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Introduction: Cleistanthus collinus (C. collinus), a well known plant toxin, contains active principles like Cleistanthin A, Cleistanthin B, Cleistanthin C and Diphyllin. Previous human case reports and animal studies have revealed that C. collinus poisoning leads to type I Distal renal tubular acidosis and type II respiratory failure. However, the mechanism of toxicity of this plant is still uncertain. Based on the hypothesis that blockade of proton channels could result in type II respiratory failure, patch clamp experiments were done to see if Cleistanthin A blocked the proton channels. Aim: To record and compare the changes in the magnitude of voltage-gated proton currents in human neutrophils, before and after addition of Cleistanthin A (test) and control solution. Materials and Methods: The test compound Cleistanthin A was isolated by partition chromatography and characterised using thin layer chromatography. Neutrophils were isolated by density gradient centrifugation method. Using voltage clamp protocol, proton currents were recorded before (pre-intervention currents) and after (post-intervention currents) the addition of Cleistanthin A or control solution. The pre and post-intervention current densities for different voltages were compared within the groups (control and test) by Wilcoxon signed-rank test and the percentage current remaining in both the groups were compared using Mann-Whitney U test, p<0.05 was considered significant. Results: Normal proton currents were recorded in human neutrophils. Comparison of the pre and post-intervention current densities within the control and test group revealed a significant depletion effect in the control group but not in the test group. However, comparison of the percentage current remaining after intervention across the groups did not show any significant difference between the control and test groups. Conclusion: Cleistanthin A does not seem to have any significant effect on the voltage-gated proton channels of human neutrophils. [ABSTRACT FROM AUTHOR]