Paracellular calcium transport across renal and intestinal epithelia

Calcium (Ca2+) is a key constituent in a myriad of physiological processes from intracellular signalling to the mineralization of bone. As a consequence, Ca2+ is maintained within narrow limits when circulating in plasma. This is accomplished via regulated interplay between intestinal absorption, renal tubular reabsorption, and exchange with bone. Many studies have focused on the highly regulated active transcellular transport pathways for Ca2+ from the duodenum of the intestine and the distal nephron of the kidney. However, comparatively little work has examined the molecular constituents creating the paracellular shunt across intestinal and renal epithelium, the transport pathway responsible for the majority of transepithelial Ca2+ flux. More specifically, passive paracellular Ca2+ absorption occurs across the majority of the intestine in addition to the renal proximal tubule and thick ascending limb of Henle’s loop. Importantly, recent studies demonstrated that Ca2+ transport through the paracellular shunt is significantly regulated. Therefore, we have summarized the evidence for different modes of paracellular Ca2+ flux across renal and intestinal epithelia and highlighted recent molecular insights into both the mechanism of secondarily active paracellular Ca2+ movement and the identity of claudins that permit the passage of Ca2+ through the tight junction of these epithelia.

Decrease in rat taste receptor cell intracellular pH is the proximate stimulus in sour taste transduction

Taste receptor cells (TRCs) respond to acid stimulation, initiating perception of sour taste. Paradoxically, the pH of weak acidic stimuli correlates poorly with the perception of their sourness. A fundamental issue surrounding sour taste reception is the identity of the sour stimulus. We tested the hypothesis that acids induce sour taste perception by penetrating plasma membranes as H+ ions or as undissociated molecules and decreasing the intracellular pH (pHi) of TRCs. Our data suggest that taste nerve responses to weak acids (acetic acid and CO2) are independent of stimulus pH but strongly correlate with the intracellular acidification of polarized TRCs. Taste nerve responses to CO2 were voltage sensitive and were blocked with MK-417, a specific blocker of carbonic anhydrase. Strong acids (HCl) decrease pHi in a subset of TRCs that contain a pathway for H+ entry. Both the apical membrane and the paracellular shunt pathway restrict H+ entry such that a large decrease in apical pH is translated into a relatively small change in TRC pHi within the physiological range. We conclude that a decrease in TRC pHi is the proximate stimulus in rat sour taste transduction.

Electrophysiology and Glucose Transport of Human Peritoneal Mesothelial Cells: Implications for Peritoneal Dialysis

Objective To elucidate ionic and glucose transport across human peritoneal mesothelium, we utilized an Ussing chamber setup and studied the electrophysiological characteristics and tissue permeabilities of human peritoneal mesothelial cells (HPMC) to l- and d-glucose. Methods Human mesothelial cells were grown on polyester filters (snapwell; Costar, Cambridge, MA, U.S.A.) that, upon confluence, were fitted into Ussing chambers. Transmesothelial resistance and resting potential were determined using electro-physiological techniques. Radiolabeled glucose was added to one side of the chamber and the permeabilities determined by serial sampling in the receptive compartment. Results The transmesothelial potential and resistance were 0.54 ± 0.07 mV (apical positive) and 20.4 ± 3.2 Ω·cm2 respectively (mean ± SEM, n = 36). The course of overall transfer of d- and l-glucose was examined using l-glucose as a positive diffusion-plus-leak marker. The permeabilities of HPMC to d-glucose were 3.00 ± 0.26 cm/sec (apical-to-basolateral) and 3.25 ± 0.27 cm/sec (basolateral-to-apical) [ n = 6 experiments, p = not significant (NS)], which were not different from those of l-glucose: 3.00 ± 0.30 cm/sec (apical-to-basolateral) and 2.71 ± 0.24 (basolateral-to-apical) ( n = 6 experiments, p = NS). Conclusions The transepithelial resistance of HPMC is low and the ionic gradient, although it exists, is small and inconsequential. Passive paracellular flow accounts for the majority of transmesothelial glucose transport. The existence of a large paracellular shunt precludes the mesothelial membrane as a clinically relevant osmotic barrier.

Changes in the apical surface of chloride cells following acclimation of lampreys to seawater

Scanning electron microscopy (SEM) was used to study the changes that occur in the morphological relationships between chloride and pavement cells in the gills during acclimation of young adult lampreys to seawater. Because chloride cells are located predominantly between lamellae and are thus obscured from view, the lamellae were removed with the use of a micromanipulator installed in a SEM. In gills of animals maintained in river water, chloride cells could then be seen to be dislike and typically to form single rows between successive lamellae. After acclimation to seawater, the apical surfaces of chloride cells lose their microvilli and change in shape from small circles to rectangles that extend the full width between successive lamellae. These changes result in an increase in the length of the paracellular pathway between chloride cells. Previous work has shown that the number of strands of the zonulae occludentes sealing this pathway declines under these conditions. This presumably leads to an increase in paracellular permeability of the gill epithelium, thereby providing the low-resistance paracellular shunt required for the passive movement of sodium into the environment during osmoregulation in seawater. The above changes are reversed by transfer of lampreys downward to 10% seawater.

Structural Elements Which Govern the Resistance of Intestinal Tissues to Compound Transport

The series of cyclized RGD peptides in this study demonstrated a very low partition into octanol as judged by HPLC. Thus, these molecules are likely to move predominantly through the paracellular pathway. Permeability across Caco-2 monolayers was determined using reversed phase HPLC and found to be restricted by molecular weight and possibly charge-charge interactions between the solute and charged moieties within the paracellular shunt. When normalized for molecular weight, molecules with a net charge between -1 and -2 demonstrated the highest permeabilities, which suggests an optimal net charge with respect to permeability.

Effect of protamine on ion conductance of upper portion of descending limb of long-looped nephron from hamsters

To estimate the contribution of paracellular shunt pathway to the cation-selective permeability in the upper portion of the descending limb of long-looped nephron (LDLu) of hamsters, we observed the effect of protamine on salt-diffusion voltage (delta VT) and transmural resistance (RT). delta VT generated on reduction of lumen NaCl concentration was decreased from 12.0 +/- 1.4 to 7.3 +/- 1.2 mV when 100 micrograms/ml protamine were added to the lumen. Although the effect of protamine persisted after removal of the agent from the lumen, addition of 30 U/ml heparin reversed the delta VT toward the control level. The effect of protamine was dose dependent in the range from 3 to 1,000 micrograms/ml. Protamine was without effect from the bath. Studies on single salt dilution voltage revealed that 100 and 300 micrograms/ml protamine inhibited relative Na+ to Cl- permeability from 4.03 +/- 0.38 to 2.14 +/- 0.21 and from 3.75 +/- 0.37 to 1.36 +/- 0.09, respectively. Protamine markedly decreased the apparent transference number for Na+ but slightly increased the value for Cl-. Protamine also inhibited permeabilities for K+, Rb+, and Li+ relative to Cl-, indicating that the inhibitory effect of protamine was not confined to Na+ but was generalized to cations. Transmural cable analysis showed that 100 micrograms/ml protamine increased RT from 14.0 +/- 1.1 to 19.3 +/- 1.2 omega.cm2, with the effect being reversed by 30 U/ml heparin. Because the effect of protamine on RT was unaffected by ouabain in the bath, changes in RT may mainly represent those of the paracellular shunt resistance.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological heterogeneity of proximal convoluted tubules in Amphiuma kidney

The present study was designed to identify functional differences between dark (early to mid) and white (late) proximal tubule segments in Amphiuma kidney. The potential difference across the peritubular cell membrane (Vb), the luminal cell membrane (Va), and the epithelium (Vte) are not significantly different between dark and white segments. Cellular and luminal cable analysis reveals that the resistance of the cell membranes in parallel is lower in dark (28.6 +/- 3.2 k omega X cm) than in white segments (63.2 +/- 5.0 k omega X cm) in contrast to the transepithelial resistance, which is higher in dark (26.6 +/- 5.5 k omega X cm) than in white (3.5 +/- 0.7 k omega X cm) segments. A step-increase of peritubular potassium (from 2.5 +/- 12.5 mmol/liter) depolarizes Vb more in white (20.1 +/- 1.2 mV) than in dark (7.2 +/- 0.4 mV) segments, whereas addition of bicarbonate to peritubular perfusate hyperpolarizes Vb more in dark (-22.4 +/- 1.6 mV) than in white (-5.9 +/- 0.7 mV) segments. An increase of luminal potassium depolarizes Va more in dark (21.3 +/- 2.0 mV) than in white (9.3 +/- 1.9 mV) segments. Similarly luminal glucose depolarizes Va more in dark (10.7 +/- 1.2 mV) than in white segments (3.2 +/- 1.4 mV). Partial peritubular replacement of NaCl and reduction of peritubular chloride polarize Vte more in white (9.6 +/- 1.0 and 28.9 +/- 2.9 mV) than in dark segments (7.0 +/- 0.5 and 15.5 +/- 1.9 mV). In conclusion, compared with white segments, dark segments have lower cell membrane and higher shunt resistances, lower potassium and higher bicarbonate conductances of the peritubular cell membrane, and a higher capacity to reabsorb glucose. Paracellular shunt chloride conductance is relatively high in both segments.