Cell Volume regulation in mammalian collecting duct: role of water channels

FORD PAULA; CHARA OSVALDO; RIVAROLA VALERIA; PARISI MARIO; CAPURRO CLAUDIA

San Luís, Argentina

Congreso; XXXV Reunión Anual de la SAFE; 2003

Sociedad Argentina de Farmacología Experimental (SAFE)

The mammalian collecting ducts (CD) play a central role in the final reguíation of urine volume and composition, as well as, salt and water balance. To accomplish this task it is crucial the presence of external osmotic gradients and arginine-vasopressin hormone modulation (AVP). Increases in external osmolality induces AVP liberation which acts increasing apical membrane water permeability, via cAMP cascade, through the insertion of specific water channels called aquaporin-2 (AQP2). in this case, water will move following the transepithelial osmotic, gradient through the apical AQP2 and basolateral resident AQP3 and AQP4. Recently it was proposed that in addition to the AVP-cAMP signaling cascade, a further pathway activated by elevated efective osmoíality is crucial for the expression of AQP2 and this response would be mediated via the tonicity-responsive elements. These osmotic water movements through epitheilal cells would result in changes, at least transients, ín cell volume. In order to maintain volume, cells require important volume-regulatory rnechanisms generally linked to ion transport activity and to water movements, Although the main ion transporters have been identified, both their activation and the mechanisms by which cells sense these changes are poorly understood. Moreover, until today no studies were performed in order lo identify water pathways during cell volume regulation and its consequences in transepithelial water transport. Consequently, the central aim of our work is to clarify the molecular mechanisms involved in osmotic water transport either at cellular and transepithelial level. Por this purpose we have used the cortical collecting duct cell line (RCCD1) wild-type and.transfected with AQPs. Our cellular studies showed that hyper-osmolic shocks induces cellular shrinkage.and activation of RVI mechanisms (regulatory volume increase) in both cells lines. However, AQPs expression clearly increascd cell volume recovery. During exposition to hypo-osmotíc shocks cell volume is rapidly increased in AQP, expressing cells but no regulatory volume decrease mechanisms were activated (RVD). Transepithelial water movement studies showed that in the absence of AQPs osmotic water movement is dependent of cellular metabolism and RVI mechanism would be crucial. Cells transfection with AQPs included an additional via to water pathway independently of cellular metabolism. Moreover CCD cells show an asymmetrical behavior to osmotic gradients: Hypertonic shocks induced Jv rectification independently of AQPs expression. Hypotonic shocks only induced Jv rectifícation in the presence of AQPs. We conclude that cell volume osmotic changes may initiate a signaling mechanism that would modulate apical or básolateral membrane permeability to protect cells from osmotic shocks.