Diffusional permeability (P) to sucrose (Psuc) and Na+ (PNa+) was determined in specimens of rabbit sternal parietal pericardium, which may be obtained without stripping. Specimens were mounted in an Ussing apparatus with 3H-labeled sucrose and 22Na+ in a luminal (L) or interstitial (I) chamber. Psuc was 2.16 +/- 0.44 for L-->I and 2.63 +/- 0.45 (SE) x 10(-5) cm/s for I-->L, i.e., approximately 10 times smaller than that previously obtained in stripped specimens of pleura despite the similarity of intercellular junctions in pericardium and pleural mesothelium of various species. These findings suggest that previous Psuc was overestimated because stripping damages the mesothelium. PNa+ (x10(-5) cm/s) was 7.07 +/- 0.71 for L-->I and 7.37 +/- 0.69 x 10(-5) cm/s for I-->L. Measurements were also done with phospholipids, which are adsorbed on the luminal side of mesothelium in vivo. With phospholipids in L, Psuc was 0.75 +/- 0.10 and 0.65 +/- 0.08 and PNa+ was 3.80 +/- 0.32 and 3.76 +/- 0.15 x 10(-5) cm/s for L-->I and I-->L, respectively, i. e., smaller than without phospholipids. With phospholipids in I (where they are not adsorbed), Psuc (2.33 +/- 0.42 x 10(-5) cm/s) and PNa+ (7.01 +/- 0.45 x 10(-5) cm/s) were similar to those values without phospholipids. Hence, adsorbed phospholipids decrease P of mesothelium. If the mesothelium were scraped away from the specimen, Psuc of the connective tissue would be 13.2 +/- 0.76 x 10(-5) cm/s. Psuc of the mesothelium, computed from Psuc of the unscraped and scraped specimens, corrected for the effect of unstirred layers (2. 54 and 19.4 x 10(-5) cm/s, respectively), was 2.92 and 0.74 x 10(-5) cm/s without and with phospholipids, respectively. Hence, most of the resistance to diffusion of the pericardium is provided by the mesothelium.

Bicarbonate presence in the bathing media doubles Na+ and fluid transepithelial transport and in parallel significantly increases Na+ and Cl- intracellular concentrations and contents, decreases K+ cell concentration without changing its amount, and causes a large cell swelling. Na+ and Cl- lumen-to-cell influxes are significantly enhanced, Na+ more so than Cl-. The stimulation does not raise any immediate change in luminal membrane potential and cannot be due to a HCO3(-)-ATPase in the brush border. The stimulation goes together with a large increase in a Na+-dependent H+ secretion into the lumen. All of these data suggests that HCO3- both activates Na+--Cl- cotransport and H+--Na+ countertransport at the luminal barrier. Thiocyanate inhibits Na+ and fluid transepithelial transport without affecting H+ secretion and HCO3(-)-dependent Na+ influx. It reduces Na+ and Cl- conentrations and contents, increases the same parameters for K+, causes a cell shrinking, and abolishes the lumen-to-cell Cl- influx. It enters the cell and is accumulated in the cytoplasm with a process which is Na+-dependent and HCO3(-)-activated. Thus SCN- is likely to compete for the Cl- site on the cotransport carrier and to be slowly transferred by the cotransport system itself.

In the rabbit gallbladder epithelium, hydrochlorothiazide (HCTZ) was shown to inhibit the transepithelial NaCl transport and the apical Na(+)-Cl- symport, to depolarize the apical membrane potential and to enhance the cell-to-lumen Cl- backflux (radiochemically measured), this increase being SITS-sensitive. To better investigate the causes of the depolarization and the Cl- backflux increase, cells were punctured with conventional microelectrodes on the luminal side (incubation in bicarbonate-free saline at 27 degrees C) and the apical membrane potential (Vm) was studied either with prolonged single impalements or with a set of short multiple impalements. The maximal depolarization was of 3-4 mV and was reached with 2.5 x 10(-4) M HCTZ. It was significantly enhanced by reducing luminal Cl- concentration to 30 mM; it was abolished by SCN-, furosemide, SITS; it was insensitive to DPC. SITS converted the depolarization into a hyperpolarization of about 4 mV; this latter was apamin, nifedipine and verapamil sensitive. It was concluded that HCTZ concomitantly opens apical Cl- and (probably) Ca2+ conductances and, indirectly, a Ca(2+)-sensitive, apamin inhibitable K+ conductance: since the intracellular Cl- activity is maintained above the value predicted at the electrochemical equilibrium, the opening of the apical Cl- conductance depolarizes Vm and enhances Cl- backflux. In the presence of apamin or verapamil, to avoid the hyperpolarizing effects due to HCTZ, the depolarization elicited by this drug was fully developed (7-10 mV) and proved to be Ca2+ insensitive. On this basis and measuring the transepithelial resistance and the apical/basolateral resistance ratio, the Cl- conductance opened by HCTZ has been estimated and the Cl- backflux increase calculate: it proved to be in the order of that observed radiochemically. The importance of this Cl- leak to the lumen in the overall inhibition of the transepithelial NaCl transport by HCTZ has been evaluated.

1. The effects of low amiloride concentrations (less than 2.5 x 10(-5) M) on the apical membrane (Vm) and transepithelial (Vms) potential differences, transepithelial resistance (Rep) and apical/basolateral membrane resistance ratio (Rm/Rs) have been determined in the gall-bladders of the rabbit and guinea-pig. 2. Vm was hyperpolarized, Vms decreased towards negative values, Rm/Rs increased, but Rep remained unchanged. 3. K+ selectivity of the apical membrane was also checked by increasing luminal K+ concentration from 5.9 to 29.4 mM; the corresponding change in the apical electromotive force was much higher in the rabbit than in the guinea-pig. 4. Pre-incubations with Cl--and HCO3--free salines did not modify K+ selectivity in the rabbit, but nearly doubled it in the guinea-pig. 5. Pre-incubations with control solutions containing 10(-5) M-amiloride in the lumen converted the apical membrane of rabbit gall-bladder into a perfectly K+-perm-selective membrane; the same results were obtained in the guinea-pig only on bathing the tissue with Cl-- and HCO3--free salines together with 10(-5) M-amiloride. 6. It is suggested that in the apical membrane of rabbit gall-bladder K+ and Na+ conductances exist, whereas in guinea-pig gall-bladder both K+, Na+ and anion (Cl- and/or HCO3-) conductances are present; in both species conductive Na+ pathways are inhibited by amiloride. .