Population growth kinetics of Tetrahymena pyriformis exposed to selected pyridines

Abstract

Summary Alterations in growth kinetics of populations of the freshwater ciliate Tetrahymena pyriformis due to exposure to six specific pyridines were investigated. Selected pyridines varied in mode of toxic action and hydrophobicity (quantified by the logarithm of the 1-octanol-water partition coefficient, log Kow). Three pyridines containing lower hydrophobicity (log Kow ≈ 0.5) and higher hydrophobicity (log Kow ≈ 1.3) were tested. Within each hydrophobicity group were representative pyridines of both non-covalent (neutral narcotics and other narcotics) and covalent modes of toxic action. Growth kinetic trends were compared and contrasted, both between and within modes of toxic action. Pyridine and 3-chloropyridine exposed populations inhibited by 32% or 50%, respectively, exhibited doubling times significantly longer than control populations. The growth kinetic trends were hydrophobicity-dependent, as established by an increased lag phase in the more hydrophobic 3-chloropyridine. The hydroxylated pyridines demonstrated similar growth kinetics independent of hydrophobicity, but distinct from the neutral narcotic pyridines. No lag phase existed and a concentration-dependent decrease in generation time occurred. This distinction suggests that hydroxylated pyridines elicit toxicity via a unique mechanism of toxic action from baseline narcotics within the non-covalent mode of action. T. pyriformis populations exposed to the nitro derivatives exhibited different trends. Two-chloro-3,5-dinitropyridine displayed concentration-dependent mortality of the initial population over the first 4 h of exposure. Remaining cells then commenced growth with generation times similar to control populations. The growth kinetic trends of 2-bromo-5-nitropyridine were a hybrid between those observed for the reactive dinitro derivative and the neutral narcotics. Thus, growth kinetic trends with the covalent mode of action may be mechanism specific as well.