AJP - Cell Physiology, Vol 236, Issue 3 117-C124, Copyright © 1979 by American Physiological Society
Thyroxine and Na+ transport in toad: role in transition from poikilo- to homeothermy
B. C. Rossier, M. Rossier and C. S. Lo
The effects of thyroxine (T4) on Na+ transport, oxygen consumption (QO2),
and Na+-K+-ATPase activity were studied in the urinary bladder and liver of
the toad Bufo marinus. In the bladder, T4 in vitro (10(-8) to 10(-6) M) had
no significant effect on these parameters during 15 h of incubation. When
injected intraperitoneally (approximately 20 microgram/(kg body wt.day) for
6 days), T4 lowered base-line, short-circuit current by 62% (P less than
0.0025) and potential difference by 37% (P less than 0.001), increasing
tissue resistance by 40% (P less than 0.02). T4 depressed QO2/DNA (-25%, P
less than 0.05) with no significant effect on Na+-K+-ATPase activity. In
liver, T4 increased the recovery per cell DNA of mitochondrial proteins by
32% (P less than 0.025), corresponding to an increased QO2 (stage IV) of
isolated mitochondria per cell DNA (+54%, P less than 0.01). There was no
significant effect on Na+-K+-ATPase activity. These results suggest that,
unlike its function in the rat, T4 in the toad does not regulate cellular
thermogenesis by inducing Na+-K+-ATPase. This major difference could
account at least in part for the transition from poikilothermy to
homeothermy. In addition, T4 has a distinct inhibitory effect on Na+
transport in the urinary bladder, which suggests an antagonism to the
action of aldosterone.
