Changes in relative cell volume in response to hypotonic solutions and glucose were studied in single isolated rat pancreatic β‐cells using a video‐imaging technique, β‐cell electrical activity was recorded under similar conditions using the perforated patch technique. Exposure of β‐cells to hypotonic solutions (10 and 33% hypotonicity) caused an immediate increase in cell volume to relative values of 1.09 and 1.33, respectively. This was followed by a gradual regulatory volume decrease. Raising the concentration of glucose from 4 to 20 mm or 12 mm (with substitution of mannitol) increased β‐cell volume by 12 and 10%, respectively. This effect of glucose persisted when Co2+ was added to inhibit insulin release. Glucose‐induced volume increases were sustained for the duration of exposure to elevated hexose concentration. The addition of 16 mm 3‐O‐methylglucose, which is transported into the β‐cell but not metabolized, produced only a transient 5% increase in β‐cell volume. Exposure of β‐cells to a 15% hypotonic solution resulted in a transient depolarization and electrical activity. Raising the glucose concentration to 20 or 12 mm caused a sustained depolarization and generation of electrical activity. However, the addition of 16 mm 3‐O‐methylglucose had no effect on β‐cell membrane potential. The glucose‐induced increase in volume and induction of electrical activity, when measured in single β‐cells simultaneously, showed comparable kinetics. The secretion of insulin from intact pancreatic islets was stimulated by exposure to hypotonic solutions (10–33% hypotonicity). A 15% hypotonic solution stimulated insulin release to a peak value comparable to that elicited by raising the glucose concentration from 4 to 20 mm. Whereas hypotonic solutions caused a transient stimulation of insulin release, the effect of glucose was sustained. It is suggested that glucose increases the volume in rat pancreatic β‐cells by a mechanism dependent upon metabolism of the sugar. The extent of cell swelling evoked by raised glucose concentrations is sufficient to depolarize the cells and induce electrical and secretory activity and may involve activation of a volume‐sensitive anion conductance.

The ciliary epithelium of the eye secretes the aqueous humor. It is a double epithelium arranged so that the apical surfaces of the nonpigmented ciliary epithelial (NPCE) and pigmented ciliary epithelial (PCE) cells face each other and the basolateral membranes face the inside of the eye and the blood, respectively. We have investigated the volume responses of both single cells and coupled pairs from this tissue to osmotic challenge. Both NPCE and PCE cells undergo regulatory volume increase (RVI) and decrease (RVD) when exposed to hyper- and hyposmotic solution, respectively. In hyposmotic solution single cells swell and return to their original volumes within ∼3 min. In nonpigmented cells RVD could be inhibited by blockers of volume-activated Cl−channels [tamoxifen (100%) > quinidine (87%) > DIDS (84%) > 5-nitro-2-(3-phenylpropylamino)benzoic acid (80%) > SITS (58%)] and K+ channels [Ba2+(31%)]. However, in PCE cells these inhibitors and additionally tetraethylammonium and Gd3+ were without effect. Only bumetanide, an inhibitor of Na+-K+-2Cl−cotransport, was found to have any effect on RVD in PCE cells. NPCE-PCE cell coupled pairs also underwent RVD, but with altered kinetics. The onset of RVD of the PCE cell in a pair occurred ≈80 s before that of the NPCE cell, and the peak swell was reduced. This is consistent with fluid movement from the PCE to the NPCE cell. The effect of the volume-activated Cl− channel inhibitor tamoxifen was to eliminate this difference in the times of onset of RVD in coupled cell pairs and to inhibit RVD in both the NPCE and PCE cells partially. On the basis of these observations we suggest that fluid is transferred from the PCE to the NPCE cell in coupled pairs during cell swelling and the subsequent RVD. Furthermore, we speculate that reciprocal RVI-RVD could underlie aqueous humor secretion.

This study investigated regulatory volume increase (RVI) in rat pancreatic beta-cells. Volume changes in isolated beta-cells were measured by a video-imaging method. Cell shrinkage was induced by exposure to solutions made hypertonic by the addition of 100 mM mannitol. In HEPES-buffered solutions, beta-cells exhibited an RVI which was almost completely abolished by 10 microM bumetanide. These data indicate that Na+-2Cl--K+ cotransporters make a major contribution to RVI in beta-cells. In HCO3--buffered solutions, however, an RVI was observed in the presence of 10 microM bumetanide. This bumetanide-insensitive component of RVI was inhibited by 100 microM amiloride or 100 microM 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS). These data suggest that, in addition to the Na+-2Cl--K+ cotransporter, functionally coupled Na+-H+ exchangers and Cl--HCO3- exchangers may also contribute to RVI in pancreatic beta-cells.

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