The Role of the Contractile Vacuole in the Osmoregulation of Tetrahymena pyriformis*

Abstract

SYNOPSIS The relationship of cell size and contractile vacuole efflux to osmotic stress was studied in Tetrahymena pyriformis strain W, after transfer into fresh solutions iso‐ or hypoosmotic to the growth medium. Microscopic measurements of the cell and contractile vacuole dimensions, made with an image‐sharing ocular at 27 C, allowed the calculation of the cell size and shape and the vacuolar efflux rate which provide a measure of osmoregulation. The contractile vacuole cycles have no homeostatic oscillations. In 0.03–0.10 osmolar solutions, the cell size and shape are constant while the vacuolar efflux rate has an inverse linear dependence upon extracellular osmolarity. Regression analyses indicate that for cells with systole faster than 0.1 sec (the major part of the population), it is only the final diastolic volume of the contractile vacuole that is related to osmotic stress while the frequency of systole is independent of osmotic stress and has a constant period of 7.7 ± 0.2 sec. Therefore, osmotic stress upon Tetrahymena is regulated by a corresponding change in the filling rate of its contractile vacuole to allow an unaltered cell size and shape. Kinetic measurements of vacuoles during diastole fit the model (dV/dt = K1‐K2A), where (dV/dt) is the vacuolar filling rate and (A) is the vacuolar surface area. This dependence of vacuolar volume upon its surface area may be ascribed either to elastic components of the vacuolar membrane or to an increasing leakiness of this membrane during diastole. Mitochondrial inhibitors were used to observe the energy requirements of vacuolar operation and of intracellular secretion of water.

Contractile Vacuoles and Associated Structures: Their Organization and Function

“…A quantitative estimate of vacuolar output must take account of the frequency of expulsions and of the amount of fluid expelled at each expulsion. This is because the amount of fluid expelled at each expulsion is not constant, nor is there any fixed relationship between frequency of expulsions and the rate of vacuolar output (Rifkin, 1973;Ahmad & Couillard, 1974; Patterson, 1976a). The size of the vacuole can be measured with any of a variety of measuring microscope oculars, but the most suitable for this purpose are imageshearing oculars (Rifkin, 1968).…”

“…However, the case for this is weakened by two observations. First, the contractile vacuole continues to function in deionized water (Reuter, 1963; Raze & Schoffeniels, 1965; Rifkin, 1973). Second, in high potassium media, transient changes in the cellular potassium levels of some species follow an increase in the external osmotic pressure (with sucrose) (Kramh~rft, 1970; Patterson, 1976a;Kirst, 1977).…”

“…3). However, such a suggestion would be premature, as this mechanism is inconsistent with the observation that the contractile vacuole complex remains fully functional when cells are placed in distilled water (Reuter, 1963 ;Raze & Schoffeniels, 1965 ;Rifkin, 1973).…”

Dispersal plasticity driven by variation in fitness across species and environmental gradients

“…For instance, thermal stress might result in increased cell metabolism and motility (Jacob & Legrand, 2021; Luan et al, 2012), which might increase dispersal rate. On the contrary, response to osmotic stress involves mechanisms such as glycerol production or regulation of intracellular sodium concentration (Dunham, 1964; Rifkin, 1973) that might trade‐off against dispersal ability.…”

Osmosensing and osmoregulation in unicellular eukaryotes

“…However it has been observed that water influx to the vacuole is the most important form of osmoregulation (Rifkin 1973).…”