The influence of thyroid state on glycerol-induced hyperpolarization of the cell membranes in isolated, perfused rat liver

Abstract

Intracellular electric potentials were measured in isolated livers from starved rats of different thyroid states. The potentials were identical in hypo-, eu- and hyperthyroid livers being −28.9, −29.1, and −29.8 mV, respectively. During a 15 min period with 2 mM glycerol in the perfusate, the intracellular negativity rose to approximately −36 mV in hypothyroid as well as in euthyroid livers, but no significant change occurred in hyperthyroid livers although these showed the highest rate of glycerol uptake. In the same period, hypo- and euthyroid livers accumulated respectively 9.0 and 4.3 μmol/g of l-glycerol 3-phosphate. The accumulation of this substance was only 0.6 μmol/g in hyperthyroid livers. The P i trapped in l-glycerol 3-phosphate balanced with the sum of P i taken up from the medium and P i made available from a decrease in the concentrations of P i and ATP in biopsies. The uptake of P i was closely accompanied by an uptake of K + which was 5.8 μequiv./g in hypothyroid livers, 3.3 μequiv./g in euthyroid livers, and 0.3 μequiv./g in hyperthyroid livers. The results seem to exclude that the glycerol-induced hyperpolarization of liver cells is an effect related to the transport of glycerol across the cell membranes. A correlation with metabolic events distinguished by their small prominence or absence in the hyperthyroid state appears more likely. However, the exact mechanism remains to be clarified since in the present experiments only a minor part of the hyperpolarization could be accounted for by the uptake of K + connected with l-glycerol 3-phosphate accumulation.