The potential role of taurine transport associated with volume regulation in renal tissue and isolated proximal renal tubules was studied in the teleost Carassius auratus (goldfish). The cellular taurine content in renal tissue fragments incubated in isosmotic solution (290 mOsm) (7.8 +/- 0.9 (SD) micromol g wet wt(-1)) decreased by 60% following exposure to hyposmotic medium (100 mOsm). The rate coefficient for [14C]taurine efflux in renal tissue and in isolated proximal renal tubules was strongly stimulated following hyposmotically or urea-activated cellular swelling. The stimulated basolateral taurine efflux pathway exhibited channel-like functional characteristics since (a) [14C]taurine influx was stimulated in parallel with the osmolality-dependent taurine efflux and (b) this efflux could not be stimulated by high medium taurine concentrations (40 mM) applied 10 min following the osmolality reduction (trans-stimulation test). Administration of the 5-lipoxygenase inhibitor ETH 615-139 (20 microM) during hyposmotic stimulation inhibited regulatory volume decreases but had no effect on taurine efflux. In addition, hyposmotically induced taurine efflux was slightly but significantly inhibited by the cyclooxygenase inhibitor indomethacin (10 microM). The taurine efflux was also dependent on both extra- and intracellular Ca2+. It is concluded that taurine is likely to coparticipate with KCl as an osmoeffector during RVD in Carassius proximal renal tubule cells. Cellular swelling seems to activate a basolateral taurine transport pathway with functional properties of a channel. This efflux mechanism appears to be partly regulated by Ca2+. Such a transport pathway could play a role in the cell volume regulatory mechanisms participating during transepithelial solute and water transport.

The objective of this work was to study cell volume regulation and transepithelial transport in renal proximal tubules. A modified stop-flow technique in in vitro perfused renal proximal tubules of Carassius auratus was used. The rate of luminal fluid absorption and the epithelial thickness were measured. Isosmotic Na+ removal from the tubule lumen or addition of the Na+/glucose co-transport blocker phloridzin (0.5 mM) to the lumen inhibited fluid absorption. Only minor effects on luminal absorption were observed following: (1) addition of the K+ channel inhibitor BaCl2 (1 mM); (2) addition of the Cl- channel inhibitor MK-196 (1 mM); (3) lowering bath and perfusate HCO3- in the presence of 0.1 mM acetazolamide; or (4) addition of the leukotriene-D4 receptor antagonist L-660, 711 (20 microM). Isosmotic addition of 40 mM taurine to the bath inhibited the rate of fluid absorption. This effect could be partially overcome with the organic acid secretion inhibitors probenecid (1 mM) and bromcresol green (0.1 mM). Finally, administration of the 5-lipoxygenase antagonist ETH 615-139 (20 microM) caused a significant reduction in the rate of luminal absorption. In summary, tubular reabsorption is, in this preparation, closely linked to sodium reabsorption. In the absence of luminal amino acids most of this sodium uptake seems to occur in co-transport with glucose followed by osmotically obligated water. Eicosanoids from the 5-lipoxygenase pathway appear to regulate this process. Finally high concentrations of taurine in the bath opposed luminal fluid absorption, at least partially, by its secretion into the tubular lumen.

The cellular response to hypotonic stimulation was studied with videometric methods in 266 proximal renal tubules dissected from Carassius auratus (goldfish). In hypotonic solutions (low NaCl), cells underwent rapid swelling followed by gradual shrinking toward isotonic volume (volume-regulatory decrease phase, VRD). Hypothermia (8 degrees C), increased extracellular potassium (15, 25, and 40 mM), quinine (0.1 mM), barium (0.5 mM), 4,4'-diisothio-cyanostilbene-2,2'-disulfonic acid (DIDS; 0.02 mM), acetazolamide (0.1 mM), decrements in extracellular bicarbonate, and increases in extracellular chloride impaired VRD. Ouabain (1.0 mM), furosemide (0.1 mM), and the chloride channel blocker 5-nitro-2-(3-phenylpropylalanine) benzoate (NPPB; 0.001 mM) had no effect. While VRD occurred in the absence of extracellular calcium influx, addition of the calcium ionophore A23187 (0.01 mM) in the presence of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; 2.0 mM) impaired this process both in acidic and alkaline media. Trifluoroperazine (0.01 mM) reversibly inhibited VRD. The effect of this calmodulin inhibitor could not be overridden with the cationic ionophore gramicidin (0.5 microM). The data suggest that Carassius proximal renal tubular cells volume regulate in hypotonic solutions by the loss of KCl and osmotically obligated water. We postulate that the main efflux of potassium is through a calcium-gated potassium channel with its counter ion extruded through a calmodulin-regulated Cl(-)-HCO3- exchanger.

Osmoregulatory Ca2+ signaling in hypotonic solutions was studied with videometric techniques in 158 proximal renal tubules isolated from the teleost Carassius auratus. Absence of extracellular Ca2+, hypoxia (23 mmHg), or NaCN (3 mM) did not alter regulatory volume decreases (RVD). Nevertheless, decrements of intracellular Ca2+ via the A23187 ionophore or after intracellular Ca2+ chelation with indo-1/AM (5 microM) inhibited RVD. In tubules depleted of Ca2+, RVD could only be fully elicited when intracellular Ca2+ pulses were given within 1 min after hypotonic stimulation. While inhibition of Ca2+ release from the endoplasmic reticulum (ER) with 8-(diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8, 50 microM) blunted RVD, some of its effects could be reversed with the anion carrier tributyltin (1 microM). Dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP, 0.5 and 1.0 mM) and forskolin (0.25 mM) also impeded RVD; however, their effects could be partially reversed with the K+ ionophore gramicidin (0.5 microM). In conclusion, in Carassius auratus proximal renal tubule cells, RVD is activated by an intracellular Ca2+ signal that likely emanates from the ER and not from the extracellular media or the mitochondrial Ca2+ pool. Ca2+ activation of a cAMP-modulated osmoregulatory K+ channel appears to play an important role.

No abstract