Measurement of intracellular and luminal K+ concentrations in a Malpighian tubule (Formica). Estimate of basal and luminal electrochemical K+ gradients

Cellular and luminal K+ concentrations were measured in different bath K+ concentrations using double-barrelled K+-selective electrodes. The electrochemical gradient for K+ across the basal and the apical cell membrane was estimated. The experiments were performed in two Cl− concentrations. In control Ringer (41 mM K+ and 57 mM Cl−) cell K+ was 82 mM and luminal K+ was 119 mM. On lowering bath K+ to 4 mM or increasing it to 90 mM cell K+ and to a lesser extent luminal K+ followed: cell K+ changed to 29 and 90 mM and luminal K+ to 106 and 132 mM, respectively. In a 143 mM Cl− Ringer solution cellular and luminal K+ concentration showed a similar change with the basal K+ concentration. The basal membrane potential difference was almost equal to the equilibrium potential for K+ in all bath K+ and Cl− concentrations tested and the estimated electrochemical gradient for K+ across the basal membrane was very small (inward, zero, or even outward in low bath K+). It tended to become more inward as the bath K+ increased. The methods for estimating such a small gradient and the idea of passive K+ uptake through K+ channels across this barrier was critically evaluated. The large transepithelial electrochemical gradient against which K+ was transported was primarily situated in the apical membrane. This gradient decreased as the bath K+ was elevated (and fluid secretion accelerated): an increase in the bath K+ reduced both the electrical and concentration step to be overcome for K+ secretion across the apical membrane. A double role for the bath K+ in the regulation of K+ secretion is proposed.