ concentration (PNa+). A proton pump would cause passive HCO-3 flux into pancreatic ducts that would vary with pancreatic jice HCO-3 concentration (CHCO-3). Because pancreatic ducts are water-permeable, CHCO-3 varies with plasma osmolarity. This phenomenon allows testing of the two hypotheses. Intravenous infusion of hypotonic salt solution to 5 anesthetized, secretin infused pigs (2.7 C.U./kg b. wt. H-1) lowered PNa+ by 20 +/- 2 mmol/l and CHCO-3 by 20 +/- 4 mmol/l and increased pancreatic HCO-3 secretion by 71 +/- 5 mumol/min. Intravenous infusion of hypertonic salt solutions to 5 other pigs raised PNa+ by 52 +/- 3 mmol/l and CHCO-3 by 54 +/- 3 mmol/l and reduced HCO-3 secretion by 86 +/0 26 mumol/min. Isotonic glucose infusion lowered PNa+ by 27 +/- 2 mmol/l and did not change CHCO-3 nor HCO-3 secretion rate in 5 pigs. These findings comply with the proton pump hypothesis and are at variance with the Na-pump hypothesis of pancreatic HCO-3 secretion.

Mechanism of osmotic diuresis

Mannitol might inhibit paracellular reabsorption of water and sodium chloride in the proximal tubules by reducing the osmotic driving force. We examined this hypothesis in anesthetized dogs. Bicarbonate reabsorption was kept constant by sodium bicarbonate infusion, and transcellular sodium chloride reabsorption was inhibited by ethacrynic acid. The glomerular filtration rate (GFR) was varied by altering renal perfusion pressure. Mannitol infusion reduced sodium chloride reabsorption from 62 +/- 5% to 33 +/- 5% of the filtered load. The calculated increase in reabsorbate osmolality, averaging 82 +/- 6 mOsm/kg H2O, was due to sodium bicarbonate and equalled the increase in plasma osmolality. Mannitol concentration averaged 81 +/- 7 mM in plasma and 101 +/- 12 mM in urine. A linear relationship between reabsorption and GFR (glomerulo-tubular balance) was maintained over the same range of GFR before and after mannitol infusion. Mannitol infusion reduced sodium chloride reabsorption from 2.6 to 1.4 moles for each mole of sodium bicarbonate reabsorbed. During mannitol infusion, acetazolamide inhibited sodium bicarbonate reabsorption as in control experiments, but reduced sodium chloride reabsorption less. We conclude that reduced water reabsorption increases sodium bicarbonate concentration in the paracellular fluid as much as mannitol concentration is raised in the plasma and glomerular filtrate. Along the proximal tubules, net osmotic force is progressively reduced as mannitol concentration rises, accounting for reduced water and sodium chloride reabsorption.

Relationship between plasma pH and pancreatic HCO3- secretion at different intravenous secretin infusion rates

The relationship between pancreatic HCO3- secretion and plasma pH during acute systemic acid-base changes was investigated in 6 anesthetized, artificially ventilated pigs (20-25 kg) at 2 different, i.v. secretion infusion rates. At 0.45 C.U./kg b. wt. h-1 secretin infusion and plasma pH 7.40 +/- 0.01 pancreatic HCO3- secretion averaged 61+/- 12 mumol/min. Stepwise lowering of plasma pH through i.v. infusion of HCl and CO2 administration to inspired air proportionately reduced secretion rate; estimated zero HCO3- secretion occurring at plasma pH 7.01. Subsequent i.v. secretin infusion at 2.70 C.U./kg b. wt. h-1 increased HCO3- secretion to 249 +/- 42 mumol/min at plasma pH 7.33 + 0.04; stepwise lowering of plasma pH proportionately reduced HCO3- secretion to estimated zero at plasma pH 6.71. A reduction of plasma pH by 0.1 pH unit reduced HCO3- secretion during low and high rate of i.v. secretin infusion by 18 +/- 3 mumol/min and 35 +/- 8 mumol/min, respectively. Secretin infusion rate did not affect pancreatic chloride excretion. These findings support the view that secretin increases HCO3- secretion, and hence proton transport to the interstitial fluid, by augmenting the proton motive force developed by HCO3- secreting cells.

Renal bicarbonate reabsorption during bicarbonate loading

To examine bicarbonate reabsorption at different GFR's, we varied the renal perfusion pressure in anesthetized dogs after inhibiting autoregulation by expanding extracellular volume and infusing ethacrynic acid. At a plasma bicarbonate concentration (PHCO3) of 28 +/- 1 mM, bicarbonate reabsorption varied in proportion to GFR (glomerulotubular balance). When PHCO3 was raised to 52 +/- 2 mM at constant PCO2 and hematocrit, bicarbonate reabsorption was reduced at all levels of filtered load. When plotted against GFR, different results were obtained dependent on the GFR level examined. At the control GFR,. bicarbonate loading reduced bicarbonate reabsorption by 30 +/- 5%. At a GFR level about 50% below the control GFR, bicarbonate loading increased reabsorption by about one third because the inhibitory effect of raising PHCO3 and extracellular pH was not sufficient to counteract the stimulatory effect of a higher filtered load. At intermediate levels of GFR, a rise in PHCO3 did not alter bicarbonate reabsorption. The finding that acetazolamide (30 mg/kg of body wt) at high PHCO3 failed to reduce bicarbonate reabsorption supports the hypothesis that the depressive effect of high extracellular pH on bicarbonate reabsorption may be attributed to reduced net tubular hydrogen ion secretion.

Oxygen requirement of bicarbonate-dependent sodium reabsorption in the dog kidney

The ratio between changes in sodium reabsorption and renal oxygen consumption (Na/O2) was measured in anesthetized dogs at high plasma bicarbonate concentration (32 +/- 1 mM); ethacrynic acid was infused continuously to prevent variations in transcellular NaCl reabsorption when sodium reabsorption was altered by varying plasma PCO2 and glomerular filtration rate (GFR). At high plasma PCO2 (110 mmHg) sodium reabsorption varied in proportion to GRF between 50 and 125% of control GFR (glomerulotubular balance). By reducing PCO2 to 20 mmHg, sodium reabsorption was reduced by 50-60% at constant GFR. The Na/O2 ratio was not significantly different during the two procedures and averaged 48 +/- 2. The ratio between changes in NaHCO3 reabsorption and oxygen consumption averaged 17 +/- 1, which is not significantly different from the Na/O2 ratio of Na-K-ATPase-dependent sodium transport. We propose that NaHCO3 is admitted to the cell by Na+/H+ exchange and that sodium is actively transported by Na-K-ATPase across the peritubular cell membrane; NaHCO3 provides the osmotic force for paracellular reabsorption of water and NaCl (bicarbonate-dependent reabsorption) without additional energy requirement.

Renal metabolic rate during changes in bicarbonate-dependent sodium reabsorption in the proximal tubules

Previous studies indicate that water and at least 2 mol NaCl are reabsorbed in the proximal tubules for each mol NaHCO3 reabsorbed. To examine the effect on cortical energy metabolism of variations in this bicarbonate-dependent sodium reabsorption, the cortical metabolic rate was examined in anaesthetized dogs by the heat production technique during continuous infusion of saline and ethacrynic acid. Sodium reabsorption was altered either by intravenous infusion of a large dose of acetazolamide (500 mg/kg body wt) or by changing plasma Pco2 during metabolic alkalosis. Acetazolamide reduced bicarbonate reabsorption by 71 +/- 2%, sodium reabsorption by 54 +/- 2% and cortical heat production by 21 +/- 3%. A rise in Pco2 to 16.4 +/- 1.3 kPa during metabolic alkalosis increased sodium reabsorption by 25 +/- 3% and cortical heat production by 14 +/- 2%. A similar elevation of plasma Pco2 during metabolic acidosis had no effect on electrolyte reabsorption or the cortical metabolic rate. A reduction in Pco2 to 2.3 +/- 0.3 kPa reduced sodium reabsorption by 40 +/- 3% and cortical heat production by 19 +/- 2%. We conclude that a rise in proximal tubular reabsorption requires energy. However, the changes in energy requirement are small, accounting for previous failures to observe significant changes in cortical energy metabolism during less extensive changes of sodium reabsorption in the proximal tubules.

Re-examination of the dose-reponse relationship for the renal effect of acetozolamide

To examine the effect of acetazolamide on tubular reabsorption of bicarbonate, plasma concentrations of bicarbonate, potassium and PCO2 were kept constant in volume expanded dogs. Intravenous infusion of acetazolamide reduced glomerular filtration rate (GFR) at all dose levels; after a dose of 30 mg/kg body wt GFR was reduced by about 30% and was not reduced further by increasing the infused dose of acetazolamide up to 500 mg/kg body wt. When control GFR was restored by raising systemic blood pressure, increments in filtered and excreted bicarbonate were not significantly different. At control GFR a linear relationship was obtained between bicarbonate reabsorption and log dose when acetazolamide was administered intravenously in doses ranging between 0.3 and 500 mg/kg body wt (r = 0.93). We conclude that a log dose-reponse relationship applies over a 25 times larger dose range than previously assumed.

Coupling of NaHCO3 and NaCl reabsorption in dog kidneys during changes in plasma PCO2

To study the relationship between proximal tubular reabsorption of bicarbonate, sodium, and chloride, the effects of changes in plasma PCO2 were examined in anesthetized dogs. Distal tubular reabsorption was inhibited by ethacrynic acid; plasma bicarbonate concentration was kept constant at 33.4 +/- 0.3 mM; glomerular filtration rate (GFR) was varied over a wide range to examine glomerulotubular balance (constant fractional reabsorption). Hypercapnia (PCO2, 112.0 +/- 2.5 mmHg) increased bicarbonate reabsorption by about 30%, and hypocapnia (PCO2, 19.8 +/- 0.6 mmHg) decreased reabsorption of bicarbonate by more than 50% and altered reabsorption of sodium, chloride, and bicarbonate in the molar ratios 2.7:1.6:1, respectively. During hypercapnia the range of glomerulotubular balance was extended to a GFR 125% of control. During hypocapnia glomerulotubular balance was present only at GFR below 50% of control; reabsorption of bicarbonate sodium, and chloride was constant at GFR exceeding 50% of control. During metabolic acidosis hypercapnia had no significant effect on reabsorption of bicarbonate, sodium, and chloride. These observations support the hypothesis that bicarbonate reabsorption is the main driving force for osmotic reabsorption of water and NaCl in the proximal tubules.

Characteristics of transcellular NaCl reabsorption in the kidney

To examine the characteristics of transcellular, energy-requiring NaCl reabsorption, increased delivery of tubular fluid of different bicarbonate and chloride composition to the outer medulla was achieved by infusion of acetazolamide (30 mg/kg body wt) or 0.9% NaCl in anaesthetized dogs. As an index of energy-requiring NaCltransport, cortical and outer medullary metabolism were determined by the heat production technique. Outer medullary metabolism was correlated to sodium excretion but not to chloride excretion. A rise in sodium excretion up to 20-25% of the filtered load during hydropenia was associated with a 70-80% increase in outer medullary metabolism. Further increments in sodium excretion induced by increasing systemic blood pressure and thereby increasing glomerular filtration rate or by infusing 2.9% NaCl did not significantly increase either reabsorption of sodium or cortical and outer medullary metabolism. By infusion of furosemide (2mg/kg body wt) sodium reabsorption and outer medullary heat production could be reduced below control values. These experiments show that sodium rather than chloride determine transcellular NaCl reabsorption. The maximal capacity of this reabsorption system is approached first at sodium excretion rates beyond the physiological range. Calculations based on clearance studies and heat production measurements, before and after furosemide infusion, indicate that transcellular NaCl reabsorption accounts for more than half of the NaCl reabsorption in the kidney.

Factors limiting renal proximal tubular reabsorption at high glomerular filtration rate

To examine the factors limiting proximal reabsorption, distal reabsorption was inhibited by continuous infusion of ethacrynic acid in anaesthetized dogs. During combined infusion of ethacrynic acid and 0.9% saline, autoregulation of renal blood flow is impaired and glomerular filtration rate (GFR) can be varied by lowering or raising renal perfusion pressure. During lowering of GFR reabsorption of bicarbonate, chloride and sodium varied in proportion to reduction in GFR (glomerulotubular balance), but during elevation of GFR the increase in the filtered load of all three ions was excreted and a maximal rate of reabsorption was approached. Administration of acetazolamide reduced the reabsorption of bicarbonate, chloride and sodium in the ratio 1:2:3, whether GFR was at control, reduced (74% of control) or increased (124% of control). Renal blood flow was 80--90% greater at high than at low GFR. These observations indicate that bicarbonate concentration increases along the proximal tubules during elevation of GFR, until plasma bicarbonate concentration is approached at the distal end of the proximal tubules. From then on, bicarbonate reabsorption cannot be further raised by increasing GFR. Proximal tubular sodium reabsoption reaches a maximum at high GFR because NaCl reabsorption varies in proportion to bicarbonate reabsorption.

Oxygen requirement of renal Na-K-ATPase-dependent sodium reabsorption

The oxygen requirement of the Na-K-ATPase-dependent sodium transport system was examined in anesthetized dogs infused with 15% mannitol-Ringer solutions at a rate of 35 ml/min. Because of renal vasodilatation and abolished autoregulation, filtered sodium (FNa) could be varied over a wide range by progressive aortic constriction. Sodium reabsorption (RNa) and renal oxygen consumption (RVO2) varied in proportion to FNa (r greater than 0.9). Ouabain, which inhibits Na-K-ATPases, reduced RVO2 by 45 +/- 6%. During subsequent aortic constriction, the ratio delta RNa/delta FNa averaged 0.45 (glomerulotubular balance) (r less than 0.9), whereas RVO2 was not significantly altered. Comparisons of deltaRNa/deltaFNa before and after ouabain administration, indicate that about half of an increase in sodium delivery to the distal nephron is reabsorbed by the Na-K-ATPase-dependent sodium transport system and that deltaRNa/deltaRVO2 (Na/O2 ratio) of this system averages 14.5 +/- 1.3. This Na/O2 ratio corresponds to 2.4 sodium ions transported per ATP dephosphorylated as found in other tissues.