ISSN:
1432-1424
Keywords:
Paracellular flow
;
Malpighian tubules
;
Secretion
;
Epithelial transport
;
Water transport
;
Convective flow
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Abstract Lumen to bath J 12/C 1 and bath to lumen J 21/ C 2 fluxes per unit concentration of 19 probes with diameters (d m) ranging from 3.0–30.0 Å (water, urea, erythritol, mannitol, sucrose, raffinose and 13 dextrans with d m 9.1–30.0 Å) were measured during volume secretion (J v ) in the upper segment of the Malpighian Tubule of Rhodnius by perfusing lumen and bath with 14C or 3H-labeled probes. J net=(J 12/C 1 − J 21/C 2) was studied as a function of J v · J v was varied by using different concentrations of 5-hydroxy tryptamine. J net for 3H-water was not different from J v We found: (i) A strong correlation between J net and J v for 8 probes d m =3.0–11.8 Å (group a probes), indicating that the convective component of J net is more important than its diffusive component and than unstirred layers effects which are negligible. Therefore group a probes are solvent dragged as they cross the epithelium, (ii) There is no correlation between J net and J v for 11 probes with d m=11.8–30 Å (group b). Therefore these probes must cross the epithelium by diffusion and not by solvent drag, (iii) In a plot of J net/J v vs. d m group a probes show a steep linear relation with a slope = −0.111, while for group b probes the slope is −0.002. Thus there is a break between groups a and b in this plot. We tried to fit the data with models for restricted diffusion and convention through cylindrical or parallel slit pathways. We conclude that (i) group a probes are dragged by water through an 11.0 Å-wide slit, (ii) Most of J v must follow an extracellular noncytosolic pathway, (iii) Group b probes must diffuse through a 42 Å-wide slit, (iv) A cylindrical pathway does not fit the data.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00235041
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