Vanadate potentiates the glycogenic action of insulin-like growth factors on isolated diaphragm.

Na3VO4 (6.5 mumol/100 g rat weight), co-injected with a trace amount of [14C]glucose, increased within 15 min the incorporation of radiolabel in diaphragmal glycogen. After 2 h the vanadate-induced increases were 12-fold in the diaphragm and 7-8-fold in heart and liver. In contrast, when added to isolated diaphragms for up to 1 h, vanadate (0.1-5 mM) had no effect on the synthesis of glycogen from 5 mM glucose. In search of a putative mediator of vanadate's action in vivo, insulin and the insulin-like growth factors (IGFs) were considered. Their plasma concentration was not affected by vanadate treatment. In isolated diaphragms, 1 mM vanadate did not potentiate insulin-induced glycogen synthesis, but it caused a several-fold increase in glycogen synthesis in the presence of concentrations of IGF-I which, alone, had no effect. A similar synergism occurred between vanadate and IGF-II. We propose that the glycogenic action of vanadate in vivo, at least in some tissues, involves a potentiation of the action of IGF-I.

Failure to relate thyroid hormones and in vitro 5'-monodeiodination activity to oocyte development and sex steroids in the giant swamp frog Dicroglossus occipitalis at the equator.

Females of the giant swamp frog Dicroglossus occipitalis were captured in Zaïre close to the equator in the course of 1 month. During this period, females with fully developed eggs were found, together with females of which the eggs were still in the first developmental stages. A close relationship was established between the maturation of the eggs and the studied gonadal factors: the gonadosomatic index, the oviduct weight, plasma estradiol-17 beta (E2) concentrations, plasma testosterone concentrations, and the total ovarian E2 concentrations. At the level of the thyroidal axis, the studied factors (plasma thyroxine (T4), plasma triiodothyronine (T3), plasma T3/T4 ratio, T4 and T3 concentrations, and the T3/T4 ratio in the thyroids and the 5'-monodeiodination activity (5'-D-activity) in the skin and kidney homogenates) did not show parallel changes with the maturation process of the eggs. These results indicate that no causal relation has to exist between the annual variation in thyroid hormones and the annual reproductive patterns as found in frogs from the tropical or temperate climatic region.

Estradiol-17 beta silastic implants in female Rana ridibunda depress thyroid hormone concentrations in plasma and the in vitro 5'-monodeiodination activity of kidney homogenates.

Estradiol-17 beta containing silastic tubings were implanted in female Rana ridibunda. Preliminary data, concerning in vitro incubations of such tubings in saline media, revealed that high concentrations of estradiol were released out of the tubings in the incubation medium. Compared to control-implanted frogs, the frogs that had the estradiol tubings implanted for 30 days showed a significant increase of the plasma estradiol concentration, the ovarian estradiol concentration, and the weight of the oviduct. Plasma triiodothyronine (T3) levels, plasma thyroxine levels, and the in vitro T3 production in the kidney homogenates were significantly decreased. These results indicate that high estradiol levels not only influence the gonadal axis, but also cause important effects on the thyroidal axis.

Luteinizing hormone-releasing hormone as a potent stimulator of the thyroidal axis in ranid frogs.

Plasma concentrations of T4, measured by radioimmunoassay, were raised significantly 2 and 4 hr after intravenous injection of synthetic luteinizing hormone-releasing hormone (LHRH) in Rana ridibunda (1 and 10 micrograms on 2 consecutive days) and in Rana esculenta (10 micrograms). A dose of 1 microgram LHRH was not so effective as 50 micrograms synthetic thyrotropin-releasing hormone (TRH) when injected in Rana ridibunda in November. However 10 micrograms LHRH was equipotent to 50 micrograms TRH. In February somewhat less than half of the Rana temporaria group was responsive to LHRH. There is no clear indication that fluctuating plasma T3 concentrations were caused by LHRH or TRH. Preinjection levels of T3 and T4 were higher during the breeding season (April) in R. esculenta (resp. 35.4 +/- 1.4 pg/ml; 744 +/- 134 pg/ml; n = 22) compared to the basal concentrations in the very closely related Rana ridibunda (November) (resp. 15.2 +/- 1.1; 162 +/- 24 pg/ml; n = 28). Four days after removal of the pars distalis plasma T4 concentrations were significantly decreased in Rana esculenta, whereas T3 could stay longer in circulation. T3 and T4 content of the thyroids was not altered by the short-term hypophysectomy. Injection of 10 micrograms LHRH had no influence on plasma T4 nor testosterone concentrations in these frogs, contrary to the sham-ectomized animals in which plasma testosterone remained elevated longer than T4. The results suggest that the stimulatory effect of intravenous injected LHRH on thyroid (and gonadal) activity in the frog is primarily mediated through the hypophysis. They also point to a possible correlation between the gonadal and thyroidal axis.

Seasonal changes of the 5'-monodeiodination activity in kidney and skin homogenates of male Rana ridibunda: relation to plasma thyroxine (T4) and testosterone.

Plasma concentrations of thyroxine (T4) and testosterone were studied together with the in vitro 5'-monodeiodination activity in liver, kidney, and skin homogenates in male Rana ridibunda during 12 consecutive months. Plasma T4 levels increase in February, drop in April, and increase again in May. In July a sharp decrease is seen and the values remain low during the rest of the year. Testosterone plasma levels also exhibit month to month variations. A maximum is found in February and March, and minimal values are measured in August, September, and October. A 5'-monodeiodination activity with a resulting 3,3',5-triiodothyronine (T3) production could be demonstrated in kidney and skin homogenates but not in liver homogenates. Addition of 1,4-dithio-DL-threitol (DTT) and EDTA to the homogenates significantly increases the T3 production. In kidney homogenates incubated with DTT and EDTA a significant annual variation in the T3 production is seen. This T3 production is very high in April, May, and June and is followed by a minimum in July and August. It is concluded that the opposite changes in 5'-monodeiodination activity and plasma T4 concentrations may indicate a peripheral monodeiodination of T4 to T3. During reproduction, which is preceded by a testosterone peak, the increase in the 5'-monodeiodination activity may reflect a greater need for T3.

Reproductive cycle, thyroxine and corticosterone in females of the giant swamp frog Dicroglossus occipitalis at the equator.

Females of the giant swamp frog Dicroglossus occipitalis were captured in the tributaries of the Zaïre River near Kisangani, a town situated close to the equator (00 degree 31' N) with virtually constant conditions as to photoperiod, temperature, precipitation, and relative humidity. An annual variation in the reproductive capacity of this species could be demonstrated. A reproductive quiescence with a low gonadal somatic index (GSI), undeveloped eggs, and minimal plasma concentrations of estradiol-17 beta (E2) and progesterone (P4) was found from October to January. E2 and P4 and thyroxine (T4) contents of the thyroids were positively correlated with GSI and egg size. Plasma T4 concentrations, however, were negatively correlated with GSI, whereas variations in corticosterone correlated only with E2. No correlation between plasma concentrations of glucose and GSI or all other hormonal parameters could be found. It is concluded that during egg maturation, increased amounts of E2 and P4 are produced. The observed variations in thyroidal and plasma T4 may be the result of an activation of thyroid function by the peripheral conversion of T4 into triiodothyronine.