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1 Division of Pediatrics Nephrology, Department of Pediatrics, Mount Sinai School of Medicine, New York, N.Y., USA
2 Division of Nephrology and Hypertension, Department of Medicine, Winthrop University Hospital, Mineola, N.Y., USA
3 Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
* To whom correspondence should be addressed. E-mail: lisa.satlin{at}mssm.edu.
High urinary flow rates stimulate K secretion in the fully differentiated but not neonatal or weanling rabbit CCD. Both small conductance secretory K (SK) and high-conductance Ca2+/stretch-activated maxi-K channels have been identified in the apical membrane of the mature CCD by patch clamp analysis. We have reported that flow-stimulated net K secretion in the adult rabbit CCD is (1) blocked by TEA and charybdotoxin, inhibitors of intermediate- and high-conductance (maxi-K) Ca2+-activated K channels, and (2) associated with increases in net Na absorption and intracellular Ca2+ concentration ([Ca2+]i). The present study examined whether the absence of flow-stimulated K secretion early in life is due to a (1) limited flow-induced rise in net Na absorption and/or [Ca2+]i, and/or (2) paucity of apical maxi-K channels. A ~6-fold increase in tubular fluid flow rate in CCDs isolated from 4-wk-old rabbits and microperfused in vitro led to an increase in net Na absorption and [Ca2+]i, similar in magnitude to the response observed in 6-wk-old tubules, but failed to generate an increase in net K secretion. By 5 wks of age, there was a small, but significant flow-stimulated rise in net K secretion that increased further by 6 wks of life. Luminal perfusion with iberiotoxin blocked the flow stimulation of net K secretion in the adult CCD, confirming the identity of the maxi-K channel in this response. Maxi-K channel 
-subunit message was consistently detected in single CCDs from animals 
4 wks of age by RT-PCR. Indirect immunofluorescence microscopy using antibodies directed against the -subunit revealed apical labeling of intercalated cells in cryosections from animals 5 wks of age; principal cell labeling was generally intracellular and punctate. We speculate that the postnatal appearance of flow-dependent K secretion is determined by the transcriptional/translational regulation of expression of maxi-K channels. Furthermore, our studies suggest a novel function for intercalated cells in mediating flow-stimulated K secretion.
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