Potassium transport in the connecting tubule

The connecting tubule (CNT) of the rabbit is a distinct anatomic and functional segment, but little is known about its role in potassium secretion. We have examined potassium secretion in the CNT using the technique of in vitro microperfusion. Chemical flux was determined using an ion-sensitive microelectrode and rubidium flux was used to estimate the unidirectional lumen-to-bath potassium flux. In control tubules, the transepithelial potential difference was -4.1 +/- 1.1 mV and there was a net potassium flux (JK) of -17.94 +/- 4. 53 pmol mm-1 min-1 that was stable for up to 3 h. The lumen-to-bath 86Rb flux was 2.78 +/- 0.38 pmol mm-1 min-1. This value represents 12% of the calculated bath-to-lumen flux of 22.32 +/- 2.51 pmol mm-1 min-1. In comparing potassium transport in the CNT to that of the cortical collecting tubule, the net flux and unidirectional fluxes were significantly greater in the CNT. In the CNT ouabain 0.01 mM decreased JK from -25.25 +/- 2.81 to -2.07 +/- 1.94 pmol mm-1 min-1, a value not significantly different from zero. Potassium flux in the CNT was flow-dependent. As the perfusion rate was increased from 5.0 +/- 0.5 to 13.9 +/- 1.1 nl/min, net JK increased from -20.65 +/- 5.47 to -44.29 +/- 13.32 pmol mm-1 min-1. When the perfusion rate was further increased to 26.1 +/- 1.9 nl/min, the net JK increased to -57.28 +/- 12.38 pmol mm-1 min-1, a value significantly greater than that obtained at a nominal perfusion rate of 5 nl/min. Both the lumen-to-bath and bath-to-lumen potassium fluxes increased as the perfusion rate increased. These studies are consistent with an important role of the CNT in renal potassium excretion.