Uptake and phosphorylation of thiamine in rat kidney cortical slices. I. Effect of ethanol

Processes involved in the disposition of thiamine within the kidney were studied in rat kidney slices. Uptake of [14C]thiamine and its metabolism to [14C]thiamine phosphate were measured with and without the presence of ethanol. Whereas slice to medium ratios of 3.41 +/- 0.11 indicated uphill movement of [14C]thiamine, metabolism to [14C]thiamine phosphate provided a metabolic sink for movement of thiamine into the cell. Accumulation was saturable and associated, in part, with the formation of thiamine phosphate. Ethanol, at 25 mM, a concentration compatible with alcohol abuse, significantly (P < .001) decreased the maximal accumulation of [14C]thiamine from 210 +/- 12.7 to 115 +/- 4.2 nmol/g and the production of thiamine phosphate from 0.44 to 0.041 nmol/g. These data indicate a facilitated uptake of thiamine and a conversion to thiamine phosphate by the kidney. The effect of ethanol to decrease thiamine accumulation in kidney tissue is suggested to be at the phosphorylating step.

Uptake and phosphorylation of thiamine in rabbit primary proximal tubule cells and Madin Darby canine kidney cells. II. Effect of ethanol

Uptake of [14C]thiamine was studied in renal primary proximal tubule cells and in Madin Darby Canine Kidney cells in culture. Findings were compared with data for the accumulation of [14C]thiamine and its phosphorylation in renal cortical slices. There was a saturable component for thiamine uptake in all cell types. When normalized for milligrams of protein, renal cortical slices accumulated 82% less 14C than renal primary proximal tubule cells and 51% less than distal tubule cells. Maximal [14C]thiamine levels accumulated by the saturable component was 1.33 nmol/g in slices and 7.02 nmol/g in proximal tubule cells. Ethanol, at 25 mM, inhibited thiamine uptake and phosphorylation in the cell cultures similar to its effect in the kidney tissue.

Brain osmoregulation during extreme and moderate dehydration in a rat model of severe DKA

To determine if osmoprotective molecules accumulate in the brain during severe DKA with extreme (DKA-E) and moderate (DKA-M) dehydration, Fischer 344 rats (250-350g) were given STZ 45 mg/kg (i.p.) and allowed food and water ad lib. DKA-M received NaCl 77 mmol/L 60 ml/kg (i.p.) q 4 hrs. on day 2. All rats were anesthetized and sacrificed at 48 hrs. Half of each brain was used to measure water content (BWC) and half to measure Na+, K+, and organic osmoles by HPLC. Just prior to expiration, values for mean concentration of serum glucose (mmol/L) percent weight loss and median blood pH for DKA-E were 33.4, 19%, 6.98; for DKA-M, 16.8, 7.5% and 6.84, respectively. Means +/- SEM were compared by Student's t-test. Percent BWC was 76.3, 77.3 and 77.6 in DKA-E, DKA-M and normal controls, respectively (NS). Brain Na+ and K+ were increased in DKA-M compared to controls (p < .05) but not significantly different in DKA-E compared to controls. Of organic osmoles measured (umol/g wet weight) taurine was significantly increased (p < .01) in DKA-E and DKA-M (8.04 +/- .39 and 9.73 +/- .78, respectively) as compared to controls (5.92 +/- .35) as was myoinositol in DKA-E compared to controls (9.96 +/- .39 vs. 8.87 +/- .28) (p < .05) and urea in DKA-E as compared to controls (14.24 +/- 3.9 vs. 4.14 +/- .52) (p < .01). DKA-M were not significantly different for brain myoinositol or urea as compared to control animals. There was no significant difference in brain glutamine between either study group and controls. Preservation of brain water despite systemic dehydration can be partly explained by increased brain concentrations of osmoprotective molecules. Such adaption in the clinical setting of DKA warrants a cautious repair of dehydration and hyperosmolality.

Osmoregulatory betaine uptake by rat renal medullary slices

Betaine is an osmolyte present in high concentrations in renal medullary cells. Betaine and other organic osmolytes, such as glycerophosphorylcholine, myo-inositol, and sorbitol, have been shown to increase in concentration during antidiuresis when the inner medullary extracellular osmolality rises. Its concentration may increase in renal cells either by betaine uptake or by choline metabolism to betaine. These studies measured the uptake of (14C)betaine into cortical, outer medullary and inner medullary slices from rat kidney. The tissue-to-medium ratio of (14C) betaine increased with increasing osmolality up to 450 mosmol/kg in outer medullary and inner medullary slices, but not in cortical slices. Betaine uptake increased when the osmolality was raised with NaCl or mannitol, but not with urea. When LiCl was substituted for NaCl in a medium of 300 mosmol/kg, there was significant inhibition of betaine uptake, although the tissue-to-medium ratios remained greater then unity. Thus, increases in osmolality stimulate betaine uptake in rat renal medullary slices and this uptake occurs by both sodium-dependent and sodium-independent betaine transport.

Effect of vanadate on renal hypertrophy and sorbitol accumulation in streptozotocin induced diabetes in rats

Vanadate has been previously shown to normalize blood glucose in streptozotocin-induced diabetic (STZ-DM) rats. The effect of a previously studied dose of vanadate (0.8 mg/ml) in drinking water on blood glucose, renal hypertrophy, and whole kidney polyol accumulation was studied in STZ-DM rats. Rats with diabetes of 5 weeks duration had higher blood glucose, greater urinary output, higher kidney weight, lower body weight, and higher kidney to body weight ratios than controls. Whole kidney sorbitol concentrations were significantly increased in diabetes but myo-inositol levels were unchanged vs control animals. After four weeks of oral vanadate treatment, blood glucose, urine volume, and kidney weights were similar to control values. Kidney to body weight ratios fell below that of the STZ-DM animals, but because body weights remained decreased, the kidney to body weight ratios were not normalized. Renal sorbitol levels returned to control values and renal myo-inositol levels remained unchanged in STZ-DM and normal animals treated with vanadate. These results provide evidence that vanadate therapy may result in regression of the hypertrophy and polyol accumulation characteristic of diabetic nephropathy in STZ-DM rats. This effect is most likely due to normalization of blood glucose by the insulin-mimetic activity of vanadate treatment.

Effect of dimethylaminoethanol, an inhibitor of betaine production, on the disposition of choline in the rat kidney

The choline metabolite betaine has been shown to be an important organic osmoregulatory solute in the kidney. The isolated perfused rat kidney and kidney slice incubations were used to investigate the effect of 2-dimethylaminoethanol (DMAE), a choline oxidase inhibitor, on the renal excretion and metabolism of choline. In the isolated perfused kidney, [14C]choline, at an initial perfusate concentration of 300 microM, was effectively removed from the perfusate over 25 min, with nearly all the 14C in the perfusate accounted for by betaine during the remainder of the 90-min perfusion. DMAE at concentrations of 3.0 or 5.0 mM significantly decreased the rate of removal of [14C]choline from the perfusate and the rate of addition of [14C]betaine to the perfusate, yet [14C]betaine remained the only metabolite of choline in perfusate and urine. In kidney tissue slice experiments, conversion of [14C]choline to [14C]betaine was found in cortical, outer medullary and inner medullary regions of rat kidney. DMAE at 5.0 mM significantly inhibited [14C]betaine production in each of the three regions studied. These data show that DMAE is an effective inhibitor of betaine production by the kidney and, as such, may be an important agent for the study of osmoregulation by the kidney.

Excretion and metabolism of nicotinic acid by the avian kidney

The renal tubular transport and metabolism of nicotinic acid (NA) were investigated using the Sperber technique in unanesthetized hens. Infusion of [14C]NA into the avian renal portal circulation at 10(-10) mol/kg/min revealed that the 14C label was actively transported into urine at a rate 74% that of simultaneously infused tetraethylammonium. Increases in NA infusion rates enhanced 14C label transport so that it eventually equalled the excretion rate of tetraethylammonium. At a NA infusion rate of 10(-8) mol/kg/min, this transport was not affected by probenecid, pyrazinoic acid or p-aminohippurate. Above infusion rates of 10(-7) mol/kg/min, saturation of 14C label transport was reached. Electrophoretic analysis of the 14C label excreted in urine at NA infusion rates less than 10(-7) mol/kg/min revealed a single, unidentified metabolite. At infusion rates greater than 10(-7) mol/kg/min, both the radiolabeled metabolite and [14C]NA were found in the urine. We conclude that NA is transported by a specific mechanism into the avian tubule cell where it is metabolized. As the body's load of this vitamin increases, NA is excreted first in the form of a metabolite and then as both metabolite and unutilized NA.

Effect of acute and chronic hypernatremia on myoinositol and sorbitol concentration in rat brain and kidney

In animal models of hypernatremia, increases in brain electrolyte content account for the entire increase in osmolality in acute but not chronic hypernatremia, suggesting that there is generation of additional intracellular solutes ("idiogenic osmoles") in chronic hypernatremic states. In the present study, the concentration of the polyols myoinositol and sorbitol and water content were determined in the brain and kidneys of rats made acutely (2 hours) and chronically (72 hours) hypernatremic by intraperitoneal injection of NaCl and water restriction. Both the brain and the kidney responded to chronic hypernatremia with increased levels of myoinositol. Sorbitol levels increased in the kidney in response to both acute and chronic hypernatremia. Water content dropped in acute hypernatremia, but remained unchanged during chronic hyperosmolar challenge. We conclude that the polyols, myoinositol and sorbitol, may play a significant role in cellular osmoregulation in brain and kidney during chronic hypernatremia in the rat.

Renal effects of prednisolone in the chicken

The effects of prednisolone on renal tubular transport of organic ions, urine flow and glomerular filtration rate were studied using the Sperber preparation in chickens. Low infusion rates of 1 and 5 nmole/min prednisolone slightly enhanced organic ion excretion. At higher infusion rates up to 1,000 nmol/min, no change in organic ion transport was observed. Urine flow and glomerular filtration rate increased at prednisolone infusion rates of 50 nmole/min and higher, independent of changes in renal blood flow.

Probenecid inhibition of the renal excretion of dyphylline in chicken, rat and man

The effect of probenecid on the renal excretion of dyphylline was studied in chicken, rat and man. Dyphylline was found to be actively excreted when measured by the Sperber preparation in hens, the isolated perfused kidney of the rat and clearance studies in man. In each study probenecid significantly decreased dyphylline excretion demonstrating that dyphylline occupied the renal organic anion transport system. This same drug interaction, at the level of the renal excretory system, in these three species occurred at comparable concentrations of dyphylline and probenecid.

Renal effects of atrial natriuretic factor in domestic fowl

The renal hemodynamic and tubular effects of ANF were investigated using the Sperber technique in chickens. This technique takes advantage of the unique portal circulation of the avian kidney and permits direct access to the renal peritubular space independent of renal arterial blood flow and glomerular filtration. Infusion of ANF into the avian renal portal system increased urine flow rate and sodium excretion by as much as 300% and 100%, respectively. These changes occurred in the absence of significant alterations in glomerular filtration rate or renal plasma flow. There was no significant difference in urine flow, sodium excretion or glomerular filtration rate between the ANF-infused kidney and the contralateral, non-infused kidney. We conclude that the diuretic and natriuretic effects of ANF do not depend on changes in glomerular filtration rate and that the site of action of ANF is the renal medulla.

Relation between transport maxima and inhibition of organic cation excretion in the chicken kidney

The ability of several organic cations to inhibit differentially the renal excretion of two prototypical organic cations, tetraethylammonium (TEA) and N1-methylnicotinamide (NMN), was investigated using the Sperber technique in chickens. TEA and NMN excretion were inhibited by the following organic cations in order of decreasing potency: quinine, TEA and NMN. The respective competitive potency of these substances was related inversely to their maximum tubular transport rates (Tm). Regardless of inhibitor used (quinine, TEA or NMN), NMN excretion was always inhibited more easily than TEA excretion. In addition, TEA excretion was suppressed more easily than cimetidine excretion by the competitive inhibitor quinine. The Tm of cimetidine was determined to be less than the Tm of TEA, which in turn is less than that of NMN. These results indicate that the Tm of an organic cation is related inversely to its inhibitory potency and related directly to its susceptibility to inhibition, reflecting different affinities of organic cations for the same carrier-mediated transport system.

Renal pressor effect of ethanol in the isolated perfused rat kidney

These experiments were performed to detect changes in renal function produced by acute infusions of small amounts of ethanol into the isolated kidney of the rat. Ethanol was infused for 10 min beginning at 40 min to reach a final concentration of approximately 80 mg/100 ml in the recirculating perfusate. Control kidneys were perfused for 90 min without the addition of ethanol. Control and ethanol infused kidneys were compared with respect to the following measurements: glomerular filtration rate, urine volume, urine protein concentration, pressure and fractional excretion of sodium, chloride, potassium, calcium and magnesium. Ethanol concentration in the perfusate was measured by gas chromatography. The only parameter affected by these concentrations of ethanol was pressure. During the ten min ethanol infusion, the pressure in the system rose significantly (P less than 0.01) from 110 +/- 0.3 to 120 +/- 2.8 mmHg. After the ethanol infusion, the pressure decreased towards pre-ethanol levels at a faster rate than the decrease in ethanol concentration in the perfusate.

Tubular transport and metabolism of cimetidine in chicken kidneys

Renal tubular transport and renal metabolism of [14C]cimetidine (CIM) were investigated by unilateral infusion into the renal portal circulation in chickens (Sperber technique). [14C]CIM was actively transported at a rate 88% that of simultaneously infused p-aminohippuric acid, and its transport was saturable. The following organic cations competitively inhibited the tubular transport of [14C]CIM with decreasing potency: CIM, ranitidine, thiamine, procainamide, guanidine and choline. CIM inhibited the transport of [14C]thiamine, [14C]amiloride and [14C]tetraethylammonium. During CIM infusion, two renal metabolites, CIM sulfoxide and hydroxymethylcimetidine, were found in urine. When CIM sulfoxide was infused, its transport efficiency was 32% and not saturable. CIM sulfoxide did ot inhibit the simultaneous renal tubular transport of p-aminohippuric acid or tetraethylammonium. CIM is transported by the organic cation transport system and the kidney metabolizes CIM. Transport of CIM and other cationic drugs could produce a drug interaction to alter drug excretion.

Effect of renal transplantation on the levels of choline in the plasma of uremic humans

Plasma choline levels were measured in patients who received a kidney transplant, in donors who underwent nephrectomy and in nonrenal surgical patients. Choline was measured using a choline kinase assay. Choline levels in patients receiving a kidney fell from 29.8 +/- 1.86 microM before transplantation to 15.7 +/- 2.32 1 day later; this normal level was maintained for at least 7 months and in a single case for 2 years. Kidney donors and nonrenal surgery patients showed a significant decrease in plasma choline on the day following surgery but choline levels returned to normal by 3 days after surgery. Thus a transplanted functional kidney reduced the high plasma choline levels, associated with uremia, to normal and maintained these normal levels throughout the period of observation.

Competition by meperidine for the organic cation renal excretory system

Renal tubular excretory transport of meperidine was studied using the Sperber preparation in chickens. When urine samples from infused and uninfused kidneys were analyzed for meperidine by gas chromatography, meperidine was always present in greater amounts in the urine from the infused kidney, demonstrating active tubular excretion. Meperidine at an infusion rate of 1 mumole/min, also inhibited the excretion of the organic cations choline and acetylcholine, indicating occupation of the renal organic cation excretory system in the chicken.

Prednisolone metabolism and excretion in the isolated perfused rat kidney

The isolated perfused rat kidney was used to identify factors responsible for the renal elimination of prednisolone (Pn). Pn was recirculated at initial concentrations varying from 100 to 1000 ng/ml for 90 min. Perfusate and urine samples were assayed for Pn and prednisone by HPLC. Protein binding of Pn was measured by using 3H-Pn and equilibrium dialysis at 37 degrees C. There were no significant differences in perfusate flow, glomerular filtration rate, urine flow, or sodium excretion between control and steroid experiments. Partial metabolism of Pn to prednisone occurred in all studies. The total kidney clearance (CIT) of Pn ranged from 0.39 to 1.24 ml/min/100 g of rat body weight with approximately half of the Pn dose unaccountable for as either Pn or prednisone. The apparent percentage of the Pn dose excreted unchanged in the urine ranged from 1.9 to 6.4% and was not related to Pn dose. The apparent urinary clearances of Pn and its metabolite, prednisone, normalized for inulin clearance (fractional excretion) were variable with means of 0.068 and 0.095, respectively. The fractional excretions of Pn and prednisone were related to the fraction of filtered water excreted but not to perfusate concentration. Thus, the extent of urinary clearance of these corticosteroids is related to glomerular filtration and passive tubular reabsorption. The perfused rat kidney reflects the urinary and renal metabolic clearance of Pn without the complication of dose-dependent disposition.

Renal N-oxidation of meperidine by the perfused kidney of the rat

Isolated perfused kidneys of rats were used to determined renal metabolism of meperidine and its relation to meperidine excretion. Starting perfusate concentrations of meperidine were 18.4 and 190 micrograms/ml, and resulted in respective nonsaturation and saturation kinetics for removal of meperidine. Disposition of 14C-meperidine was followed and supplemented with measurements of meperidine by gas chromatography. A major renal metabolite was shown to facilitate the renal excretion of meperidine. This metabolite was identified by gas chromatography/mass spectrometry as the N-oxide of meperidine.

Renal tubular transport and nephrotoxicity of DDA

Since DDA [bis(p-chlorophenyl)acetic acid] has been shown to be transported and concentrated by the renal proximal tubule, this metabolite of DDT has been postulated to be a potential nephrotoxic agent. The present study explored the renal transport of DDA in the isolated, perfused rat kidney and the effects of DDA on renal function. When DDA (0.6 microM) was present in a dextran perfusate which eliminated DDA-colloid binding, the DDA/inulin clearance ratio was congruent to 0.05; however, some metabolism of DDA was apparent. During these studies, DDA had no effect on the glomerular filtration rate and the fractional reabsorption of Na, K or H2O. To determine the concentration of DDA which would produce an effect on renal cellular function, studies were performed with renal cortical slices. DDA at media concentrations greater than or equal to 0.1 mM were needed to produce significant alterations in tetraethylammonium transport, tissue oxygen consumption and intracellular electrolyte composition; however, no effect was demonstrated on Na-K-ATPase activity although DDA did affect Mg-ATPase activity. In conclusion, DDA at a 0.6 microM perfusate concentration undergoes net tubular reabsorption and metabolism without affecting the function of the perfused kidney. Only high concentrations of DDA were shown to produce alterations in cellular function.

Effect of DIDS on renal tubular transport

Two stilbene derivates that had been used to covalently label the Cl- carrier in the erythrocyte were investigated for reactivity with the renal organic anion system. These compounds, 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) and (4,4'-diisothiocyano)-dihydrostilbene-2,2'-disulfonate (H2DIDS), were found to be potent inhibitors (Ki congruent to 35 microM) of p-aminohippurate (PAH) transport in the renal cortical slice without affecting tetraethylammonium (TEA) transport or tissue viability. During renal clearance studies performed in the perfused kidney, DIDS decreased the PAH/inulin clearance ratio to congruent to 1. When the possible renal transport of [3H]H2DIDS was investigated, the renal slice transport or binding of [3H]H2DIDS reached a slice-to-medium ratio of congruent to 6, and this accumulation was decreased by probenecid. In perfused kidney experiments, the [3H]H2DIDS/inulin clearance ratio was congruent to 0.8. Since probenecid reduced this clearance ratio to congruent to 0.5, there was the possibility that H2DIDS underwent tubular secretion. In conclusion, DIDS and H2DIDS interacted with the renal organic anion transport system, which indicated that these compounds were possible probes for this transport system.

Isoproterenol excretion and metabolism in the isolated perfused rat kidney

[3H]isoproterenol excretion and metabolism were studied in the isolated perfused rat kidney using a one-pass, non-recirculating perfusion system with constant infusion rates of [3H]isoproterenol. The [3H]isoproterenol (U/P) to inulin (U/P) ratio was approximately 15 indicating extensive tubular secretion. A major renal metabolite, 3-O-methylisoproterenol, appeared in the urine and renal vein perfusate and also accumulated in the renal tissue. The fractional excretion of isoproterenol decreased with time while fractional excretion of p-aminohippurate remained stable. The observed decreasing urinary clearance and percent extraction of isoproterenol with time may be due to the progressive intrarenal accumulation of 3-O-methylisoproterenol.

Effect of ethanol on the renal excretion and metabolism of choline in the isolated perfused rat kidney

The isolated perfused rat kidney was used to investigate the effect of ethanol on the renal excretion and metabolism of choline. Choline at an initial perfusate concentration of 2.8 mM, with tracer amounts of [methyl-14C]choline, was recirculated through kidneys and radioactivity measured in perfusate, urine, and kidney. 14C-Choline and its metabolites were identified by chromatographic and electrophoretic procedures. Tubular excretion of choline was demonstrated and a transport maximum (Tm) of 1.6 mumol/kidney/min was reached at a choline perfusate concentration of 1.2 mM. Addition of 50 mM ethanol resulted in a 56% increase in the choline Tm and 100 mM ethanol decreased the choline Tm by 25%. The rate of loss of 14C-choline from the perfusate was increased by the lower ethanol concentration and decreased by the higher ethanol concentration. Ethanol at both concentrations diminished the amount of 14C remaining in the kidney. 14C-Betaine was the major choline metabolite and the only 14C-metabolite present in perfusate or urine. Addition of either 50 or 100 mM ethanol increased both glomerular filtration rate and urine volume.

Relations of renal transport rate, transport maximum, and competitor potency for tetraethylammonium and choline

By use of the Sperber technique in chickens, the renal tubule transport maximum (Tm) for the organic cation tetraethylammonium (TEA) was determined in vivo. The tubular transport rate, the Tm, and the competitor potency are assumed to have causal relationships. It was demonstrated that the two cations TEA and choline compete for the tubular transport of [14C]TEA. The saturating load of one cation was reduced by the simultaneous presence of the other. The Tm for these two cations was different by a factor of 2.4 and their respective competitor potency was inversely related to their Tm.

Renal N-oxidation of trimethylamine in the chicken during tubular excretion

The Sperber technique of infusion into the renal portal circulation in chickens was used to investigate in vivo the renal tubular transport and renal metabolism of trimethylamine (TMA). When 14C-TMA was infused at a rate of 1 x 10(-9) mol/min the transport efficiency (TE), that is, the tubular excretion of the 14C-label relative to excretion of simultaneously infused paminohippuric acid, was 0.70. Progressive addition of unlabeled TMA up to infusion rates of 1 x 10(-5) mol/min produced a progressive fall in the TE of the 14C-label. Identification of the 14C-label excreted in the urine revealed that approximately 85% of the infused 14C-TMA was excreted by the infused kidney as a single metabolite over the entire range of infusions. By use of the techniques of low-voltage electrophoresis, high-voltage electrophoretic mobility-pH profile, and gas chromatography/mass spectrometry, the renal metabolite was found to be identical with standard 14C-trimethylamine oxide (TMAO). At a TMA infusion rate of 1.5 x 10(-6) mol/kg/min reaching the infused kidney, the rate at which TMAO was formed and excreted by the kidney was 0.12 x 10(-6) mol per g of kidney per min. When 14C-TMAO was infused into chickens its TE was 0.11, which was not evidence for active excretory transport. Infused TMA was almost entirely metabolized in vivo to its N-oxide, TMAO, which then entered the urine. The renal tubular excretion of 14C during infusion of 14C-TMA was inhibited by the cationic blocker of transport, quinine, and by the anionic blocker of transport, probenecid.

Choline loss during hemodialysis: homeostatic control of plasma choline concentrations

Endogenous concentrations of free choline in plasma were measured in azotemic subjects receiving repetitive hemodialysis and excretion of free choline into the dialysate was determined. Chemical choline in plasma and dialysate was measured by adding choline kinase and measuring the production of radiolabelled phosphorycholine in the presence of radiolabelled adenosine triphosphate (ATP). Mean free choline concentration in plasma of azotemic subjects receiving hemodialysis was found to be 37 muM, which is about twice that of normal persons. The total excretion of choline into the dialysate during 360 min averaged 730 mumoles +/- 69 (SEM). Levels of free choline in plasma fell during hemodialysis at two hours but recovered toward predialysis values at six hours. The return of plasma choline concentrations toward control values during dialysis suggests that a feedback mechanism exists which was activated rapidly to produce homeostasis of plasma choline concentrations. In these patients, the degree of peripheral neuropathy as judged by measurement of nerve conduction velocities showed a significant inverse correlation with levels of free choline in plasma.

The biphasic effect of organic cations on the excretion of other organic cations

The renal excretion of 14C-choline or 14C-acetylcholine was increased by the infusion of another organic cation at low rates but was decreased by infusion of the same added organic cation at higher rates with the Sperber technique in hens. The range of low rates of infusion was from 1 X 10(-15) to 1 X 10(-8) mol/min. At infusion rates greater than 1 X 10(-8) mol/min, inhibition of tubular excretion was found. At the low infusion rates, thiamine, lysine, quinine, atropine, acetylcholine and methylguanidine were found to increase 14C-choline excretion. The same compounds with the exception of lysine and acetylcholine inhibited 14C-choline excretion at the higher infusion rates. A biphasic effect on 14C-acetylcholine excretion was also observed with added atropine, thiamine and choline over the same infusion range. Increases in 14C-choline excretion occurred during a choline infusion rate that normally produced an excretory tubular maximum for choline whereas increases in 14C-acetylcholine excretion occurred during infusion of tracer amounts of 14C-acetylcholine. The effect of the addition of organic cations was selective for cations since the tubular excretion of organic anions was not affected by the addition of organic cations. The tubular excretion ratio of 14C-thiamine/p-aminohippuric acid increased from 0.25 to 0.95 when the infusion rate of added unlabeled thiamine was increased from 1 X 10(-11) to 1 X 10(-8) mol/min. Enhanced tubular excretion of 14C-thiamine may represent the effect of the increased load of unlabeled thiamine to protect the labeled thiamine from conversion to a nontransportable metabolite. Enhancement of excretion of 14C-choline and 14C-acetyocholine produced by very small amounts of other organic cations may represent either inhibition of tubular reabsorptive transport or induction of tubular excretory transport.

Renal tubular excretion of triethylcholine (TEC) in the chicken: enhancement and inhibition of renal excretion of choline and acetylcholine by TEC

1. [3H]-triethylcholine (TEC) was actively transported by the renal tubule of the chicken at a rate 85% that of simultaneously administered p-aminohippuric acid (PAH). 2. TEC was demonstrated to be transported by the organic cation transport system in the kidney through inhibition with quinine and the bio-cation choline. 3. When the infusion of TEC was increased to 2 times 10(-6) mol kg(-1) min(-1) reaching the infused kidney, the transport of [3H]-TEC was inhibited, suggesting that an excretory transport maximum for TEC in the renal tubules had been reached. 4. The excretion of both choline and acetylcholine was enhanced by TEC loads as low as 1 times 10(-18) mol kg(-1) min(-1). Enhancement continued as TEC infusion was increased up to approximately 1 times 10(-7) mol kg(-1) min(-1) at which point this enhancement was converted to inhibition. 5. Possible mechanisms for the biphasic effect of TEC on organic cation transport are discussed.

The kidney in regulation of plasma choline in the chicken

Exogenous choline was administered into the wing vein of chickens until steady-state plasma choline levels were achieved. Both plasma and urine were analyzed for free choline by a choline kinase, radiochemical microassay that did not require prior extraction of choline from the biological fluids. Choline was infused at rates from 0.5 to 20.0 mumol/kg-min. Total and urinary clearance were assessed at the steady state reached in each experiment. At the endogenous level of plasma choline of 0.019 mM, total clearance of choline from the plasma was about 50 ml/kg-min and urinary clearance was 0.06 ml/kg-min. There was no significant increase in urinary clearance of choline produced by infusion loads from 0.5 to 1.0 mumol/kg-min. However, as the infusion of choline was increased further, extraurinary clearance decreased while the contribution of the kidneys to total clearance of choline from the plasma increased. At the choline infusion rate of 12.5 mumol/kg-min, plasma choline was 0.5 mM and urinary choline clearance had reached a maximum value of 18.5 ml/kg-min for two kidneys, removing more than 70% of the infused choline.