A comparative study of collagen induced thromboxane release from platelets of different species: Implications for human athero sclerosis models

The role of thromboxane in human, rabbit, dog, pig and sheep platelet aggregation has been studied. Each of these species showed a concentration-dependent relationship between collagen concentrations and the thromboxane B2 (TXB2) released from aggregating platelets, however sheep platelets produced only 17.5% of the amount of TXB2 released from human platelets under the same collagen stimulus. Indomethacin did not inhibit sheep platelet aggregation in the concentration range 0.114 mM - 0.114 μM, yet proved to be a potent inhibitor of human platelet aggregation. This prostaglandin endoperoxide analogue, U.46619, induced human platelet aggregation at a dose of 100 ng, however the analouge, at doses as high as 500 μg was much less active on sheep platelets when compared to human platelets. Thus we have demonstrated that there are differences in the extent of thromboxane involvement in platelet aggregation between different species, which must be considered with other criteria, before choosing an animal model for studying human atherosclerosis

Inhibition of progesterone secretion by a 3β-hydroxysteroid dehydrogenase inhibitor in late pregnant sheep

The role of progesterone in the initiation of parturition in the sheep is unclear. Whether a decrease in plasma progesterone is the essential prerequisite for the initiation of parturition or whether other factors also maintain uterine quiescence until delivery is not known. The effect of withdrawal of progesterone on the initiation of parturition has been investigated by intravenous administration of trilostane, a 3β-hydroxysteroid dehydrogenase Δ isomerase inhibitor, to late pregnant sheep. Twenty-five or 100 mg trilostane caused a precipitous decrease in plasma progesterone to about 30% of preinjection levels. Progesterone remained depressed for up to 7 days after treatment. 13,14-Dihydro-15-keto-prostaglandin F(α) (PGFM) became elevated between 7 and 36 h after trilostane injection but gradually returned to preinjection levels during the subsequent 36 h, at a time when plasma progesterone was still depressed. Four of 11 animals treated with 100 or 200 mg trilostane aborted prematurely at a time when plasma PGFM was maximal and plasma progesterone minimal. There were no consistent changes in plasma estradiol-17β or ovine placental lactogen concentrations after treatment with trilostane. It is suggested that a decrease in plasma progesterone will cause a transient increase in plasma PGFM concentrations which can lead to the premature initiation of parturition. In some instances the myometrium does not appear to respond to the elevated PGFM concentrations even when the estrogen:progesterone ratio is elevated by a decrease in plasma progesterone

Induction of transient functional luteolysis in cyclic sheep by a 3β-hydroxysteroid dehydrogenase inhibitor

The mechanism by which prostaglandin F(α) terminates luteal function in the sheep is unclear even though it is used extensively in animal husbandry. At the time of luteal regression, a decrease in 3β-hydroxysteroid dehydrogenase (3β-HSD) activity is apparent in the corpus luteum, but is is not known whether the decrease in enzyme activity is the primary cause of structural luteolysis. The effect of trilostane, a 3β-HSD inhibitor, on luteal function and morphology has therefore been investigated. Intravenous injection of trilostane in the mid-luteal phase of the oestrous cycle caused a decrease in ovarian tissue progesterone content. A transient decrease in peripheral and utero-ovarian vein plasma progesterone was observed but there was no significant effect on the length of the luteal phase of the cycle. There was no significant change in plasma 13,14-dihydro-15-oxo-prostaglandin F(α) during the period when plasma progesterone was depressed. Morphological examination of the corpora lutea revealed a decrease in the concentration of electron-dense granules without any other features of impending luteal regression. When plasma progesterone was reduced for more than 10 h by two injections of trilostane 4 h apart, there was again no subsequent effect on the length of the oestrous cycle or on the return to oestrus. Plasma progesterone returned to preinjection levels within 24 h of injection. This evidence suggests that competitive inhibition of 3β-HSD activity, per se, is ineffective in bringing about structural luteolysis

The effects of certain vasodilating prostaglandins on the coronary and hindlimb vascular beds of the conscious sheep

1. Vasodilating prostaglandins were injected, in bolus doses, into the lower abdominal aorta or left circumflex coronary artery (LCCA) of conscious sheep. 2. Local blood flow. mean arterial pressure (MAP), heart rate (HR) and ECG were monitored continuously. 3. 6-Keto PGF1α had no effect on either vascular bed in doses up to 100 μg. PGE2 was more potent than PGI2in dilating hindlimb vasculature and PGE2 induced a more persistent hyperaemia whereas PGD2 elicited a biphasic response (constriction -dilation). 4. PGE1, PGE2, PGD2 and PGI2 all produced dose-dependent vasodilation, the order of potency being PGD2 > PGI2>PGE1≥ PGE2. The effect of PGI2 was more transient and PGE1 and PGD2 caused small but consistent decreases in MAP and HR, respectively

Prostaglandins in fetal tracheal and amniotic fluid from late pregnant sheep

Concentrations of prostaglandin E (PGE), prostaglandin F (PGF) and 13,14 dihydro 15 keto prostaglandin F (PGFM) have been measured in fetal tracheal and amniotic fluid from chronically catheterized sheep during late pregnancy. Amniotic fluid contained significantly greater concentrations of these prostaglandins than tracheal fluid (p < 0.01); there was no correlation between the level of prostaglandins found in each fluid. In tracheal fluid concentrations of PGE and PGFM exceeded those of PGF (P < 0.01) whereas no significant differences were found in amniotic fluid. The levels of prostaglandins in these fluids were similar in ewes bearing hypophysectomized fetuses

The interaction of human endometrial and myometrial steroid receptors with danazol

The affinity of danazol for oestrogen, androgen and progesterone receptors in human endometrium and myometrium was determined, to study the mechanism of action of this drug in the treatment of endometriosis. The ability of danazol to combine with each of the three types of receptor was similar in both endometrium and myometrium. The capacity of danazol to compete with oestradiol‐17β for the oestrogen receptor was very low (1·72 ± 0·48 ± 10 cross reaction, mean ± SEM) and danazol, at the maximum concentration used, was unable to saturate the receptor; but danazol's ability to compete with progesterone for its receptor was considerably higher (8·41 ± 1·65% using progesterone, 1·95 ± 0·41% using R5020) and was saturable. Danazol was also able to displace dihydrotestosterone from the cytosol androgen receptor (6·29·1±82% cross reaction). The association constant of oestradiol for the endometrial and myometrial oestrogen receptors was 2·19 ± 10M and 7·45 ± 10M respectively, while that of progesterone and dihydrotestosterone for their receptors was similar in endometrium and myometrium (mean 0·25 ± 0·06 ± 10 M and 3·62·1±67·10 M respectively). Using R5020, the association constant for the myometrial progesterone receptor was 2·50 ± 0·73 ± 10 M. We conclude that, in view of the high circulating levels of danazol present in patients being treated for endometriosis, it is possible that danazol may bind to, and partly saturate, endometrial and myometrial oestrogen, progesterone and androgen receptors during treatment. An explanation may thus be provided for some of the diverse actions of this drug. Copyrigh

Effects of hydroxysteroid dehydrogenase inhibitors on in-vitro steroidogenesis in the ovine adrenal gland

The effectiveness of trilostane and azastene as inhibitors of adrenal steroidogenesis was compared by in-vitro and in-vivo methods. A radioimmunoassay was developed for the measurement of cortisol in ovine plasma, incubation medium and tissue extract using a specific antiserum raised against cortisol 21-acetate,3-carboxymethyloxime:bovine serum albumin. Trilostane (20 μmol/l) decreased cortisol synthesis and release both in unstimulated and in ACTH-stimulated adrenal tissues in vitro. The same concentration of azastene had a lesser effect on unstimulated adrenals and was completely ineffective in blocking the stimulatory action of ACTH. In vivo, trilostane suppressed adrenal steroidogenesis in pregnant and cyclic ewes but the suppression in pregnant ewes was over a longer period, and after lower doses. It is concluded that trilostane had an inhibitory effect on ovine adrenal steroidogenesis both in vitro and in vivo