Symmetry and non-linearity of osmotic flow across rectal cuticle of the desert locust.

Ionic and osmotic regulation in the desert locust (Phillips, 1964-70) as in most insects studied to date (reviewed by Stobbart & Shaw, 1964) is ultimately achieved by selective re-absorption of ions and water in the rectum. Re-absorbed substances must first cross the chitinous cuticle or intima before active transport across the epithelial layer can occur. This paper continues a systematic study of the rectal intima (Phillips & Dockrill, 1968) to determine the relative role of this membrane and the epithelial layer in selective re-absorption. From a knowledge of transfer across the intima alone and across the whole rectal wall, the properties of transfer across the epithelial layer may be deduced. A previous paper (Phillips & Dockrill, 1968) was concerned with relative permeability (as measured by isotopic flux in the absence of net movement) of the intima to a series of hydrophilic non-ionic molecules of graded hydrated size, as well as to tritiated water. The intima was found to behave as a molecular sieve with a uniform population of pores, 6-8 A in radius. Net absorption of water occurs continuously from the locust rectum in vivo (Phillips, 1964 a, c). Permeability to water as estimated by isotopic flux across most biological membranes is generally much lower than the value calculated from osmotic flow (reviewed by Dick, 1966). Various explanations for this discrepancy have been suggested (e.g. bulk flow through pores, Paganelli & Solomon, 1957; single-file diffusion of isotopes, discussed by Dick, 1966; and unstirred layers, Dainty, 1963, 1965). Assuming bulk flow through pores, this discrepancy has been used to provide an independent estimation of pore size (e.g. Paganelli & Solomon, 1957). One objective of the present study was to measure the osmotic permeability of the rectal intima for comparison with that of the whole rectal wall (Phillips, 1964 a, 1970) and to see whether rates were consistent with the hypothesis of pores of 6-8 A radius (Phillips & Dockrill, 1968). The insect cuticle is a heterogeneous multilaminar membrane, the layers of which might be expected to have different physical characteristics. Patlak, Goldstein & Hoffman (1963) have predicted that such membranes may exhibit rectification; that is, osmotic permeability may be different in opposite directions of flow, so that the intima might act as a value. This is a well-established property of integumentary cuticle (Beament, 1961, 1964, 1965). In the latter membrane, the asymmetry to water movement is associated with a continuous superficial monolayer of wax molecules. While available evidence indicates the absence of such a continuous wax monolayer