Some Properties of a Model Assay for Ciliary Contractility

The response of glycerinated cilia from Tetrahymena to the addition of adenosine triphosphate (ATP) can be assayed by measuring the size of the ciliary pellet obtained after brief, low-speed centrifugation in hematocrit tubes (1) . Upon the addition of ATP, cilia swell, resulting in pellets up to threefold larger than pellets of control cilia . There are two interdependent hydrodynamic factors which determine pellet size : (a) the density of the pellet, which depends on the packing arrangement as well as particle shape, and (b) the percent of total cilia sedimented, which depends on the rate of sedimentation of the cilia . Concentrations of ATP from 0 .1 to 1 .0 mat (but not other nucleoside triphosphates) cause an increase in pellet size relative to control cilia both by increasing the percent cilia sedimented and by decreasing the pellet density, apparently by an ATP-induced hydration (1-3) . Measurement of the parameters of the assay (per cent cilia sedimented and pellet density) may reveal changes even when the pellet height is unchanged by a particular experimental treatment . In this study, the pellet height assay was employed in conjunction with ATPase activity measurements to investigate certain key features of the interaction of glycerinated cilia with ATP and potential denaturing or uncoupling agents, which have been shown by other workers to alter or inhibit the beating of ciliary models or of cilia in situ . The evidence presented below supports the view that the ATP-induced swelling of cilia measured in the present assay, although not identical to beating, is closely related to ciliary contractile process . METHODS