AJP - Endocrinology and Metabolism, Vol 240, Issue 1 37-E42, Copyright © 1981 by American Physiological Society
Autoregulation of thyroid iodide transport: possible mediation by modification in sodium cotransport
M. Berkowitz, D. Daughtridge and J. R. Sherwin
Cat thyroid slices were employed to determine whether preincubation with
excess iodide, a procedure previously reported to depress subsequently
measured iodide transport activity, is associated with any alteration in
the relationship between sodium and iodide transport. Kinetic analysis
revealed that the inhibition of iodide-concentrating activity by iodide
pretreatment was accompanied by a reduction in the apparent affinity of the
iodide transport for iodide as reflected by increase in the value of KA,
the concentration of iodide required to achieve half-maximal transport
activity. A reduction in extracellular sodium concentration was also
accompanied by an increase in the value of KA for iodide transport. Thus,
the KA of iodide transport in control thyroid tissue was increased from a
value of 8.6 +/- 1.2 to 16.5 +/- 2.4 microM, by a reduction in
extracellular sodium from 144 to 52 mM. In contrast, in thyroid tissue
subjected to a 2-h preincubation with 30 microM NaI, a similar reduction in
extracellular sodium concentration was associated with only a 20% increase
in KA from a value of 17.7 +/- 2.4 to 21.7 +/- 2.2 microM. Analysis of the
kinetic data suggests that the autoregulatory effect of excess iodide
results in a decline in the sodium dependency of iodide transport. This
loss of sodium dependency is accompanied by an increase in the affinity of
the process for sodium, as well as a reduction in the affinity of the
transport system for iodide.
