MATURATIONAL RESPONSE OF COLONIC AND RENAL NA-ATPASE ACTIVITY TO K+ LOAD AND BETAMETHASONE IN PRETERM RATS(,K+)

Preterm human infants demonstrate high fecal K+/Na+ ratio that is inve rsely related to maturation. Renal and colonic basolateral membrane-lo cated Na+,K+-adenosine triphosphatase (Na+,K+-ATPase) governs K+ excre tion and is enhanced by both steroid administration and K+ loading. Ho wever, the response of premature kidney or colon to these stimuli is n ot well studied. We measured basal as well as stimulated levels of ren al and colonic Na+,K+-ATPase activity in response to betamethasone and K+ load given separately and betamethasone given after K+ load in pre mature, mature, and 4-day-old Sprague-Dawley rats. Although preterm ra ts showed a higher basal level in the kidneys (p < 0.004), an increase in enzyme activity in response to the stimuli was achieved only in th e colon (p < 0.001 for betamethasone, p < 0.0001 for K+ load, and p < 0.0007 for the combination). At term and at 4 days of postnatal age, h owever, the kidneys showed much higher levels than the colon in respon se to stimuli (p < 0.0001 for all) except for betamethasone at term. T he response of colonic tissue of preterm and term rats to betamethason e given 4 days after a K+ load was less marked than response to betame thasone or K+ load given separately (p < 0.001). The stimulated levels achieved in the preterm colon were higher than the basal preterm rena l values (p <0.001). The tissue Kf content increased in response to K load in only the preterm colon (p < 0.05). We have demonstrated a tis sue specificity in the development of Na+,K+-ATPase activity in K+ exc retory organs of rats. This difference in the ontogeny of Nat,Kt-ATPas e activity in the colonic and renal tissues might have implications in the electrolyte regulation of very low birth weight infants. Basal en zyme levels that were higher in the preterm kidney do not explain high fecal K+/Na+ ratios seen in the preterm neonates, whereas it appears that only the colon and not the kidneys could act as a vehicle for Kexcretion in the K+-loaded states in prematurity. Pretreatment with gl ucocorticoids might mitigate the tendency for hyperkalemia in such inf ants. Our findings suggest that development of Na+,K+-ATPase enzyme in K+ excretory organs is tissue specific, is ontogenically determined, and can be modulated.