K depletion and NH4Cl acidosis increase apical insertion of studded membrane in type A intercalated cells

To examine the effects of K depletion and metabolic acidosis on the prevalence and distribution of H+-adenosinetriphosphatase (H+-ATPase)-related studs at the luminal plasma membrane of A-type intercalated cells (A-ICs), we conducted a quantitative electron microscopical study on the cortical collecting ducts (CCDs) of control, NH4Cl-loaded, and K-depleted rats. The percentage of A-ICs was slightly increased in the K-depleted but not in the acidotic rats. A-ICs were considered ''active'' when they presented a semicontinuous row of 9- to 10-nm studs at the cytoplasmic face of the apical membrane and ''inactive'' when all of the apical membrane was devoid of studs. The percentage of active A-ICs was greatly increased in acidotic (87.2%) and K-depleted (79.3%) rats compared with controls (41.6%). These results give a quantitative expression to the general view that acidosis elicits insertion of studded membrane in the apical plasma membrane of A-ICs. Furthermore, we show, for the first time, that an increase in the membrane insertion of H+-ATPase is also part of the response to K depletion.