Red cell membrane Ca-ATPase in cystic fibrosis: increased activation by Na

In the present study, we compared the kinetics of activation by Na, of red cell membrane Ca-ATPase of cystic fibrosis (CF) patients and healthy volunteers (controls). Calmodulin (membrane-free hemolysate) was included in the assay medium to promote maximal Ca-ATPase activation. There were no significant differences between the red cell Ca-ATpase activities of the two groups, assayed either in the absence or in the presence of optimal concentrations of Na. There were also no significant differences between the apparent dissociation constants or Hill coefficients for activation of red cell Ca-ATPase by Na. On the other hand, the percent activation by Na of red cell Ca-ATPase activity of the CF patients was approximately 40% greater than that of the controls. The additional increment of Ca-ATPase activity due to the elevated percent activation was about 14% of the total red cell Ca-ATPase activity of the CF patients. Although this increment of Ca-ATPase activity is relatively small, the increased percent activation by Na does suggest an alteration in the enzyme's response to Na. At present we do not know whether or not this alteration applies to Ca-transport or if it is of any pathophysiological importance.