The Passage of Ammonia Across the Blood-Brain-Barrier and its Relation to Blood pH

Abstract

Ammonium salts have been shown to have significantly different toxicities (LD,0's) when administered intravenously under controlled conditions (1). This differential toxicity appeared to be related to the change in blood pH induced by each salt.The normal blood pH (7.4) is close enough to the pKbh of ammonia,2 approximately 8.90 at 370 C. (2), so that any change in blood pH would significantly affect the ratio of unionized (NH8) to ionized (NH4+) ammonia. Based on two factors, the greater tissue permeability to unionized particles (3-6) and the marked limitation of passage of ionized particles by the blood-brain-barrier (7-9), it was postulated that a greater percentage of a given dose of ammonia might enter the brain as the blood pH rose because of an increase in the amount present as NH3.In order to test this hypothesis, blood pH and simultaneous blood and brain ammonia concentrations were determined following the intravenous administration of LD50 doses of several ammonium salts. This was done on the assumption that since the primary toxic effect of ammonia appeared to be located in the central nervous system the brain levels would be relatively similar despite different LD50 dosages of each salt. MATERIALS AND METHODSLD.4 doses (1) of five ammonium salts dissolved in 1 ml. of isotonic saline were rapidly injected into the tail veins of 18 to 22 Gm. Swiss albino mice. Heart blood was drawn at various intervals after injection. Immediately thereafter the mice were frozen in liquid nitrogen.The blood ammonia concentration in a protein free preparation of 0.5 to 1.0 ml. of heart blood was determined by. a microdiffusion technique (10). The control blood 1 Present address: Peter Bent Brigham Hospital, Boston, Mass.2The term "ammonia" refers to ammonium ion plus free ammonia.ammonia concentration was 0.6 Iug. per ml. Brain ammonia levels were determined by a modification (11) of this technique utilizing the entire frozen mouse brain. The brain was shelled out, ground to a fine powder with cold instruments, and added to 3 ml. of 3 per cent cold trichloracetic acid. The tubes were weighed before and after the brain was added, and were centrifuged at 3°C. The ammonia concentration of a 1 ml. aliquot of the supernatant was then measured by the previously mentioned method (10) and the brain ammonia calculated from these data. Glutamine amide error was a constant 22 per cent of the control ammonia concentration of 4.0 ug. per Gm. of wet brain. The pH of heparinized heart blood obtained from live unanaesthetized animals was immediately determined at room temperature with a Beckman Model GS meter equipped with a 290-80 blood electrode.Electrocardiograms were obtained from pentobarbital anaesthetized mice during the intravenous injection of LD, doses of the five ammonium compounds. Electrodes were implanted subcutaneously as limb leads, and the recording was made on a Sanborn polyviso recorder. RESULTSMice injected with ammonium chloride immediately demonstrated agitated movements followed rapidly by the onset of coma. ...

The Comparative Effect of Chlorothiazide and Meralluride on the Development of Hepatic Encephalopathy in Cirrhotic Patients

“…Although we did not measure blood pH, these findings are consistent with the fre quent dem onstration of metabolic alkalosis secondary to urinary po tassium and chloride loss following chlorothiazide and meralluride [2,4,11,14,17,18,20,21,27]. Since metabolic alkalosis has been shown to increase the toxicity of a given ammonia load [28], our results suggest th at the production of encephalopathy following diu retics, especially chlorothiazide, is due to the interrelated effects of hyperam monem ia, hypokalemia and alkalosis.…”

THE ROLE OF pH GRADIENT IN THE DISTRIBUTION OF AMMONIA BETWEEN BLOOD AND CEREBROSPINAL FLUID, BRAIN AND MUSCLE*

“…During metabolic and respiratory alkalosis brain and muscle ammonia concentrations increased two-to threefold, while during metabolic and respiratory acidosis brain and muscle concentrations remained at or decreased below control concentrations. 4. Explanation for these findings may be found in the pH gradient drug distribution hypothesis.…”

“…Recently a direct relationship between an increase in blood pH and the diffusion of ammonia from blood into tissues has been suggested (2). Other investigators have demonstrated greater toxicity of the more alkaline ammonium salts (3) and that an increase in blood pH enhanced passage of ammonia into the brain of mice (4).…”

The pathogenesis and medical treatment of hepatic encephalopathy

“…Extracellular metabolic alkalosis is detrimental to the patient because it encourages ammonia to enter cells. Warren & Nathan (1958) have demonstrated the increased toxicity in mice of alkaline as opposed to acid ammonium salts. Patients have been treated by acid infusions and though blood and CSF ammonia levels respond appropriately no clinical improvement results.…”