Isolated mucosal sheets of guinea pig distal colon were studied in vitro in Ussing-type chambers, using a computer-controlled voltage clamp. A conductance of 8-12 mS/cm2 and spontaneous variations of the potential difference (Vt, -4 to +6 mV) and the short-circuit current (Isc, -1.6 to +1.5 mu eq X cm-2 X h-1) were observed. With use of a green feed diet these variations could be entirely attributed to the rate of Na transport. Unidirectional Na and Cl fluxes were measured, and for Na, K, and Cl transport the individual conductances and directions were estimated from the changes in Vt and Isc, using the appropriate blockers amiloride, barium, and piretanide. The sum of the electrogenic Na, K, and Cl transport determines the spontaneous electrical behavior of this epithelium. Na transport was further characterized with transepithelial and transapical current-voltage relations. Apical Na entry occurred by diffusion, intracellular Na activity was 12 mM, and apical Na permeability was calculated as 0.02 cm/h. This study represents the first in vitro investigation of electrogenic transport in this epithelium and shows that it closely resembles transport mechanisms found in rabbit colon.

The effects of pre-treatment in vivo with aldosterone and dexamethasone were investigated on rabbit distal colon. Apical Na-permeability and net sodium transport were measured in vitro. In this epithelium, Na-transport is entirely electrogenic. It can therefore be measured electrically as the fraction of short circuit current which is blockable by amiloride. The epithelia were studied in an Ussing chamber and the electrical values recorded by a computerized digital voltage clamp. Transepithelial parameters, and the transapical membrane parameters (in preparations depolarized from the serosal side) were investigated after treatment with the two hormones. Under transepithelial conditions, aldosterone and dexamethasone stimulated the short circuit current (Isc) from control (17.4 microA/cm2) to a similar degree (86.6 and 93.8 microA/cm2). However, whereas aldosterone did not alter the transepithelial resistance (RT) significantly, dexamethasone reduced RT from 357 to 167 omega X cm2. The stimulation of the potential difference (VT) under control condition (6.6 mV) was therefore significantly different between aldosterone (28.7 mV) and dexamethasone (16 mV). Mucosal amiloride (0.1 mM) inhibited Isc and VT completely under all conditions. Steady state current-voltage relations were obtained by voltage clamping the tissues in "staircase" increments before and after mucosal treatment with amiloride. As measured by the difference between these two states, Na-currents were calculated both for the transepithelial and the transapical condition. Intracellular Na-activity and apical Na-permeability were then calculated by the Nernst and Goldman-Hodgkin-Katz equations. These values were found to be increased after treatment with both hormones. Dexamethasone was a more potent stimulator of both values.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo and in vitro studies showed that electrogenic sodium transport in rabbit distal colon is modulated by aldosterone. It varies in a circadian rhythm; the external synchronizer is the light-dark cycle. The site of regulation was found to be in the apical membrane of colonic epithelial cells, in which the number of conducting sodium-channels is increased by aldosterone.

The influence of adrenal steroids on sodium transport in hen coprodeum was investigated by electrophysiological methods. Laying hens were maintained on low-NaCl diet (LS), or on high-NaCl diet (HS). HS hens were pretreated with aldosterone (128 micrograms/kg) or dexamethasone (1 mg/kg) before experiment. A group of LS hens received spironolactone (70 or 160 mg/kg, for three days). The effects of these dietary and hormonal manipulations on the amiloride-sensitive part of the short-circuit current were examined. This part is in excellent agreement with the net Na flux, and therefore a direct electrical measurement for Na transport. After depolarizing the basolateral membrane potential with a high K concentration, the apical Na permeability and the intracellular Na activity were investigated by current-voltage relations for the different experimental conditions. Plasma aldosterone concentrations (PA) were low in HS hens, dexamethasone-treated HS hens and spironolactone-treated LS hens (less than 70 pM). In contrast LS hens and aldosterone-treated HS hens had a PA concentration of 596 +/- 70 and 583 +/- 172 pM, respectively. LS diet (chronic stimulation) had the largest stimulatory effect on Na transport and apical Na permeability. Hormone-treated animals had three- to fourfold lower values. Spironolactone supply in LS hens decreased Na transport and apical Na permeability about 50%. The results provide evidence that both mineralo- and gluco-corticoids stimulate Na transport in this tissue by increasing the apical Na permeability. Quantitative differences between acute and chronic stimulation reveal a secondary slower adaptation in apical membrane properties.

The role of renal prostaglandins (PGE2 and PGF2\g=a\) and kallikrein in the renal escape from excess mineralocorticoid has been evaluated in 10 normal volunteers on a high sodium diet. Escape from 9-\ g=a\ \ x=r eq-\ fluorohydrocortisone (9-\g=a\-FF)administered for 12 days (0.6 mg daily) occurred within 4\p=n-\5 days when 345 \ m=+-\50 mmol sodium had accumulated. Indomethacin, a cyclooxygenase inhibitor, was then administered for 3 days while maintaining 9-\g=a\-FF.This was followed by a further cumulative gain of 105 \m=+-\28 mmol sodium after which escape resumed. Potassium balance remained negative