Plasma ACTH, arginine vasopressin (AVP), and catecholamines were measured at 5-min intervals in the pituitary venous effluent of the unanesthetized horse. Pulses of ACTH and AVP were found to be surprisingly brief (usually of less than 10-min duration) and frequent (averaging between 15-25 min). A highly significant relationship in the changes in concentration of these two hormones was demonstrated (P less than 0.0002) both at rest and after a mild hypoglycemic stimulus. Although there was also a significant correlation (P less than 0.005) between simultaneous plasma ACTH and AVP values the pulse amplitude ratio of AVP to ACTH showed a considerable variation. A rise in cortisol appeared to have a greater suppressive effect on the amplitude of ACTH than AVP pulses. The gradient in hormonal concentration between pituitary effluent and jugular plasma was at times over 50-fold for ACTH, and 500-fold for AVP. A gradient was also found for epinephrine, norepinephrine, and dopamine. A highly significant correlation (P less than 0.005) was demonstrated between changes in norepinephrine, ACTH, and AVP concentrations, but no such relationship could be shown for epinephrine and dopamine. It is concluded that there is a close temporal relationship between changes in ACTH, AVP, and norepinephrine concentrations. Pulses of these hormones are greater in amplitude and more frequent than would have been suspected from sampling peripheral plasma. The variability in the pulse amplitude ratio of ACTH and AVP may suggest that other factors are affecting ACTH secretion. The ability to sample frequently for several hormones and to obtain a marked gradient in hormonal secretion between the pituitary venous effluent and jugular plasma suggest that the horse should provide an excellent animal model in which to study the regulation of hypothalamic and pituitary hormone secretion.

I. Kale (Brassica oleracea) and ryegrass (Lolium perenne)-clover (Trijolium repens) pasture, grown under similar soil conditions, were grazed in the vegetative state by growing lambs of 23.6 kg initial live weight for 24 weeks. The kale and pasture contained respectively 20 and 270pg iodine/kg dry matter (DM). The kale also contained 8 pmol total glucosinolates/g DM and 11.5 g S-methyl-L-cysteine sulphoxide (SMCO)/kg DM, both of which were nondetectable in the pasture diet.2. Intramuscular injections of I (475 mg) were given during weeks 1 and 12 to half the forty-eight lambs grazing each forage. Wool growth, live-weight gain and cytochrome oxidase (EC 1 .9.3.1) activity of biopsied hind-limb muscle were measured at 6-week intervals. Jugular blood samples were removed every 6 weeks for the determination of haematological factors and serum thyroid hormone concentrations. All animals were slaughtered at the end of the experiment and thyroid weight, thyroid I content, and the weight and cytochrome oxidase activity of heart muscle determined.3. Serum concentrations of thyroxine (T,) increased from 20 to 48 nmol/I during the 24 weeks that control lambs grazed ryegrass-clover pasture. I supplementation increased the concentration and total amount of I in the thyroid gland and increased serum T, concentration, but did not affect any other values measured in the lambs grazing the pasture herbage. Serum concentrations of triiodothyronine (T,) were stable at 2 nmol/l for both groups.4. Control lambs grazing kale for 24 weeks showed marked thyroid enlargement and depletion of thyroid I. By week 6, serum T, and T, concentrations had declined to 2-5 nmol/l and 1 nmol/l respectively and were stable at these values for the remainder of the experiment. I supplementation eliminated the thyroid depletion of this element, caused serum T, concentration to rise and stabilize at 90 nmol/l by week 18, and T, concentration to stabilize at 2 nmol/l by week 6. From week 6 onwards, wool growth was increased 13% by I supplementation, whereas empty body growth was unaffected.5. Lambs grazing kale developed haemolytic anaemia, due to rumen fermentation of SMCO. I supplementation enabled the lambs to resist the anaemia better by increasing erythrocyte reduced gluthathione (GSH) content. Relative to pasture-fed animals, lambs grazing kale and supplemented with I showed increased heart muscle weight and cytochrome oxidase activity. This represented a compensatory mechanism for the reduced blood oxygen-carrying capacity caused by the anaemia. I-deficient (1.e. control) lambs grazing kale showed reduced cytochrome oxidase activity in both heart and hind-limb muscle.6. The findings are in accord with T3 having a greater biological potency than T4 for regulating rates of body and wool growth. Increases in heart weight, heart cytochrome oxidase content and erythrocyte GSH content of kale-fed lambs were, however, associated with elevation in serum T, and not T, concentration.7. I requirements of growing sheep and cattle consuming the pasture diets ar...

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