METABOLIC-ACIDOSIS STIMULATES CARBONIC-ANHYDRASE ACTIVITY IN RABBIT PROXIMAL TUBULE AND MEDULLARY COLLECTING DUCT

Both membrane-bound carbonic anhydrase (CA) (isozyme type IV) and cyto solic CA (type II) activities enhance urinary acidification. We have p reviously shown that chronic metabolic acidosis (CMA) accomplished by NH4Cl loading with food restriction induces soluble CA activity in rab bit renal cortical homogenates. The present study was designed to asse ss the effect of CMA on the activity of CA isozymes in cortical and ou ter medullary homogenates, as well as in major proton-secreting segmen ts of the kidney. Segments were microdissected from proximal convolute d tubules (PCT), proximal straight tubules, cortical collecting ducts, and outer medullary collecting ducts (OMCD). Total CA activity was me asured by a colorimetric endpoint method, and CA TV activity was asses sed from the sodium dodecyl sulfate-resistant hydratase activity. In c ontrols, CA IV activity accounted for 3% of total CA activity in tissu e homogenates. CMA induced a threefold increase in CA IV activity in c ortical homogenates, in the absence of renal or tubular hypertrophy. I n the PCT, CMA induced a 78% increase in total CA activity, which comp rised a 178% increase in CA TV activity, and a 58% increase in CA II a ctivity. In the OMCD, CMA induced a 53% increase in total CA (probably CA II) activity. We conclude that CMA induces CA activity in the PCT (CA II and CA TV) and the OMCD (most likely CA II) of adult rabbit kid neys. The induction of CA activity accompanies the increase in urinary acidification observed in CMA.