Physiological Discontinuity in An Epithelium with An Apparently Uniform Structure

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Abstract
The lower Malpighian tubule of Rhodnius is made up of cells of apparently uniform structure. Functionally, however, this length of the Malpighian tubule consists of two distinct regions. The upper region is osmotically very permeable and plays no part in reabsorbing KC1 from the primary excretory fluid. By contrast, the lowermost 30% of the length of the lower tubule is osmotically impermeable and when stimulated absorbs KCl from the lumen at the high rate of about 0.80 μmol cm−2 min−1. In other respects such as permeability to other solutes and the ability to transport alkaloids the two regions scarcely differ.