457 Introduction Intentional self-poisoning is a major public health prob- lem in the Asia Pacific region, with at least 300 000 deaths a year. 1 Most of these deaths occur in rural communities, with self-poisoning being the commonest cause of in-patient death in rural Sri Lankan communities and the incidence of fatal self-harm being three to five times higher in rural areas when compared to urban regions in China. 2,3 Ingestion of poisonous plants as a method of suicide is common in India and among the plants encountered in clinical and forensic toxicology in India, Cleistanthus collinus is highly toxic. 4 It is a shrub of the Euphorbiaceae family native to India and is locally known as Oduvan. It ranks second in causing deaths due to accidental/suicidal ingestion, 5 and again, inci- dences of C. collinus poisoning are frequent in rural areas, especially in Southern India. 6 The leaf of the plant is mainly ingested and is highly toxic when taken in decoction form. 7 The toxin causes cardiac arrhythmias, metabolic acidosis, hypokalemia, acute respiratory distress syndrome, and distal renal tubular acidosis (dRTA). 5 Approximately one-third of the patients had renal failure, 7 with dRTA being one of the probable explanations for renal failure. 8 Acid–base homeostasis is critical for normal cellular function and the mammalian kidney maintains acid–base balance by secreting acid into the urine as it is produced by cellular metabolism. 9 The kidney achieves normal acid–base balance using three main mechanisms: (a) proximal tubular reabsorption of HCO 3 , (b) proximal tubular synthesis and subsequent net excretion of ammonium ions, and (c) distal secretion of H  ions. Disorders affecting the overall ability of the renal tubules either to secrete hydrogen ions (H  ) or to retain bicarbonate ions (HCO 3 ) result in renal tubular acidosis (RTA). 10 Although the bulk of hydrogen ion secre- tion occurs in the proximal tubule, the ultimate regulation of net acid excretion is probably achieved in the collecting duct. 9 The plasma membrane vacuolar type H  -ATPase (V-ATPase) 11 in alpha-intercalated cells of the distal tubule and collecting duct, are involved in secreting acid into the urine. Failure of this urinary acidification process results in dRTA. 12 Current management of patients with C. collinus poison- ing is purely supportive due to lack of information on the molecular mechanism of action of the toxin. Better insight into the mechanistic aspects of C. collinus poisoning is important to develop targeted treatment modalities. Based on Clinical Toxicology (2011), 49, 457–463 Copyright © 2011 Informa Healthcare USA, Inc. ISSN: 1556-3650 print / 1556-9519 online DOI: 10.3109/15563650.2011.590939 RESEARCH ARTICLE Received 4 April 2011; accepted 21 May 2011. Address correspondence to Anup Ramachandran, PhD, The Wellcome Trust Research Laboratory, Department of Gastrointestinal Sciences, Christian Medical College, Vellore-632004, India. Tel: 91-0416-2282052. Fax: 91-0416-2282486. E-mail: anup@cmcvellore.ac.in Mechanisms of toxicity of Cleistanthus collinus: Vacuolar ATPases are a putative target KAVITHA PRIYA KETTIMUTHU 1 , A. AMALA LOURTHURAJ 2 , A. SOOSAI MANICKAM 2 , SATHYA SUBRAMANI 2 , and ANUP RAMACHANDRAN 1 1 The Wellcome Trust Research Laboratory, Department of Gastrointestinal Sciences, Christian Medical College, Vellore, India 2 Department of Physiology, Christian Medical College, Vellore, India Ingestion of Cleistanthus collinus, a shrub native to South India, either intentionally or accidentally, is a common cause of death in the area. Consumption of a boiled decoction of leaves is highly toxic, but medical management of patients is mainly supportive because the molecular mechanisms of toxin action are unknown. Distal renal tubular acidosis is one of the symptoms of poisoning in patients and adenosine triphosphate (ATP) requiring proton pumps is important for acid secretion in the kidney. Hence, we hypothesized that these may be putative targets for C. collinus action and we tested this by exposing rat renal brush border membrane (BBM) as well as cultured kidney cells to a boiled decoction of C. collinus. Exposure to the C. collinus decoction resulted in significant inhibition of vacuolar type H  -ATPase (V-ATPase) activity in renal BBM as well as blocking of the proton pump in renal BBM vesicles. C. collinus decoction was also found to inhibit acidification of intracellular organelles in cells in culture, similar to the effect seen with either bafilomycin or concanamycin – specific inhibitors of the V-ATPase. This was accompanied by a decrease in V-ATPase activity, but an increase in protein levels. These results demonstrate that the V-ATPase in renal cells is a putative target for the toxins in C. collinus and the inhibition of this important proton pump probably plays a role in the development of distal renal tubular acidosis and subsequent renal failure seen in poisoned patients. Keywords Cleistanthus collinus; Distal renal tubular acidosis; Vacuolar type H  -ATPases; Renal brush border membrane; Bafilomycin A1 Clinical Toxicology Downloaded from informahealthcare.com by University of Colorado on 08/22/11 For personal use only.