An improved quantitative assay for chemokinesis in Tetrahymena

This paper presents a quantitative and sensitive assay for the measurement of chemosensory behavior in Tetrahymena. The two-phase assay is easy to perform in large quantities, so a variety of compounds can be screened under comparable conditions. A suspension of 2 x 10(5) cells ml-1 (the upper phase) is starved for 20-40 h and then gently placed on top of a 5% solution of Metrizamide (the lower phase) in a disposable microcuvette. The optical density of the lower phase is monitored at 600 nm with an automated spectrophotometer at selected time points. Optimum sensitivity of the assay is achieved when the cells slowly but continuously enter the lower phase, so that about 5% of them will be in the lower phase within 30 min. Optimal chemosensory responses occurred in Tetrahymena thermophila at about 25 degrees C. The response was delayed at 15 degrees C and markedly reduced at 35 degrees C. The data suggest three bases for quantifying the response in the assay: (1) initial slope of the absorbance versus time; (2) final maximal absorbance within the time period of measurement; and (3) signal-to-noise ratio (S/N) at a fixed time. We have quantified--in terms of S/N--the chemosensory responses in Tetrahymena for the following compounds: beta-endorphin, fibroblast growth factor, insulin, and platelet-derived growth factor (PDGF); these substances were active in nanomolar concentrations, and the maximal S/N was between 3 and 5.1. Acetylcholine was active only in millimolar concentrations; maximal S/N was 4.1 at 1 mM.(ABSTRACT TRUNCATED AT 250 WORDS)