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 (I sc N ) did not change during postnatal life or after adrenalectomy performed in 16-day-old rats.I sc N was entirely inhibited by ouabain; the inhibitory constant was 0.1mm in 10-day-old (young) and 0.4mm in 90-day-old (adult) rats. The affinity of the Na, K pump for Na+ was higher in young (11mm) than in adult animals (19mm). 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 (K I ) was not changed during development (0.1–0.3mm). Specific [3H]ouabain binding to isolated colonocytes increased during development (19 and 82 pmol/mg protein), the dissociation constant (K D ) was 8 and 21 μm in young and adult rats, respectively. The Na+ turnover rate per single Na, K pump, which was calculated fromI sc N 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 the 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.