Evidence of the receptor nature of the binding sites induced in Tetrahymena by insulin treatment. A quantitative cytofluorimetric technique for the study of binding kinetics

Tetrahymena pyriformis GL cells pretreated (imprinted) and not pretreated with insulin showed dissimilar quantitative relations of FITC-insulin binding. Displacement of FITC-insulin by unlabelled insulin was considerably less in the control than in the imprinted series. The curve for saturation of the binding sites with FITC-insulin resembled a true saturation curve. The imprinted cells bound considerably more hormone in a shorter time than the control cells at identical levels of exposure. The dissociation of bound hormone from the imprinted cells increased over the control at 23 degrees C, and to a still greater degree at 4 degrees C. The effect of the pH of the medium on the dissociation of bound FITC-insulin also differed between the imprinted and not imprinted cells. Thus the proposed cytofluorimetric assay of binding kinetics demonstrated the actual conditions of receptor activity, and indicated that the induced insulin binding sites of Tetrahymena behaved similarly to 'classical' receptors.