The colonic H+,K+-ATPase functions as a Na+-dependent K+(NH4+)-ATPase in apical membranes from rat distal colon

Recent studies have suggested that the colonic H+,K+ ATPase (HK alpha(2)) c an secrete either Na+ or H+ in exchange for K+. If correct, this view would indicate that the transporter could function as either a Na+ or a H+ pump. To investigate this possibility a series of experiments was performed usin g apical membranes from rat colon which were enriched in colonic H+,K+-ATPa se protein. An antibody specific for HK alpha(2) was employed to determine whether HK alpha(2), functions under physiological conditions as a Na+-depe ndent or Na+-independent K+-ATPase in this same membrane fraction. K+-ATPas e activity was measured as [gamma-P-32]ATP hydrolysis, The Na+-dependent K-ATPase accounted for approximately 80% of overall K+-ATPase activity and w as characterized by insensitivity to Sch-28080 but partial sensitivity to o uabain. The Na+-independent K+-ATPase activity was insensitive to both Sch- 28080 and ouabain, Both types of K+-ATPase activity substituted NH4+ for K in a similar manner. Furthermore, our results demonstrate that when incuba ted with native distal colon membranes, the blocking antibody inhibited dra matically Na+-dependent K+-ATPase activity. Therefore, these data demonstra te that HK alpha(2), can function in native distal colon apical membranes a s a Na+-dependent K+-ATPase. Elucidation of the role of the pump as a trans porter of Na+ versus H+ or NH4+ versus K+ in vivo will require additional s tudies.