Cardiotonic Steroids Inhibit NHE3 in Renal Proximal Tubules by a Src Kinase Dependent Pathway

Abstract

Cardiotonic steroids (CE) are known as sodium‐pump inhibitors. The natriuretic property of these compounds is related to the inhibition of the α1 isoform of Na+/K+ATPase (NKA) in renal tubules. NHE3, the main route for Na+ and bicarbonate reabsorption in proximal tubules, depends on the Na+ gradient generated by the NKA. The present study aimed to investigate and compare the effects and mechanisms of 50 nM bufalin (BUF), telocinobufagenin (TBG), marinobufagenin (MBG) and ouabain (OUA) on NHE3 activity by the stationary renal microperfusion technique. By this method we are able to measure the H+ secretion in nephron segments through H+ sensitive microelectrodes. We observed that all compounds studied promoted a inhibition of proximal H+ secretion / bicarbonate reabsorption when compared to the control, being BUF the most potent compound (JHCO3‐,nmol.cm‐2.s‐1–control ethanol ‐2,11±0,16(10); BUF 1.04 ± 0.07 (8)*; TBG 0.077 ± 1.36(10)*; OUA 1.48 ± 0.10 (11)* and MBG 1.63 ± 0.08 (10)*. Our data also showed that this effect involved the inhibition of NHE3, since in the presence of NHE3 inhibitor (S3226), BUF did not promote additional inhibition. Furthermore, we demonstrated the involvement of NKA‐Src kinase pathway in the action signaling mechanism of these steroids on NHE3 activity, since in the presence of Src kinase specific inhibitor (SU6656), the inhibitory effect promoted by the CE was abolished.