Inhibitory effects of amiloride and its analogues on prostaglandin E2-stimulated fluid transport by cultured porcine thyroid cells: evidence for apical membrane Na+ channels

Activation of sodium transport mediates regulation of thyroid follicle volume in response to hypotonic media

The thyroid epithelium possesses a bidirectional fluid transport system capable of absorbing Na+ and secreting Cl-. In the present studies we have examined its possible role in the regulation of thyroid follicular size. When exposed to hypotonic media (200 mosM) cultured porcine thyroid follicles first swelled and then displayed a regulatory volume decrease (RVD) over 60 min. This was associated with a transient depolarization of the transepithelial potential difference (TEP) and subsequent hyperpolarization with a time course similar to RVD. Phenamil (1 microM), an antagonist of epithelial Na+ channels, did not affect initial swelling but prevented the subsequent follicular RVD. Phenamil abolished hyperpolarization of TEP, but the loop diuretic bumetanide, which inhibits Cl- secretion in thyroid cells, did not prevent it. Exposure to hypotonic medium produced a slow hyperpolarization of the intracellular potential (basolateral membrane potential) consistent with an increase in basolateral membrane K+ conductance. Ba2+ and quinidine, which are known to inhibit K+ channels in epithelia, prevented RVD. Addition of the K+ ionophore valinomycin (1 microM) caused follicle shrinkage that was prevented by phenamil (1 microM). We conclude that cultured follicles respond to hypotonically induced stretch by activating outwardly directed Na+ transport through a mechanism which involves change in the basolateral K+ conductance. This response would be characteristic of a system that controlled follicle volume. However, it is not clear from these studies whether the cells responded primarily to the increase in follicle volume or to the change in cell volume that is expected to accompany hypotonic challenge.

Bidirectional ion transport in thyroid: secretion of anions by monolayer cultures that absorb sodium

Cultured porcine thyroid cell monolayers transport Na+ in an apical-to-basal direction, resulting in the development of a basal-positive transepithelial potential difference (TEP) and the formation of domes (fluid-filled elevations of the cell layer above the culture dish substrate). Stimulation by prostaglandin E2 (PGE2) increases the magnitude of the TEP, the short-circuit current (Isc) measured in Transwell Ussing chambers, and the height of domes in cultures grown on impermeable substrates. A phenamil-resistant, PGE2-stimulated component of the Isc in Transwells and of the TEP in monolayers in conventional culture dishes was inhibitable by bumetanide, a diuretic drug that blocks NaKCl2 symporters, mediating active transport of Cl-. The rate of decrease in height of domes in cultures after addition of phenamil, presumably indicative of transport of fluid in a basal-to-apical direction, was also reduced by bumetanide. Studies with Transwells in Cl(-)-free, HCO(3-)-free or Cl(-)- and HCO(3-)-free media indicated that thyroid cells transported HCO3- as well as Cl- in a basal-to-apical direction. It was concluded that the thyroid epithelium is both sodium absorbing and anion secreting.

Regulation of thyroid follicular volume by bidirectional transepithelial ion transport

Previous studies have shown that thyroid cells in monolayer culture exhibit bidirectional ion transport comprising apical-to-basal amiloride-sensitive Na+ transport and oppositely directed bumetanide-sensitive Cl- transport. We have now investigated the role of ion transport in the regulation of thyroid follicular size using follicular primary porcine thyroid cell cultures. Bumetanide (10 microM) added at the beginning of culture inhibited the formation of follicular lumina and caused a fall in follicle height when added to 3-day-old cultures. In contrast, phenamil (1 microM; an amiloride analog) increased follicle size both in freshly isolated and 3-day-old cultures. The effect of bumetanide was prevented by the prior addition of phenamil. Micropuncture studies showed that follicles had a lumen-negative, basal-positive transepithelial potential difference which was progressively reduced in magnitude by the serial addition of bumetanide (10 microM) and phenamil (1 microM). We conclude that thyroid follicles possess a bidirectional ion transport system which transports Na+ in an apical-to-basal direction and Cl- in the opposite direction. The balance between these two processes determines net solute flux and hence follicular size. A physiological role of ion transport in the thyroid may be to regulate follicular volume suggesting that abnormalities of ion transport may be responsible for disorders of follicular size.

Amiloride analogues induce responses in isolated rat cardiovascular tissues by inhibition of Na+/Ca2+ exchange

The role of inhibition of Na+/Ca2+ exchange in the positive inotropic, negative chronotropic and vasorelaxant responses to amiloride and some of its analogues was investigated in isolated cardiovascular tissues from female Wistar rats. The compounds tested were amiloride, 5-(N-ethyl-N-isopropyl)amiloride (EIPA, a potent inhibitor of Na+/H+ exchange), phenamil and 2',4'-dimethylbenzamil (DMB), both potent Na+ channel inhibitors with activity against Na+/Ca2+ exchange, and 5-(N-4-chlorobenzyl)-2',4'-dimethylbenzamil (CBDMB), a potent inhibitor of Na+/Ca2+ exchange with reduced activity against Na+ channels compared with its parent compound DMB. Phenamil, DMB and CBDMB increased the force of contraction of right ventricular papillary muscles with similar potencies (-log EC50 values: 4.77 +/- 0.06, 5.09 +/- 0.09, 4.97 +/- 0.17 respectively), while amiloride and EIPA gave small negative inotropic responses. All compounds gave negative chronotropic responses at similar concentrations to those which exerted inotropic effects. Inhibition of KCl contraction of endothelium-free aortic rings was observed with all compounds tested. Phenamil, DMB and CBDMB but not amiloride or EIPA showed a shift to the left of the concentration-response curves in the presence of intact endothelium. These results provide further evidence for positive inotropic and endothelium-dependent vasorelaxant effects of amiloride analogues mediated by inhibition of Na+/Ca2+ exchange.