F, W. LENGEMANN
That the thyroxine took approximately two
days to produce its effect upon milk radio-
the low milk/plasma ratios sometimes encountered in normal cows, and in cows given sub-
iodine is evidence that its mechanism of action
may differ in some way from that of the inor-
stances such as perchlorate, the limited effect
plained at this time, but preliminary experiments upon three cows seem to indicate that it
tration mechanism.
ganic anions. This action of thyroxine is unex-
may not be mediated through thyroid-stimulating hormone. Daily injection of up to ten
units of TSH had no consistent effect upon the
milk/plasma I™ ratio in these cows. The lack
of effect of 2-4 dinitropheno]l means that more
than a phenolic group is necessary to elicit
the thyroxine response. That p-thyroxine increased the milk/plasma ratio, in contrast to
the action of L-thyroxine, suggests a specificity
for L-thyroxine.
Data of Figure 3 show that thyroxine can
reduce the movement of iodide from milk back
to plasma. For thyroxine to produce an overall reduction in the accumulation of iodide in
milk, it is also necessary for the transfer of
iodide from plasma to milk to be reduced.
This evidence is only qualitative; quantitative
measurements have yet to be made.
An interesting observation during the thyroxine experiments was that there was a definite
maximum response that could beelicited by the
exogenous thyroxine. The data show that the
per cent of I per liter of milk and the milk/
plasma I" ratio could be decreased to only
about 50% of the control value.
In view of
[6 |
of thyroxine suggests that the hormone is not
the sole control of the mammary iodide-concenREFERENCES
(1) Brown-Grant, K. 1957. The Iodide Concentrating Mechanism of the Mammary Gland.
J. Physiol. 135: 644.
(2) Brown-GrantT, K. 1961. Extrathyroidal Iodide Coneentrating Mechanisms. Physiol.
Rey., 41: 189.
(3) Garner, R. J., Sansom, B. F., anD JONES,
H. G.
1960. Fission Products and the
Dairy Cow. III.
Transfer of ™-Iodine to
Milk Following Single and Daily Dosing.
J. Agr. Sei., 55: 283.
(4) LENGEMANN, F, W. 1963. *"I Concentrations
in Blood, Milk, Urine, and Faeces of Dairy
Cows Following a Single Dose of Radioiodine. J, Agr. Sei., 61: 375.
(5) LENGEMANN, P, W., anD Comar, C. L. 1964.
Metabolism of I" by Dairy Cows During
Long Term Daily Administration of the
Radioisotope. Health Physics, 10: 55.
(6) LeNGEMANN, F. W., anp Swanson, E. W.
1957. A Study of the Secretion of Iodine
in Milk of Dairy Cows Using Daily Oral
Doses of I. J. Dairy Sci., 40: 216.
(7) PremacHanpra, B. N., aNnD TURNER, C. W.
1961. Effect of Milk Hyperthyroidism upon
Secretion of I in Milk of Dairy Cattle.
J. Dairy Sei., 44: 2035.