Dimensions of polar pathways through rabbit gallbladder epithelium

Abstract

Summary. The permeability of rabbit gallbladder to hydrophilic nonelectrolytes, with molecular weights from 20 to 60,000, has]been studied. Restriction in the diffusion of the small electrolytes is very significant up to glycerol, which suggests permeation through aqueous pores with equivalent radii of 4/~. An extracellular pathway is responsible for the permeation of the larger solutes. This extracellular pathway shows no restriction in diffusion of molecules up to the size of inulin. Dextran (15,000 to 17,000 mol wt) is significantly restricted. Albumin permeability is < 10 .8 cm sec -1. These observations can be equated with equivalent pore radii of ~40 • for the shunt pathway.Increasing osmolarities of the incubation medium cause decreased cell-membrane permeability and increased shunt permeability. 0.5 mM phloretin induces a 60 % reduction in urea permeability and a 168 % increase in antipyrine permeability. No effect on the osmotic water permeability or on the shunt permeability is observed in the presence of phloretin. The apparent activation energy of urea permeation changes from values consistent with diffusion in bulk water, to values consistent with diffusion through hydrocarbon regions. This suggests that the polar route for urea permeation is blocked by phloretin.The contribution of the shunt pathway to osmotic flow induced by sucrose or NaC1 gradients is smaller than 16% according to Poiseuille's flow calculations. Tetraethylammoniumchloride and albumin have been shown to be osmotically more effective than sucrose, suggesting a greater shunt contribution to the total water flow.Extensive studies on nonelectrolyte permeability of rabbit gallbladder epithelium have led to the postulation of small aqueous pores in the cell membranes and a small number of larger pores somewhere in the epithelium [13,48,56]. In this respect gallbladder epithelium resembles the epithelium of kidney proximal tubules [5,23], stomach [1] and small intestine [33]. In all these tissues the anomalous permeability of relatively large hydrophilic solutes, such as sucrose, inulin and even bigger molecules, is explained by postulating the existence of some large aqueous pores in the epithelium.

Routes of nonelectrolyte permeability in gallbladder. Effects of 2,4,6-triaminopyrimidinium (TAP).

“…The transepithelial permeability coefficients P's, obtained for frog and rabbit gallbladders during the control period and after 12 mM TAP was added to 2 This is probably the reason why the permeability values for sucrose in rabbit gallbladder reported here (Table I) are more than two times smaller than those reported by Smulders and Wright (1971) and van Os et al (1974). The value of the permeability coefficient for sucrose is, however, similar to the one obtained from Fig.…”

“…1 of Smulders et al, 1972 (P = 2.3 X 10 --6 cm/s) when a more careful cheek of the preparations was made. The greater permeability, coefficients for sucrose obtained by van Os et al (1974) could be due to the use of sac preparations by these authors. I20 THE JOURNAL OF GENERAL PHYSIOLOGY " VOLUME 66 .…”

“…~ a The value of the steady-state P$ reported here for frog gallbladder is close to that found under similar circumstances for rabbit gallbladder (1.7)< 10 -2 cm/s, Wright et al, 1972;van Os and Sleegers, 1973). These values of steady-state Pf are probably gross underestimations of the instantaneous, real one (Wright et al, 1972;van Os, 1974). The reason for that is the solute polarization due to the "sweeping away effect" (Dainty, 1963) which led to actual steady-state osmotic gradients…”

Selective Decrease in Paracellular Conductance of Tight Junctions: Role of the First Extracellular Domain of Claudin-5

“…Phloretin and 25 mM glucose were added to the incubation buffer to inhibit the transcellular transport of methylglucose and glucosamine (46); chloroquine was used to prevent the vesicular traffic of sialic acid (3). These substances had no effect on the paracellular pathway, as found in previous studies (6,55).…”

Developmental changes in epithelial transport characteristics of preimplantation rabbit blastocysts.

“…blastocysts (45 -60%: Table 1 and Fig. 2) was comparable to that produced by the same concentration of phloretin in rabbit gall-bladder (45-55 % Van Os, de Jong &r Slegers, 1974). It would appear, therefore, that the rabbit blastocyst acquires a phloretin-inhibited urea entry pathway between 6 and 7 days after fertilization.…”