Aldosterone increases the maximal turnover rate of the sodium pump

These experiments were performed to determine whether aldosterone-dependent effects in apical and basolateral membranes could be temporarily dissociated and whether aldosterone increases the maximal capacity, or maximum turnover rate of the sodium pump. Tissue from rat distal colon exposed to the action of high plasma levels of aldosterone for 4 h, 24 h and 7-10 days was compared to control tissue in a modified Ussing chamber, before and after addition of nystatin to the mucosal solution to remove the apical barrier to the cell entry of sodium. The maximum turnover rate of the sodium pump was represented by the equivalent short circuit current, Isc, after the addition of nystatin. After 4 h of aldosterone basal Isc increased 2.6-fold above control (43 +/- 34 microA.cm-2, p less than 0.05) and transepithelial PD, VT, increased 2-fold over control (-7.0 +/- 5.5 mV, lumen negative, p less than 0.05). Administration of aldosterone for 24 h caused further marked increases in Isc (11-fold) and a fall in RT to 50% of control. Similar changes were observed after 7-10 days on low sodium diet, and at all time intervals the changes were completely inhibited by amiloride (10(-4)M). Although aldosterone stimulated Isc within 4 h, there was no further increase in Isc in the presence of nystatin during the same time period compared to the control post-nystatin Isc of 419 +/- 79 microA.cm-2. However, after 24 h aldosterone caused approximately a 40% increase in the maximal turnover rate of the sodium pump, which persisted for 7-10 days.(ABSTRACT TRUNCATED AT 250 WORDS)