Gluconeogenesis from glutamine and lactate in the isolated human renal proximal tubule: longitudinal heterogeneity and lack of response to adrenaline

Recent studies in vivo have suggested that, in humans in the postabsorptive state, the kidneys contribute a significant fraction of systemic gluconeogenesis, and that the stimulation of renal gluconeogenesis may fully explain the increase in systemic gluconeogenesis during adrenaline infusion. Given the potential importance of human renal gluconeogenesis in various physiological and pathophysiological situations, we have conducted a study in vitro to further characterize this metabolic process and its regulation. For this, successive segments (S1, S2 and S3) of human proximal tubules were dissected and incubated with physiological concentrations of glutamine or lactate, two potential gluconeogenic substrates that are taken up by the human kidney in vivo, and glucose production was measured. The effects of adrenaline, noradrenaline and cAMP, a well established stimulator of gluconeogenesis in animal kidney tubules, were also studied in suspensions of human renal proximal tubules. The results indicate that the three successive segments have about the same capacity to synthesize glucose from glutamine; by contrast, the S2 and S3 segments synthesize more glucose from lactate than the S1 segment. In the S2 and S3 segments, lactate appears to be a better gluconeogenic precursor than glutamine. The addition of cAMP, but not of adrenaline or noradrenaline, led to the stimulation of gluconeogenesis from lactate and glutamine by human proximal tubules. These results indicate that, in the human kidney in vivo, lactate might be the main gluconeogenic precursor, and that the stimulation of renal gluconeogenesis observed in vivo upon adrenaline infusion may result from an indirect action on the renal proximal tubule.

Health care system reform. Ontario family physicians' reactions

OBJECTIVE

To determine the effect on a cohort of family physicians of health care system reforms in Ontario and the relationship of reforms to their career satisfaction.

DESIGN

Follow-up survey in 1999 of a cohort initially studied in 1993, posing many of the original questions along with some new ones. Four focus groups of other Ontario family physicians.

SETTING

Family practices in Ontario.

PARTICIPANTS

All family physicians who had received certification after completing a family medicine residency between 1989 and 1991 and were practising in Ontario in 1993. This report addresses only those members of the cohort who continued to practise family medicine in Ontario (N = 236). Four focus groups with a total of 27 family physicians.

MAIN OUTCOME MEASURES

Reaction to health care system reforms. Perceived effect of reforms on practice. Current perception of quality of health care system and level of career satisfaction and changes in these variables since 1993.

RESULTS

Response rate was 53% of original cohort. Only three of 13 selected health reforms were believed to have had a favourable effect. Physicians reported lower levels of satisfaction with their careers. Overall quality of the health care system was perceived by both respondents and focus group members to have declined. Several difficulties affected practice and personal life.

CONCLUSION

Family physicians viewed the effect of health care reforms negatively and were significantly less satisfied with their careers than they were in 1993. Better consultation with stakeholders before implementation of reforms is needed to ensure that these stakeholders understand the likely effects of these reforms.

Professional activity. How is family physicians' work time changing?

OBJECTIVE

To examine hours worked professionally, work preferences, and changes in both of these and their correlates.

DESIGN

Repeated surveys done in 1993 and 1999.

SETTING

Ontario family practices.

PARTICIPANTS

Cohort of physicians certified in family medicine between 1989 and 1991 after family medicine residency who were surveyed in 1993 when they resided in Ontario.

MAIN OUTCOME MEASURES

Self-reported hours spent weekly on professional activities, desired hours of professional work, and balance between work and other activities.

RESULTS

Fifty-three percent (293) of 553 physicians responded to the 1999 survey; 91% had remained family physicians; 85% of these had participated in the 1993 survey. The difference between the hours that family physicians preferred to work professionally and their actual hours of work had increased since 1993. Childless physicians, women physicians with preschool children, and women physicians married to other physicians worked fewer hours professionally than other physicians in 1999. Female physicians and physicians without children worked closer to their preferred hours than other physicians. Reporting a preference to work fewer hours professionally in 1993 was linked with a reduction in professional activities by 1999.

CONCLUSION

Greater attention should be paid in physician resource planning to the family life cycle of female physicians. Lifestyle changes could lead to a reduction in professional activity among these physicians.

Physicians certified in family medicine. What are they doing 8 to 10 years later?

OBJECTIVE

To determine field of medicine and location of a cohort of physicians certified in family medicine between 1989 and 1991 and residing in Ontario in 1993 and to gather information on the scope of practice of family physicians in the cohort in 1999.

DESIGN

Responses to a mailed questionnaire sent in 1999 were compared with responses to a 1993 survey of this group.

SETTING AND PARTICIPANTS

All family physicians in Ontario in 1993 who received certification in 1989, 1990, or 1991 after completing a family medicine residency. Seven of 557 respondents to the 1993 survey were ineligible; 293 physicians (53%) responded to the 1999 survey.

MAIN OUTCOME MEASURES

Field, location, and scope of practice.

RESULTS

About 91% of the cohort were still practising family medicine, although 11% of these had restricted their practices to certain areas within family medicine. Physicians migrated from Ontario (6%) in nearly equal numbers to other provinces and other countries, predominantly the United States. More family physicians offered counseling, shared antenatal care, and newborn care in 1999 than in 1993. Those with restricted family practices provided fewer types of services and were less likely to provide antenatal or intrapartum care or to provide in-hospital services.

CONCLUSION

Receiving certification in family medicine does not guarantee that physicians will remain in family practice 8 to 10 years later. Loss from general family medicine to restricted practices within family medicine and specialization was greater than loss from migration.

Glutamine synthesis is heterogeneous and differentially regulated along the rabbit renal proximal tubule

Glutamine synthesis, a major process for ammonia detoxification and the control of acid-base balance, occurs from various precursors in suspensions of rabbit proximal tubules. However, no data are currently available on the distribution of glutamine synthesis along the rabbit proximal tubule, and its modulation by changes of substrate concentration. Therefore we have microdissected and incubated the three parts (S1, S2 and S3) of rabbit proximal tubules and measured glutamine synthesis from alanine and aspartate. With a physiological concentration of alanine (0.25 mM) or aspartate (0.05 mM), glutamine synthesis in the S1 segment was about half of that in the S2 and S3 segments, and was greater from alanine than from aspartate along the entire proximal tubule. Elevation of alanine and aspartate concentrations to 5 mM increased glutamine synthesis in both a substrate- and segment-dependent manner. It is concluded that glutamine synthesis occurs from alanine and aspartate along the entire rabbit proximal tubule; however, contrary to what might have been expected on the basis of measurement of glutamine synthetase activity, the basal rate of glutamine synthesis and its adaptation to increased substrate availability are heterogeneous along this nephron segment.

Preoperative assessment for difficult intubation in general and ENT surgery: predictive value of a clinical multivariate risk index

Several clinical multifactorial indexes have been described for predicting difficult laryngoscopy or intubation, or both, mostly in general surgery, and less frequently in ENT surgery. The objective of this study was to develop and validate a single clinical index for prediction of difficulty in tracheal intubation in both ENT and general surgery. We studied a population of 1200 consecutive ENT and general surgical patients. Clinical criteria were tested using univariate and multivariate analysis. Difficult intubation was defined as requiring unusual techniques. Logistic regression identified seven criteria as independent predictors of difficult tracheal intubation; previous history of difficult intubation; pathologies associated with difficult intubation; clinical symptoms of pathological airway; inter-incisor gap and mandible luxation; thyromental distance; head and neck movement; and Mallampati's modified test. Point values were assigned to each of these factors in proportion to regression coefficients representing the relative weight of each predictive intubation difficulty factor, the sum comprising the score. The best predictive threshold was chosen using a receiver operating characteristic curve. We then prospectively studied and validated the score in a population of 1090 consecutive ENT and general surgery patients. The sensitivity and specificity of the predictions were 94% and 96% in general surgery, 90% and 93% in non-cancer ENT surgery, and 92% and 66% in ENT cancer surgery, respectively.

Closed and restricted practices

OBJECTIVE

To determine the proportion of recently certificated Ontario family physicians who have closed their practices to new patients or restricted their services.

DESIGN

Cross-sectional survey mailed between September 1993 and January 1994.

SETTING

Ontario family practices.

PARTICIPANTS

All family medicine residency-trained certificants of the College of Family Physicians of Canada from 1989 to 1991 currently practising in Ontario. Response rate was 70% (395 of 564 eligible physicians). Otherwise eligible physicians practising as locums, emergency room physicians, or military physicians were excluded.

MAIN OUTCOME MEASURES

Self-report of practices being closed to new patients and of various restrictions placed on practices.

RESULTS

Nearly one third of respondents had closed their practices to new patients. Although the decision to close a practice correlated with length of time in practice, physicians in metropolitan Toronto were significantly less likely to report closed practices than physicians practising in other regions of Ontario. Restrictions reported related to patients and problems, geographic area, and type of setting(s) serviced. About 45% of respondents did not provide one or more of a defined set of five services.

CONCLUSIONS

Results of this study suggest that family physicians restrict their practices in various ways within the first 5 years after certification.

Increase in oxidative key enzymes in a case of muscle ubiquinol-cytochrome c reductase deficiency

In a 29-year-old patient suffering from exertional muscle intolerance with a ubiquinol-cytochrome c reductase deficiency related to a cytochrome b gene point mutation of the mitochondrial DNA, we conducted a study of the aims of which were: (1) to test whether changes in the maximum activities of muscle key enzymes of the main energy-producing pathways occur, (2) to address the issue of whether fibers of different types are equally affected in their enzymatic machinery involved in energy production, and (3) to correlate the results obtained with histochemical and 31P NMR spectroscopy data. When compared to results obtained in six normal subjects, our study clearly shows that the type I fibers of the patient virtually all contained subsarcolemmal mitochondrial aggregates and increased activities of succinate dehydrogenase and cytochrome c oxidase; microdissected type I fibers also displayed a significant increase in both citrate synthase and beta-hydroxyacyl-CoA dehydrogenase, two key enzymes of mitochondrial oxidative metabolism. Despite these changes in the patient's muscle, its whole energy-producing machinery remained impaired as revealed by a slowed post-exercise recovery of phosphocreatine.

Cohort, gender and practice organization: examining the bounds of collaborative medicine among newly established female and male family physicians in Ontario

This paper analyzes data from a 1993 survey of 395 newly established female and male family physicians in Ontario, Canada, to examine the relationship between practice organization and gender. Previous research suggests that younger physicians, particularly women, tend to enter group practice. Compared to solo practice, groups may offer more predictable incomes, more manageable workloads, peer collaboration and review, and economies of scale. Further, female physicians in groups may develop distinctive styles of collaborative medicine. The results show that a majority of physicians in our cohort are in private community-based group practice. However, while many groups share premises, staff and expenses, and many have common charts and practice guidelines, only a minority incorporate regular meetings to discuss business or patient care, have shared care of hospitalized patients, or audits of physicians' practices. Few gender differences are observed in private group practice: although women physicians attract larger proportions of female patients than do their male colleagues, women and men organize their groups in similar ways and have similarity strong patient-centered attitudes.

Advantages and limitations of the use of isolated kidney tubules in pharmacotoxicology

Among the cellular models used in in vitro renal pharmacotoxicology, isolated kidney tubules, used as suspensions mainly of proximal tubules, offer important advantages. They can be prepared in large amounts under nonsterile conditions within 1-2 h; thus, it is possible to employ a great number of experimental conditions simultaneously and to obtain rapidly many experimental results. Kidney tubules can be prepared from the kidney of many animal species and also from the human kidney; given the very limited availability of healthy human renal tissue, it is therefore possible to choose the most appropriate species for the study of a particular problem encountered in man. Kidney tubules can be used for screening and prevention of nephrotoxic effects and to identify their mechanisms as well as to study the renal metabolism of xenobiotics. When compared with cultured renal cell, a major advantage of kidney tubules is that they remain differentiated. The main limitations of the use of kidney tubules in pharmacotoxicology are (1) the necessity to prepare them as soon as the renal tissue sample is obtained; (2) their limited viability, which is restricted to 2-3 h; (3) the inability to expose them chronically to a potential nephrotoxic drug; (4) the inability to study transepithelial transport; and (5) the uncertainty in the extrapolation to man of the results obtained using animal kidney tubules. These advantages and limitations of the use of human and animal kidney tubules in pharmacotoxicology are illustrated mainly by the results of experiments performed with valproate, an antiepileptic and moderately hyperammonemic agent. The fact that kidney tubules, unlike cultured renal cells, retain key metabolic properties is also shown to be of the utmost importance in detecting certain nephrotoxic effects.

Interest in different types of patients. What factors influence new-to-practice family physicians?

OBJECTIVE

To identify factors that influence new-to-practice family physicians to be particularly interested in certain types of patients.

DESIGN

Qualitative study and cross-sectional survey.

SETTING

Ontario family practices.

PARTICIPANTS

Seven focus groups involved a volunteer sample of 34 physicians who completed family medicine residency training between 1984 and 1989. A convenience sample of 43 physicians who had completed their residencies between 1990 and 1992 were interviewed. All certificates of the College of Family Physicians of Canada currently practising in Ontario who received certification between 1989 and 1991 were surveyed.

MAIN OUTCOME MEASURES

Physician interest as determined by scores on two scales: one labeled "Chronic/Older Patient," designed to assess special interest in geriatric patients, chronic pain patients, palliative care patients, and chronically ill patients, and one labeled "Young Patient," designed to assess special interest in young families and adolescents.

RESULTS

In general, new-to-practice physicians had little interest in caring for older or chronic patients; older physicians and male physicians had greater interest in caring for chronic or older patients. Women physicians, physicians rating higher on the "empathy" and "interest in counseling" scale, and physicians receiving primarily fee-for-service remuneration showed greater interest in caring for young families and adolescents than other physicians.

CONCLUSIONS

Physicians' age, sex, attitudes to patient care, method of remuneration, undergraduate and postgraduate schools of medical training, and the age and sex composition of their practices all influenced their interest in caring for different types of patients.

Time spent on professional activities and unwaged domestic work. Is it different for male and female primary care physicians who have children at home?

OBJECTIVE

To examine how having children affects the hours spent by male and female family physicians on professional activities and on unwaged domestic work.

DESIGN

Survey fielded between September 1993 and February 1994.

SETTING

Ontario.

PARTICIPANTS

All Ontario-based physicians certificated by the College of Family Physicians of Canada between 1989 and 1991 after completing a family medicine residency.

MAIN OUTCOME MEASURES

Self-reported hours spent per week on professional activities and unwaged domestic work.

RESULTS

Response rate was 70%; men and women were equally likely to respond. About half (47.7%) had children at home. Women with children at home spent fewer hours on professional activities (P < 0.001) than men with children, whose hours of professional activity were similar to hours of men without children. Both women and men with children reported spending more time on household maintenance than did those without children. Among physicians with children, although men spent time on child care (mean time 11.4 hours; SD 11), women spent much more time on it (mean time 39.7 hours; SD 21; P < 0.001). The women worked an average of 90.5 hours per week in professional and unwaged activities; men averaged 68.6 hours. Childless physicians worked fewer hours: men 54.1, women 52.6.

CONCLUSIONS

Female physicians with children at home spend more time on child care and household maintenance than their male partners. These responsibilities reduce professional work time (at least until all children are at school full time) and might deter women from active involvement in professional organizations.

Current health policy initiatives and options. New-to-practice family physicians' attitudes

OBJECTIVE

To obtain information about new-to-practice family physicians' attitudes toward current health policy options and initiatives.

DESIGN

Cross-sectional, mailed survey.

SETTING

Ontario family practices.

PARTICIPANTS

Residency-trained Ontario family physicians (395 of 564 eligible physicians replied) who were certified between 1989 and 1991.

MAIN OUTCOME MEASURES

Extent of approval or disapproval for 14 health policy options and initiatives.

RESULTS

A 70% response rate was achieved. More than half of surveyed physicians expressed approval for shifting resources from acute care into preventive care and health promotion (71.6% approved), stricter immigration requirements to limit licensing of foreign physicians in Canada (60.4%), offering physicians salaries as an alternative to fee-for-service (54.0%), and incentives to physicians who wish to practise in community health centres or other forms of salaried group practice (51.1%). Some diversity of opinion was associated with sex, type of practice, primary source of remuneration, and practice location.

CONCLUSIONS

These new-to-practice family physicians display diverse views and should not be seen as sharing a single opinion of health care policy options and initiatives. Many approve of changes to the health care system or are willing to consider policy alternatives.

Sanctions against sexual abuse of patients by doctors: sex differences in attitudes among young family physicians

OBJECTIVE

To explore attitudes of new-to-practice certified family physicians in Ontario concerning sanctions against sexual abuse of patients by physicians and to assess the importance of concern about accusations of sexual abuse in influencing clinical decisions.

DESIGN

Qualitative study and cross-sectional survey.

SETTING

Ontario.

PARTICIPANTS

Focus groups: 34 physicians who completed family medicine residency training between 1984 and 1989 participated in seven focus groups between June and October 1992.

SURVEY

all certificants of the College of Family Physicians of Canada who received certification between 1989 and 1991 and were currently practising in Ontario. Of the 564 eligible physicians 395 (184 men and 211 women) responded, for an overall response rate of 70.0%. The response rates among the male and female physicians were 70.5% and 69.6% respectively.

OUTCOME MEASURES

Physicians' attitudes toward restricting physical examinations done by physicians to same-sex patients, mandatory reporting of sexual impropriety and loss of licence in cases of sexual violation and the perceived importance of concern about accusations of sexual abuse as an influence on clinical decisions.

RESULTS

During the focus groups male physicians in particular expressed concerns about the effect on their practice patterns of the current climate regarding sexual abuse of patients. Female physicians were less concerned about possible accusations of sexual abuse but expressed concerns regarding possible sexualization of the clinical encounter by male patients. In the survey equal proportions of men (163 [93.7%]) and women (191 [92.3%]) disagreed with restricting examinations to same-sex patients. The women were more likely than the men to agree that all suspected cases of sexual impropriety committed by other physicians should be reported (121 [58.7%] v. 86 [50.0%]), whereas the men were more likely to disagree (48 [27.9%] v. 32 [15.5%]) (p = 0.008). The women were also more likely than the men to agree that physicians should lose their licence permanently if they were found guilty of sexual violation (125 [62.2%] v. 73 [43.5%]), whereas the men were more likely to disagree (61 [36.3%] v. 37 [18.4%]) (p < 0.001). Almost half of the men (80 [46.5%]) but only 28 women (14.1%) reported that concerns about accusations of sexual abuse were of importance in their clinical decisions (p < 0.001).

CONCLUSIONS

Young female family physicians practising in Ontario are much more likely than their male counterparts to endorse permanent loss of licence for physicians who sexually abuse patients and are significantly less concerned about accusations against themselves. Neither sex endorses only same-sex examinations by physicians. Educational approaches to protect patients while ensuring that appropriate care continues to be delivered are essential.

Effects of chronic renal failure on enzymes of energy metabolism in individual human muscle fibers

In order to improve knowledge about the mechanisms underlying the alterations of energy metabolism recently observed in the skeletal muscle of patients suffering from chronic renal failure, this study was designed to test (1) whether changes in the activity of key enzymes of energy metabolism occur in the muscle of these patients, and if so (2) whether the different muscle fiber types are equally altered in their metabolic machinery. For this, the maximum activities of 14 enzymes were measured in individual muscle fibers microdissected from biopsies of rectus abdominis muscle obtained from seven normal subjects and seven patients with end-stage renal failure before renal replacement therapy. A large decrease in the activities of beta-hydroxyacyl-coenzyme A dehydrogenase, a key enzyme of the beta-oxidation pathway, of citrate synthase, which initiates the tricarboxylic acid cycle, and of fructose-1,6-bisphosphatase, which contributes to the synthesis of glycogen from lactate, was observed in the three fiber types (slow-twitch oxidative, fast-twitch oxidative-glycolytic, and fast-twitch glycolytic). A smaller reduction of the activities of phosphofructokinase and/or pyruvate kinase, two key enzymes of glycolysis, was also observed in slow-twitch oxidative and/or fast-twitch oxidative-glycolytic fibers. These results demonstrate that the abnormalities of muscle energy metabolism observed in patients with chronic renal failure are due, at least in part, to intrinsic changes in the key enzymes of major energy-providing pathways; they also offer a satisfactory explanation for the defect of oxidative metabolism recently demonstrated in the muscle of these patients.

[Cocaine contact anesthesia for endonasal surgery. Kinetics and clinical tolerance of a concentrated solution]

OBJECTIVE

To assess the pharmacokinetics and clinical tolerance of a 33% cocaine solution administered topically for intranasal surgery.

STUDY DESIGN

Clinical prospective open trial.

PATIENTS AND METHODS

Twelve ASA I patients scheduled for intranasal surgery were sedated with midazolam 2 mg and fentanyl 50 micrograms. Topical anaesthesia was obtained with aqueous 33% cocaine HCl 360 mg, lidocaine HCl 140 mg, adrenaline 0.04 mg and naphazoline 0.4 mg. Venous blood samples were taken before cocaine application and 15, 30, 45, 60, 90, 120, 150, 180, 240 min later. The plasma was immediately separated and the samples were frozen. The concentration of cocaine was measured by HPLC. Potential cardiotoxic and neurotoxic effects were clinically monitored.

RESULTS

The mean dose of cocaine applied was 5.85 +/- 1.3 mg.kg-1 and the dose actually delivered was 4 +/- 1.5 mg.kg-1. The Cmax was 859 +/- 503 ng.mL-1 after a Tmax to 47 +/- 17 min. The mean elimination half-life was 87 +/- 19 min (mean +/- SD). The total clearance and the volume of distribution were respectively 4,521 +/- 1,858 mL.min-1 and 568 +/- 273 L. No clinical evidence of toxicity was found.

CONCLUSIONS

This study shows that it is possible to perform major intranasal surgery under topical anaesthesia with a concentrated solution (33%) of cocaine at a high dose (6 mg.kg-1). These results differ completely with data obtained in addicts.

Substrate uptake and utilization by the kidney of fed and starved rats in vivo

In order to obtain information (1) on the quantitative contribution of various circulating substrates to renal metabolism and (2) on the relative importance of net luminal and basolateral transport for substrate uptake, we have precisely quantified the renal blood flow, the urinary flow, and the rates of substrate handling by the kidney of anesthetized fed and 72-hour-starved rats. For this, the concentration of twelve metabolites were simultaneously measured in arterial and venous whole blood and plasma as well as in urine of each rat thanks to the use of microassays based on enzymatic cycling. In fed rats, the main potential energy sources were glucose and lactate followed by fatty acids, ketone bodies, citrate and glycerol. Starvation caused a large increase in renal uptake and metabolism of fatty acids, ketone bodies, glutamine and glycerol, and a large inhibition of lactate utilization. The net peritubular uptake of acetoacetate, citrate, glycerol and free fatty acids demonstrated in both nutritional states was increased by starvation only for glycerol and free fatty acids; net peritubular efflux of both beta-hydroxybutyrate and ammonium ions was stimulated whereas that of glutamine was converted into net peritubular uptake by starvation.

Effect of valproate, sodium 2-propyl-4-pentenoate and sodium 2-propyl-2-pentenoate on renal substrate uptake and ammoniagenesis in the rat

Experiments were carried out in the intact functioning rat kidney to study the effect of valproate (VPA), a widely used antiepileptic drug and an hyperammonemic agent, but usually without clinical relevance, and of two of its metabolites, sodium 2-propyl-4-pentenoate (4-en-VPA) and sodium 2-propyl-2-pentenoate (2-en-VPA), on the renal production of ammonia and on the renal uptake of glutamine, glutamate and of inhibitors of renal ammoniagenesis; mainly lactate, fatty acids, ketone bodies and alpha-ketoglutarate. Administration of VPA and 4-en-VPA stimulated the uptake of glutamine and glutamate and the production of ammonia by the rat kidney, resulting in an increase in the renal venous release of ammonia and in a hyperammonemia. By contrast, no hyperammonemia was observed after the administration of 2-en-VPA which stimulated renal ammoniagenesis to a lesser extent than VPA and 4-en-VPA, resulting in no stimulation of the renal venous release of ammonia. The three compounds tested caused, in a qualitatively different but, in terms of substrate carbons, in a quantitatively similar manner, a significant diminution of the renal uptake of fatty acids, ketone bodies and alpha-ketoglutarate. These results suggest that, in the rat kidney, VPA, 4-en-VPA and 2-en-VPA stimulate the production of ammonia at least in part by reducing the renal uptake and metabolism of ammoniagenesis inhibitors; the more potent stimulation of renal ammoniagenesis caused by VPA and 4-en-VPA also suggest that these compounds exert their stimulatory effect by an additional mechanism.

Transport of beta-hydroxybutyrate and acetoacetate along rat nephrons: a micropuncture study

The transport of ketone bodies across the luminal membrane of the nephron was studied by means of micropuncture techniques in rats in normal acid-base state. The concentration of beta-hydroxybutyrate (beta-HB) and acetoacetate (AcAc) in plasma, tubular fluid and urine was measured by an ultramicromethod using enzymatic cycling. At endogenous plasma ketone body concentration, approximately 80% of the filtered load of beta-HB and AcAc was reabsorbed in the proximal convoluted tubule, the remaining fraction being almost completely reabsorbed between the late proximal convoluted and the distal tubule; under these conditions, the urinary excretion of ketone bodies was less than 1% of the filtered load. A progressive elevation to steady-state levels of plasma beta-HB resulted in a progressive reduction of the fractional reabsorption of beta-HB and AcAc in the proximal convoluted tubule, which means that reabsorption of ketone bodies in this nephron segment is saturable. No net secretion of ketone bodies could be demonstrated along the nephron even at the highest plasma ketone body concentrations reached. In clearance experiments, the capacity of the rat kidney for reabsorbing both beta-HB and AcAc was found to be limited by a maximal tubular capacity (Tm). The data suggest that, in the young Wistar rat nephron, most of the reabsorption of ketone bodies is carrier mediated.

An enzymatic cycling method for 3-acetylpyridine adenine dinucleotide to increase the sensitivity of enzymatic methods which employ this NAD analog

The NAD analog 3-acetylpyridine adenine nucleotide (APAD), because of its higher oxidation potential, has proven useful for the direct enzymatic measurement of such compounds as lactate, malate, glutamate, etc., for which the equilibrium with NAD+ as oxidant is unfavorable. An enzymatic cycling method which is capable of increasing the sensitivity of such reactions 10,000-fold or more is described. The APADH produced in the original stoichiometric reaction is used to catalyze a cycling reaction that employs lactate and malate dehydrogenases (EC 1.1.1.27 and EC 1.1.1.37) to generate (from lactate plus oxalacetate) very large quantities of pyruvate and malate. After the cycling step, the malate formed is measured with NAD+ and with malate dehydrogenase, plus aspartate aminotransferase, and oxaloacetate to pull this indicator reaction to completion. The application of this cycling method is illustrated by analysis of malate in the range 1 to 10 pmol.

Effect of lithium on renal transport and utilization of alpha-ketoglutarate in the rat

Experiments were carried out in the intact functioning rat kidney to study the effect of lithium on both the renal transport of alpha-ketoglutarate (alpha-KG) along the nephron by micropuncture techniques and the renal uptake and peritubular transport of alpha-KG by measuring the renal blood flow, the urinary flow and the rate of renal alpha-KG delivery, filtration, reabsorption or secretion and excretion. At endogenous plasma alpha-KG concentration, 2.3 mM plasma lithium caused an increase in the fractional excretion of alpha-KG, whereas 4.6 mM plasma lithium led to a net secretion of alpha-KG. The micropuncture data indicate that this secretion occurred between the late proximal and the distal tubule, i.e., in the pars recta and/or in the loop of Henle. When plasma alpha-KG concentration was elevated, the two doses of lithium used inhibited the reabsorption of alpha-KG both in the proximal tubule and in the pars recta and/or the loop of Henle. Renal arteriovenous measurements reveal that, at low plasma alpha-KG concentrations, lithium caused a significant decrease in both the renal uptake of alpha-KG and the peritubular transport of this organic anion. These results suggest that the alpha-KG secreted in the pars recta and/or the loop of Henle was synthesized within the renal cells of the latter segments and not transported from the blood to the tubular lumen. At higher plasma alpha-KG concentrations, both the peritubular transport and the renal reabsorption were reduced by lithium.

Effect of microgravity on metabolic enzymes of individual muscle fibers

Eleven enzymes were measured in individual fibers of soleus and tibialis anterior (TA) muscles from two flight and two control (synchronous) animals. There were five enzymes of glycogenolytic metabolism: phosphorylase, glucose-6-phosphate isomerase, glycerol-3-phosphate dehydrogenase, pyruvate kinase, and lactate dehydrogenase (group GLY); five of oxidative metabolism: citrate synthase, malate dehydrogenase, beta-hydroxyacyl-CoA dehydrogenase, 3-ketoacid CoA-transferase, and mitochondrial thiolase (group OX); and hexokinase, subserving both groups. Fiber size (dry weight per unit length) was reduced about 35% in both muscles. On a dry weight basis, hexokinase levels were increased 100% or more in flight fibers from both soleus and TA. Group OX enzymes increased 56-193% in TA without significant change in soleus. Group GLY enzymes increased an average of 28% in soleus fibers but underwent, if anything, a modest decrease (20%) in TA fibers. These changes in composition of TA fibers were those anticipated for a conversion of about half of the originally predominant fast glycolytic fibers into fast oxidative glycolytic fibers. Calculation on the basis of fiber length, rather than dry weight, gave an estimate of absolute enzyme changes: hexokinase was still calculated to have increased in both soleus and TA fibers, but only by 50 and 25%, respectively. Three of the OX enzymes were, on this basis, unchanged in TA fibers, but 3-ketoacid CoA-transferase and thiolase had still nearly doubled, whereas TA GLY enzymes had fallen about 40%. In soleus fibers, absolute levels of OX enzymes had decreased an average of 25% and GLY enzymes were marginally decreased.

Transport and utilization of alpha-ketoglutarate by the rat kidney in vivo

In order to establish the characteristics of net renal transport and utilization of alpha-ketoglutarate (alpha-KG) in the rat, we have precisely quantified the renal blood flow, the urinary flow and the rates of alpha-KG delivery, filtration, reabsorption or secretion, excretion, uptake or production by an in vivo rat kidney preparation. In normal rats, alpha-KG uptake was higher than alpha-KG reabsorption at both endogenous and elevated plasma alpha-KG concentrations; thus, a net peritubular transport, which was the main supplier of alpha-KG to the renal cells, took place. Saturation of reabsorption and peritubular transport of alpha-KG occurred at blood alpha-KG concentrations about 30 and 150 times above normal, respectively. Acute metabolic acidosis was found to have no effect on renal handling of alpha-KG. At endogenous plasma alpha-KG concentrations, alkalosis converted net renal uptake into net renal production of alpha-KG resulting in addition of alpha-KG by the renal cells both to blood and to the luminal fluid. Elevation of blood alpha-KG concentration restored the renal uptake of alpha-KG. This uptake, which was entirely accounted for by the peritubular transport of alpha-KG, reached a maximum which was lower than that observed in normal and acidotic rats.

Valproate-induced stimulation of renal ammonia production and excretion in the rat

Administration of valproate, a widely used antiepileptic drug, markedly stimulated the production of ammonia by the rat kidney, resulting in an increase in both the renal venous release and the urinary excretion of ammonia. These effects were associated with a diminution of the plasma concentration of urea which was not accompanied by a stimulation of the urinary excretion of urea. These results indicate that the kidneys, together with the liver, might contribute to the hyperammonaemia caused by valproate.

Reabsorption and secretion of alpha-ketoglutarate along the rat nephron: a micropuncture study

The transport of alpha-ketoglutarate (alpha-KG) across the luminal membrane of the rat nephron was studied by micropuncture and microassay techniques. In normal and acidotic rats, approximately 75% of the filtered alpha-KG was reabsorbed in the proximal tubule and 20% in the pars recta and/or loop of Henle at endogenous plasma concentration of alpha-KG. A progressive elevation to steady-state levels of plasma alpha-KG resulted in a progressive reduction of the fractional reabsorption of alpha-KG in the proximal tubule as well as in a progressive increase in the fractional reabsorption of alpha-KG in the pars recta and/or loop of Henle. At plasma alpha-KG concentration 20-40 times above normal, reabsorption of alpha-KG was found to be limited by a maximal tubular capacity. In alkalotic rats, net secretion of alpha-KG in the early proximal convoluted tubule, net reabsorption in the remainder of the proximal convoluted tubule, and net secretion in the pars recta and/or loop of Henle were observed. These micropuncture data indicate that, depending on the acid-base conditions, net reabsorption or net secretion of alpha-KG may occur in at least two distinct sites along the rat nephron.

Renal excretion of ascorbic acid in the rat: a micropuncture study

Ascorbate concentration was measured by high-performance liquid chromatography and amperometry in plasma, tubular fluid, and urine from rats infused with ascorbic acid to steady-state levels. At a low concentration of ascorbate in plasma (Pasc = 0.2 mM) reabsorption occurred along the proximal convoluted tubule (PCT). The fractional delivery (FD) of ascorbate [(TF/P)asc/(TF/P)polyfructosan] to the late proximal convoluted tubule was 0.64 +/- 0.04, and the fractional excretion of ascorbate (FEasc) was 0.56 +/- 0.01. At higher Pasc (0.9 mM) net secretion occurred in the PCT, while the FDasc was 1.5 +/- 0.2 to the early and 1.8 +/- 0.2 to the late PCT. At still higher Pasc the secretory and the reabsorptive transports were saturated and the FDasc and FEasc approached unity, indicating that reabsorptive as well as secretory transport occurs in the proximal tubule. In clearance experiments the reabsorptive transport and secretory transport were inhibited by 2-nitroprobenecid. The drug induced a fall of FEasc when infused at a low rate (0.9 mumol X kg body wt-1 . min-1), which was followed by an increase in FEasc when the rate of infusion of 2-nitroprobenecid was increased to 3 mumol . kg body wt-1 X min-1.

Effects of p-aminohippurate and pyrazinoate on the renal excretion of salicylate in the rat: a micropuncture study

The inhibitory effects of p-aminohippurate and pyrazinoate (PZA) on the transport of salicylate were studied by free-flow micropuncture in the rat. p-Aminohippurate (5, 10 or 25 mumol/kg X min) and PZA (10 or 25 mumol/kg X min) inhibited proximal tubular secretion of salicylate; they induced decreases in the fractional delivery of salicylate to the late proximal tubules. In alkalotic rats, the late proximal decrease in fractional delivery of salicylate was accompanied by a decreased fractional excretion of salicylate. In contrast, such a decrease in fractional excretion of salicylate was not observed in rats in normal acid-base balance. A greater rate of PZA infusion (25 mumol/kg X min) not only inhibited secretion, but also depressed a reabsorptive carrier-mediated transport of salicylate. Thus, in alkalotic rats, fractional delivery of salicylate to late proximal tubules were 1.90 +/- 0.15, 0.90 +/- 0.10 and 1.34 +/- 0.13, respectively, for control rats and rats infused with 10 or 25 mumol/kg X min of PZA. The corresponding values of fractional excretion of salicylate were 1.21 +/- 0.09, 0.61 +/- 0.05 and 0.08 +/- 0.09, respectively. The data suggest that salicylate is secreted as well as reabsorbed by carrier-mediated mechanisms and that there may be two secretory mechanisms.

The neurotoxicity of valine deficiency in rats

When valine, an essential amino acid, was withdrawn from the diet of weanling rats, the animals rapidly developed a unique pattern of neurological symptoms characterized by head retraction, staggering and aimless circling. At necropsy degenerative changes were most prominent in the neurons of the red nuclei, brain stem structures which modulate motor function. To explore the pathogenesis of the neurotoxicity associated with valine deficiency, we fed rats purified diets deficient in valine alone or in valine plus other branched chain and neutral amino acids, and we examined brain tissues by light microscopy. Motor disfunction and red nuclei damage occurred only in rats fed diets lacking valine alone and not in rats fed diets lacking all three branched chain amino acids. These results suggest that the neurotoxicity of valine deficiency results from amino acid imbalance rather than from lack of dietary valine per se.

Intrarenal distribution of citrate in the dog during antidiuresis and diuresis

The intrarenal distribution of citrate was evaluated in the dog during antidiuresis and osmotic diuresis, by using the specific citrate assay method of Moellering and Gruber. The measurements were made on tissue samples taken from four different regions throughout the kidney: cortex, outer and inner medulla, and papilla. During antidiuresis, a characteristic distribution of citrate was observed with highest levels in the papilla and lowest ones in the outer medulla. A medullary concentration gradient for citrate was found. Mannitol greatly decreased papillary citrate and sodium, but no changes in outer and inner medullary citrate occured. The results could not be explained by the citrate contained either in the trapped urine or blood in the tissue. It is suggested that citrate accumulation in the inner regions of renal medulla may be accounted for by countercurrent mechanisms or regional differences in renal citrate metabolism.