Flow dependence of chloride transport in rat S1 proximal tubules

Abstract

These studies examined whether the luminal flow dependency of chloride absorption in the S1 proximal tubule during glomerulotubular balance was due to change in active and/or passive transport of chloride. Using in vivo microperfusion in the Munich-Wistar rat and an essentially pure sodium chloride perfusate (devoid of bicarbonate and organic solutes), we found that an increase in luminal perfusion rate from 30 to 45 nl/min caused stimulation of total chloride absorption (active plus passive) by 87 peq.mm-1.min-1 (632 +/- 17 to 719 +/- 11, P < 0.001). When cyanide was added to this perfusate to eliminate active transport, the flow-induced change in passive transport was 58 peq.mm-1.min-1 (479 +/- 9 to 537 +/- 11, P < 0.001). The cyanide-inhibitable active transport component was therefore 29 peq.mm-1.min-1. With elimination of the transepithelial chloride gradient and, hence, passive transport by isethionate substitution, active transport increased by 63 peq.mm-1.min-1 (121 +/- 4 to 184 +/- 7, P < 0.001) as flow rate rose from 30 to 45nl/min. Removal of organic solutes from a glomerular ultrafiltrate-like perfusate had a minimal effect on flow-induced change in chloride transport (190 vs. 207 peq.mm-1.min-1). In conclusion, flow-dependent active and passive chloride transport in the S1 proximal tubule may both contribute to normal glomerulotubular balance.