The effect of diester diterpenoid alkaloids (DDA) from aconiti class on the growth of Tetrahymena thermophila BF5 was investigated by means of the TAM III isothermal microcalorimetric system. The extent and duration of toxic effects on the metabolism were evaluated by studying the growth rate constant (k), half inhibitory ratio (IC₅₀), maximum heat-output power (P_max), peak time of maximum heat-output power (t_p) and total heat production (Q_t). Experimental results showed that the value of t_p increased with increasing the concentration of DDA. Moreover, the value of k and P_max were reduced with increasing the concentrations of the three drugs. Furthermore, the growth rate constant (k) was linked to the concentration of DDA: r_aconitine＝0.9910 (IC₅₀＝207.7 μg•mL^－1); r_mesaconitine＝0.9923 (IC₅₀＝412.5 μg•mL^－1); r_hypaconitine＝0.9977 (IC₅₀＝1497 μg•mL^－1). The sequence of toxic effects of the three DDA is aconitine＞mesaconitine＞hypaconitine. It was suggested that the hydroxy group at C(3) and ethyl group connectted with nitrogen could possibly improve the toxic effects of DDA on Tetrahymena thermophila BF5 growth.

Key words: microcalorimetry, diester diterpenoid alkaloid, Tetrahymena thermophila BF5, growth, metabolism