The influence of the thyroid gland on the functional and histochemical development of fast- and slow-twitch skeletal muscle of fetal sheep has been studied in euthyroid fetal sheep (n = 6) and athyroid fetuses (n = 4) surgically thyroid-ectomized at 70-75 days of gestation. Two fast-twitch muscles, the medial gastrocnemius and extensor digitorum longus, and the slow-twitch soleus muscle were studied at the fetal age of 140 days gestation. The athyroid fetuses had significantly slower twitch contraction and relaxation times in both the medial gastrocnemius and extensor digitorum longus muscles compared with the euthyroid fetuses. Twitch contraction and relaxation times of the soleus were not different in the two groups. Thyroidectomy resulted in an increase in the proportion of fast (type II) muscle fibers and myosin, as shown histochemically and by gel electrophoresis of heavy-chain myosins. These results indicate that the functional maturation of the fast-twitch muscles of sheep is influenced by the presence of an intact thyroid gland from at least 70 days of gestation. In contrast, the slow-twitch soleus muscle fiber diameter and twitch contraction and relaxation times were not different in the two groups.

In this study, plasma thyroxine, contractile and histochemical (adenosinetriphosphatase and NADH) characteristics of soleus (SOL), medial gastrocnemius (MG), and extensor digitorum longus (EDL) were examined in 140-day-gestation fetal sheep and in 2-, 5-, and 30-day-old lambs and adult ewes. Electrophoretic separation of myosin heavy chains was also done on all muscles and the diaphragm. There were no differences in the twitch contraction and relaxation times of MG and EDL at the different ages; in contrast SOL contraction times were significantly shorter in the fetus and newborn than in the adult. Fast glycolytic fibers first appeared in EDL, MG, and diaphragm at 5, 30, and 5 days after birth, respectively. The proportion of slow oxidative fibers decreased after birth and with postnatal development in EDL, whereas they increased in MG and diaphragm. Plasma thyroxine concentrations were higher in the fetus and day-old lambs than in 2-, 5-, and 30-day-old lambs or adult sheep. It is suggested that contractile specialization of the fast-twitch diaphragm, MG, and EDL is largely achieved in utero and is probably mediated by thyroid hormone. In contrast, SOL changed postnatally, probably influenced by the altered neural drive.

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Uterine and umbilical blood flows were measured in pregnant sheep (125-142 days gestation) under normothermic and hyperthermic conditions using the Fick principle with [14C]antipyrine as the indicator. Exposure of the sheep to an ambient temperature of 43 +/- 1 degrees C (25-30% relative humidity) for 8 h increased maternal and fetal core temperatures 1.19 +/- 0.15 and 1.39 +/- 0.12 degrees C respectively. Maternal hyperventilation caused a significant decrease of both maternal and fetal arterial partial pressure of CO2 (Pa,CO2) and increase of arterial pH. Uterine blood flow increased significantly during the hyperthermia (+54.2 +/- 14.1%), the increase being correlated with the magnitude of the decrease of maternal Pa,CO2 (r = -0.84, P < 0.05) but not with the increase of maternal core temperature. The increase of uterine blood flow was not associated with a concomitant increase in the placental clearance of [14C]antipyrine, a result which could arise if the increase of blood flow was non-placental (i.e. did not occur within the cotyledons), or occurred through uterine arteriovenous shunts during the period of heat stress. Hyperthermia was not associated with a significant change of umbilical blood flow, placental transfer of glucose, or fetal glucose uptake. Since the loss of heat from the fetus occurs mainly across the placenta, we speculate that the apparent increase of uterine blood flow during maternal hyperthermia has an adaptive significance by maintaining conductive heat flux in a fetomaternal direction, even though cotyledonary (placental) blood flows did not increase.

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