Na,K-ATPase and the development of Na+ transport in rat distal colon

Na,K-ATPase function was studied in order to evaluate the mechanism of increased colonic Na+ transport during early postnatal development. The maximum Na(+)-pumping activity that was represented by the equivalent short-circuit current after addition of nystatin (ISCN) did not change during postnatal life or after adrenalectomy performed in 16-day-old rats. ISCN was entirely inhibited by ouabain; the inhibitory constant was 0.1 mM in 10-day-old (young) and 0.4 mM in 90-day-old (adult) rats. The affinity of the Na,K pump for Na+ was higher in young (11 mM) than in adult animals (19 mM). The Na,K-ATPase activity (measured after unmasking of latent activity by treatment with sodium dodecylsulfate) increased during development and was also not influenced by adrenalectomy of 16-day-old rats. The inhibitory constant for ouabain (KI) was not changed during development (0.1-0.3 mM). Specific [3H]ouabain binding to isolated colonocytes increased during development (19 and 82 pmol/mg protein), the dissociation constant (KD) was 8 and 21 microM in young and adult rats, respectively. The Na+ turnover rate per single Na,K pump, which was calculated from ISCN and estimated density of binding sites per cm2 of tissue was 500 in adult and 6400 Na+/min.site in young rats. These data indicate that they very high Na+ transport during early postnatal life reflects an elevated turnover rate and increased affinity for Na+ of a single isoform of the Na,K pump. The development of Na+ extrusion across the basolateral membrane is not directly regulated by corticosteroids.