And function and plasma corticosteroids and are clearly in a hypoadrenal, sodium- and waterwasting state. In the only other study directly investigating ENaC activity with patch-clamp analysis in Adx animals, no significant ENaC activity was observed in cortical collecting tubules from Adx rats maintained with a get Flagecidin normal sodium diet (12). This experimental condition, in general, is similar to that used in the present studies. There are potentially important differences, however. For instance, rodents were maintained with some glucocorticoid replacement in the earlier study but not in the present study. As discussed below, glucocorticoids can influence the circulating concentration of AVP. Interestingly, a high K+ diet increased ENaC activity in Adx and control rats in the absence of significant changes in plasma aldosterone levels in the earlier study (12). It is not clear yet whether these discrepancies represent true incongruence or whether they reflect rather slight variations in experimental conditions or differences in sensitivity. Our rationale is that we consistently find significant ENaC activity, albeit at lower levels compared with a sodium deficient diet, in ASDN isolated from rats and mice maintained with regular-sodium (0.32 [Na+]) and high-sodium (2 [Na+]) diets (14, 20, 21). Aldosterone should be low (extremely so in the latter case) with these feeding regimens. In contrast, these others find no significant ENaC activity in cortical collecting tubules isolated from rodents maintained with normal and high-sodium diets (11, 30). We interpret our results as showing that the activity of ENaC in the ASDN is high in the presence of aldosterone and low, but significant, in the absence of this hormone in normal animals. Nevertheless, it is accepted that in ASDN from normal animals, ENaC activity is related in a positive manner with aldosterone levels (11, 12, 14, 21). Adx mice, which are not normal (see below), represent an exception, then, where ENaC activity is high in the absence of aldosterone and other adrenal steroids. This exception demonstrates that ENaC can be active in the absence of aldosterone and, thus, that aldosterone is not necessary for the activity of this channel in the ASDN. This interpretation is consistent with what has been reported for ENaC activity in neonatal MR-null mice (13). These mice do not survive long after the first week of life without sodium supplementation due to pathological renal sodium excretion. During this critical phase of early life, renal sodium loss cannot be compensated by nursing pups because of the low sodium and water content of mother’s milk. However, neonatal MR-null mice retain residual, but significant, ENaC activity–24 of normal–as extrapolated from amiloride-sensitive fractional Na+ excretion and transport across isolated, perfused collecting ducts. Knockout of aldosterone synthase (AS) agrees with findings from MR-null mice (25, 32). AS-null mice have pronounced renal sodium and water wasting, which cannot be compensated during the critical neonatal period. Sodium and water wasting results in dehydration, failure to thrive, and death of 1/3 of ASnull mice in the first weeks of life. Sodium restriction exacerbates renal salt and water wasting in both AS- and MR-null mice compared with control animals, which have appropriate feedback TGR-1202 biological activity regulation by RAAS of ENaC and other mechanism for decreasing renal Na+ excretion. These observations are reminiscent of human infants carryi.And function and plasma corticosteroids and are clearly in a hypoadrenal, sodium- and waterwasting state. In the only other study directly investigating ENaC activity with patch-clamp analysis in Adx animals, no significant ENaC activity was observed in cortical collecting tubules from Adx rats maintained with a normal sodium diet (12). This experimental condition, in general, is similar to that used in the present studies. There are potentially important differences, however. For instance, rodents were maintained with some glucocorticoid replacement in the earlier study but not in the present study. As discussed below, glucocorticoids can influence the circulating concentration of AVP. Interestingly, a high K+ diet increased ENaC activity in Adx and control rats in the absence of significant changes in plasma aldosterone levels in the earlier study (12). It is not clear yet whether these discrepancies represent true incongruence or whether they reflect rather slight variations in experimental conditions or differences in sensitivity. Our rationale is that we consistently find significant ENaC activity, albeit at lower levels compared with a sodium deficient diet, in ASDN isolated from rats and mice maintained with regular-sodium (0.32 [Na+]) and high-sodium (2 [Na+]) diets (14, 20, 21). Aldosterone should be low (extremely so in the latter case) with these feeding regimens. In contrast, these others find no significant ENaC activity in cortical collecting tubules isolated from rodents maintained with normal and high-sodium diets (11, 30). We interpret our results as showing that the activity of ENaC in the ASDN is high in the presence of aldosterone and low, but significant, in the absence of this hormone in normal animals. Nevertheless, it is accepted that in ASDN from normal animals, ENaC activity is related in a positive manner with aldosterone levels (11, 12, 14, 21). Adx mice, which are not normal (see below), represent an exception, then, where ENaC activity is high in the absence of aldosterone and other adrenal steroids. This exception demonstrates that ENaC can be active in the absence of aldosterone and, thus, that aldosterone is not necessary for the activity of this channel in the ASDN. This interpretation is consistent with what has been reported for ENaC activity in neonatal MR-null mice (13). These mice do not survive long after the first week of life without sodium supplementation due to pathological renal sodium excretion. During this critical phase of early life, renal sodium loss cannot be compensated by nursing pups because of the low sodium and water content of mother’s milk. However, neonatal MR-null mice retain residual, but significant, ENaC activity–24 of normal–as extrapolated from amiloride-sensitive fractional Na+ excretion and transport across isolated, perfused collecting ducts. Knockout of aldosterone synthase (AS) agrees with findings from MR-null mice (25, 32). AS-null mice have pronounced renal sodium and water wasting, which cannot be compensated during the critical neonatal period. Sodium and water wasting results in dehydration, failure to thrive, and death of 1/3 of ASnull mice in the first weeks of life. Sodium restriction exacerbates renal salt and water wasting in both AS- and MR-null mice compared with control animals, which have appropriate feedback regulation by RAAS of ENaC and other mechanism for decreasing renal Na+ excretion. These observations are reminiscent of human infants carryi.