CONTROL OF ION-TRANSPORT IN THE THYROID - PROSTAGLANDIN-E2 ACTIVATES CATION-TRANSPORT ON THE BASAL MEMBRANE OF CULTURED PORCINE THYROID-CELL MONOLAYERS

Confluent monolayers of cultured porcine thyroid cells transport fluid from the apical to the basal surface, forming circumscribed zones of detachment (domes) from the culture dish substrate. Stimulation of fluid transport by prostaglandin E2 (PGE2: 1 μmol/l) was associated with an increase in transepithelial potential (TEP). Intracellular potentials (equal to the potential difference across the apical membrane of the cell, E(apical)) and the TEP were measured in individual domes so that the potential difference across the basal membrane of the cell (E(basal)) could be calculated from the relationship TEP = E(apical) - E(basal). The PGE2-induced increase in TEP was associated with hyperpolarization of the basal membrane, accompanied by a slight depolarization of the apical membrane. Lines of best fit by least-squares regression showed E(apical) = -20.3 mV + 0.219 TEP (correlation coefficient r = 0.627, P < 0.001) and E(basal) = -20.3 mV - 0.781 TEP (r = 0.944; P < 0.001). Phenamil (1 μmol/l), a Na+ channel selective amiloride analogue, reduced the TEP from 13.25 ± 0.58 (S.E.M.; n = 56) to 2.39 ± 0.16 mV (n = 51; P < 0.001) and hyperpolarized the apical membrane potential from -20.7 ± 0.68 (n = 60) to -32.2 ± 0.83 mV (n = 105; P < 0.001). The response of the TEP to phenamil was immediate, and was promptly reversed on washing; in contrast, addition of 5-(N-ethyl-N-isopropyl)amiloride (20 μmol/l; selective for Na+/H+ antiporters) resulted in a slow depolarization over 30 min with a slow recovery after washout. Exposure of the cultures to media of pH 7.04 (compared with the normal pH of 7.34) resulted in a reduced response to PGE2, and a reduction in magnitude of E(basal). It was concluded that stimulation of ion transport by PGE2 in thyroid monolayers involves activation of cation transport across the basal membrane.