Hypotonicity-Activated Efflux of Taurine and Myo-lnositol in Rat Inner Medullary Collecting Duct Cells: Evidence for a Major Common Pathway

Abstract

To further characterize the hypotonicity-activated efflux pathways for the organic osmolytes taurine and myo-inositol in inner medullary collecting duct (IMCD) cells tracer fluxes of taurine and myo-inositol were investigated. The time course of activation of both fluxes after exposure of cells isolated at 600 mosm to a hypotonic medium (300 mosm by omission of sucrose) was identical with a major increase of release within the first 10 min. All ‘anion channel blockers’ employed proved to be strong inhibitors of both fluxes. Inhibition of myo-inositol efflux by 0.5 mM NPPB and 0.1 mM dideoxyforskolin was not significantly different from that of taurine efflux (87.7 ± 11.4 compared to 94.6 ± 4.6% and 98.8 ± 2.0 compared to 95.9 ± 3.7%). However, SITS (0.5 and 0.01 mM), DIDS (0.5 and 0.01 mM), and niflumic acid (0.5 mM) inhibited myo-inositol efflux more strongly than taurine efflux. The respective values were 65.4 ± 4 vs. 42.9 ± 3.6% for 0.01 mM SITS, 65.7 ± 4.2 vs. 45.8 ± 2.0% for 0.01 mM DIDS, and 79.5 ± 3.5 vs. 54.2 ± 2.5% for 0.5 mM niflumic acid. Taurine as well as myo-inositol efflux were decreased to a similar extent by 10 mM extracellular ATP (26.9 ± 6.3 vs. 29.8 ± 17.7% inhibition), by 10 mM extracellular cAMP (52.8 ± 9.8 vs. 60.1+17.2% inhibition) and by reduction of the intracellular ATP content employing 2-deoxy-D-glucose (31.9 ± 5.9 vs. 40.4 ± 13.6% inhibition). In polarized primary cell cultures taurine and myo-inositol were released during a hypotonic shock primarily across the basal-lateral membrane, the ratio of basolateral versus apical efflux was 4.1 for taurine and 3.9 for myo-inositol. Apical fluxes were more sensitive to 0.01 mM SITS or DIDS; this was particularly evident for apical myo-inositol efflux which was inhibited by 0.01 mM SITS by 84.1 ± 5.9% compared to 43.5 ± 13.1% inhibition of the basolateral efflux. Thus, taurine and myo-inositol efflux show to a great extent a similar cellular distribution, intracellular regulation and pharmacological inhibition profile. This similarity suggests that the two osmolytes share an efflux pathway that might be identical with the swelling-activated taurine conductance described previously. Additional minor pathways can, however, not be excluded.

Taurine secretion in primary monolayer cultures of flounder renal epithelium: stimulation by low osmolality

“…An almost complete RVD was observed after 5 min in isolated perfused rabbit proximal convoluted tubules following a 40 mosmol/kgH 2 O osmolality decrease (6). Short time courses for RVD were similarly observed in hypotonic-stressed renal cells of the distal nephron, Madin-Darby canine kidney cells (21), and rat inner medullary collecting duct cells (20). In the present study, when the flux measurements were carried out after a 2-h exposure to hyposmotic medium, the cell volume of the flounder PTCs (as indicated by the water content) was not different from that of the cultures in isosmotic medium.…”

“…Other lines of evidence further indicate that the response of the PTCs to hyposmotic medium was not a short-term RVD. Typical RVD responses of different renal cells include lowered cellular content of taurine in hyposmotic medium secondary to enhanced taurine efflux (8,20,21). In contrast, intracellular content of taurine in flounder PTCs exposed to hyposmotic solution was not significantly different from those in isosmotic medium.…”

Inositol and higher inositol phosphates in neural tissues: homeostasis, metabolism and functional significance

“…In response to hypotonic stress, inositol efflux occurs from both neurons and glia via a facilitated diffusion mechanism that involves a non‐specific Cl – channel. Based upon electrophysiological studies, the latter has been designated as the volume‐sensitive organic osmolyte anion channel (VSOAC) and mediates the rapid efflux of triethylamines and amino acids, in addition to inositol (Jackson and Strange 1993; Goldstein and Davis 1994; Ruhfus and Kinne 1996; Jackson and Madsen 1997). VSOAC‐mediated inositol efflux is dependent upon the non‐hydrolytic binding of ATP and can be regulated by G αi , protein kinases A and C, tyrosine kinases and lipoxygenases (Strange et al .…”

The effects of thawing on the plasma metabolome: evaluating differences between thawed plasma and multi-organ samples

“…In addition, myo-inositol and taurine concentrations were found to increase in plasma as a response to thawing while the thawed kidney samples displayed decreased concentration of these metabolites. Taurine and myo-inositol function as osmolytes in the kidneys and have been found to co-ordinately respond to aid in cellular protection, which may explain the high concentration of these metabolites (Ruhfus and Kinne 1996). Muscle was the only organ that displayed decreased uracil concentration in response to thawing.…”