Interaction of intracellular electrolytes and tubular transport

To disclose possible regulatory mechanisms, the potential difference across the peritubular cell membrane (PD_pt) and intracellular activities of sodium (Na(Formula presented.)), potassium (K(Formula presented.)), calcium (Ca(Formula presented.)), bicarbonate (HCO(Formula presented.)) and chloride (Cl(Formula presented.)) have been traced continuously during inhibition of Na⁺/K⁺-ATPase with ouabain. Within 31±4 min following application of ouabain, PD_pt decreases (from 57±2 mV) to half and K(Formula presented.) by 37.7±2.2 mmol/l (from 63.5±1.9 mmol/l), Na(Formula presented.) increases by 35.1±4.1 mmol/l (from 13.2±2.4 mmol/l), Ca(Formula presented.) by 0.17 ± 0.2 µmol/l (from 0.09 µmol/l), HCO(Formula presented.) by 3.0±1.1 mmol/l (from 15.3±2.0 mmol/l) and Cl(Formula presented.) by 6.2±1.0 mmol/l (from 14.4±1.6 mmol/l). Within the same time the luminal and peritubular cell membrane resistances increase 45±15% and 53±17%, respectively. The increase of the resistances is mainly due to a decrease of K⁺ conductance, which in turn mainly accounts for the depolarisation of PD_pt. Additional experiments demonstrate that the K⁺ conductance of the peritubular cell membrane is sensitive to the cell membrane potential difference and possibly linked to Na⁺/K⁺-ATPase activity. The decline of PD_pt probably accounts for intracellular alkalinisation which in turn reduces Na⁺/H⁺ exchange. Na⁺-coupled transport of glucose and phenylalanine decrease in linear proportion to PD_pt. The transport of these and probably of similar substances represents the main threat to electrolyte homeostasis of the cells.