Thyroxine and Na+ transport in toad: role in transition from poikilo- to homeothermy.
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.
D000818:Animals; D005075:Biological Evolution; D001833:Body Temperature Regulation; D002024:Bufo marinus / Q000502:physiology; D005260:Female; D008099:Liver / Q000378:metabolism; D008297:Male; D010101:Oxygen Consumption / Q000187:drug effects*; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013974:Thyroxine / Q000494:pharmacology*; D001743:Urinary Bladder / Q000378:metabolism
D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D013974:Thyroxine