MEASUREMENT OF INTRACELLULAR AND LUMINAL K- ESTIMATE OF BASAL AND LUMINAL ELECTROCHEMICAL K+ GRADIENTS( CONCENTRATIONS IN A MALPIGHIAN TUBULE (FORMICA) )

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- conc entrations. 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 increasin g 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, respecti vely. In a 143 mM Cl- Ringer solution cellular and luminal K+ concentr ation showed a similar change with the basal K+ concentration. The bas al membrane potential difference was almost equal to the equilibrium p otential for K+ in all bath K+ and Cl- concentrations tested and the e stimated 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 estima ting such a small gradient and the idea of passive K+ uptake through K + channels across this barrier was critically evaluated. The large tra nsepithelial electrochemical gradient against which K+ was transported was primarily situated in the apical membrane. This gradient decrease d as the bath K+ was elevated (and fluid secretion accelerated): an in crease in the bath K+ reduced both the electrical and concentration st ep to be overcome for K+ secretion across the apical membrane. A doubl e role for the bath K+ in the regulation of K+ secretion is proposed.