POSTNATAL MATURATION OF TISSUE KALLIKREIN-PRODUCING CELLS (CONNECTINGTUBULE CELLS) IN THE RAT-KIDNEY - A MORPHOMETRIC AND IMMUNOHISTOCHEMICAL STUDY

The mature, fully differentiated connecting tubule (CNT) cell plays an important role in the regulation of serum potassium levels and synthe sizes the enzyme tissue kallikrein, a main component of a renal vasoac tive system, the kallikrein-kinin system. To characterize the growth o f CNT cells (tissue kallikrein-producing cells), we studied the rat ki dney at three different time points of postnatal development: at day 5 , day 15, and day 30. The CNT cells were identified on tissue sections by a standardized immunohistochemical procedure. The tissue kallikrei n content was determined by radioimmunoassay and the activity of the e nzyme in kidney homogenates was measured using a selective synthetic s ubstrate. The number of immunolabeled CNT and CNT cells per cortex are a gradually increased from day 5 to day 30. A similar rise in the cont ent and activity of tissue kallikrein was observed when the enzyme lev els were determined by radioimmunoassay or by the enzymatic method. In addition, the morphometric analysis showed that the distal end of CNT had larger cells that displayed a more intense tissue kallikrein stai ning than those present in the proximal end, suggesting that the postn atal development of CNT is induced from its juxtamedullary portion. Ou r results show that tissue kallikrein expression is very low in the ne wborn rat, increasing gradually with age to reach adult levels at day 30. This finding, together with the morphometric data, suggests immatu rity of CNT cells in newborn rats, a fact that could contribute to exp laining the high serum potassium levels reported at this stage. In add ition, the contrasting behavior of kallikrein and renin in the postnat al development (kallikrein increasing and renin decreasing) could expl ain the gradual decrease in renal vascular resistance and increase in renal blood flow observed after birth.