The precise relationship between the rate of contractile vacuole fluid discharge and the vacuole diameter at the start of systole was determined in cells of Paramecium multimicronucleatum subjected to various external conditions. The rate of discharge was higher when the diameter was larger. When the rate of discharge was plotted against the diameter, the points fell around a single parabolic line passing through the origin and were independent of the external conditions employed. This implies that the rate of discharge is proportional to the square of the vacuole diameter. We have previously proposed a hypothesis in which membrane tension in the contractile vacuole is altered as its planar membrane becomes tubular or as tubules become planar membrane (termed the membrane area-proportional tension hypothesis). We propose here that it is this change in membrane tension which determines the vacuole pore shape and sets the subsequent rate of fluid discharge.

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