AJP - Endocrinology and Metabolism, Vol 264, Issue 4 E644-E649, Copyright © 1993 by American Physiological Society
Activation of sodium transport mediates regulation of thyroid follicle volume in response to hypotonic media
A. S. Yap, J. W. Armstrong, E. J. Cragoe Jr, J. R. Bourke, G. J. Huxham and S. W. Manley
Department of Physiology and Pharmacology, University of Queensland, St. Lucia, Australia.
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.
