CONTRACTILE VACUOLES1
Authors: KITCHING J. A. 
Published: January 2008

Although comparatively little has been added to our knowledge of the structure of contractile vacuoles since the publication of Lloyd's (1928) review,1 the work of Gelei has thrown doubt upon previous interpretations of the functioning of the feeder canals and central vesicle of Paramecium. Moreover, much work with methods of osmic impregnation has suggested that the vacuolar membrane contains fats or lipoids.

The idea that marine and parasitic Protozoa rarely or never have contractile vacuoles is incorrect; contractile vacuoles occur in a large number of marine and parasitic ciliates. However, the rate of output of fluid is much lower in marine than in fresh‐water ciliates of the same size. In some Protozoa–and probably in most fresh‐water ones–the water to be evacuated enters mainly by the general body surface. However, in a few cases the water taken in with the food provides the greater share. Nothing is known of the composition of the vacuolar fluid apart from its hydrogen‐ion concentration. Thus circumstantial evidence suggests that in many fresh‐water Protozoa osmoregulation is one of the functions of the contractile vacuole.

The osmoregulatory theory has been given support by the results of many experiments. The rate of vacuolar output is usually decreased by an increase, and increased by a decrease, in the concentration of (suitable) solutes in the external medium. More direct evidence is provided by the use of agents, such as cyanide, which inhibit vacuolar activity. When vacuolar activity is stopped the body of fresh‐water Peritricha swells; and removal of the cyanide leads to recovery of the contractile vacuole and shrinkage of the body. A similar effect of cyanide has been found in euryhaline marine Peritricha placed in dilute sea water. It is concluded that the contractile vacuole controls the body volume by baling out water as fast as this comes in by osmosis through the body surface. In fact the contractile vacuole may be compared with the bilge pump of a rather leaky ship. However, it is clearly necessary that an osmoregulatory organ should conserve the internal salts of the organism; therefore according to the osmoregulatory theory the contractile vacuole should separate solutes from water. It is not known whether any other part of the body, such as the body surface, is concerned in osmoregulation. It should also be emphasized that the contractile vacuole does not maintain the internal concentration of the organism absolutely constant in spite of changes in the external osmotic pressure. The contractile vacuole merely has an ameliorating effect, so that the internal changes are less drastic.

The contractile vacuole might perform other functions besides osmoregulation, and an excretory function has often been suggested for it. It is believed that in some cases granules disappear in the neighbourhood of the contractile vacuole during diastole, but it is by no means certain that their substance is removed in solution in the vacuolar fluid. There is no chemical evidence to show that the contractile vacuole eliminates nitrogenous waste matter. In certain Protozoa which take in large quantities of fluid food at one “mouthful”, the contractile vacuole gets rid of the excess water. The function of the contractile vacuoles of marine and parasitic Protozoa is unknown.

Much speculation must enter into any discussion of the mechanisms of diastole and systole. The inadequacy of all other theories gives indirect support to the view that diastole involves an active secretion of water, and retention of solutes, by the vacuolar membrane. This view alone meets the requirements of the osmoregulatory theory of vacuolar function. Systole may be brought about by body turgor, but it appears from a consideration of multi‐vacuoled species that there must in addition be some mechanism which ensures that a vacuole does not discharge until it reaches a certain size. Many abnormalities of the ultimate vacuolar diameter may be attributed to a change in the structure of the surface membrane of the body.

Many other external conditions influence vacuolar activity. Rise of temperature leads to an increase of frequency and to a decrease in ultimate diameter. The effects of neutral salts, hydrogen‐ion concentration, and heavy water are probably only indirect. Experiments on the effects of glandular extracts are so far inconclusive. Feeding and division rate are said to influence vacuolar frequency.

According to the secretion theory, which is supported in this review, vacuolar activity involves the performance of osmotic work, the energy for which must ultimately be derived from cell respiration. It is not yet known whether the adverse effect of cyanide on vacuolar activity is due to inhibition of cell respiration, or to some independent poisoning action on the vacuolar mechanism.

1 Biological Reviews