Prostaglandins and myogenic control of tension in lower esophageal sphincter in vitro

Active tension is produced by the lower esophageal sphincter (LES) of North American opossum $\text{in vitro}$ by a myogenic mechanism. Strips of LES, but not those from the esophageal body, contracted to prostaglandin (PG)F_2α, stable expoxymethano derivatives of PGH₂ and to thromboxane B₂. Stable endoperoxides were more than 500 times more potent than PGF_2α. PGI₂ and 6-keto PGF_1α were weak relaxants of LES strips. LES strips transformed arachidonic acid into contractile substances. This transformation was prevented by agents which interfere with PG synthesis by inhibiting cyclo-oxygenase [indomethacin (IDM), 5,8,11,14-eicosatetraynoic acid (ETA) or thromboxane synthetase [imidazole]. Tranylcypromine 500 μg/ml also inhibited contractions to arachidonic acid. These agents also reduced muscle tone, so that endogenous PG formation may contribute to active tension in the LES. ETA and IDM increased tone before inhibiting it, and this effect was prevented by prior treatment with ETA or imidazole. There may also be an endogenous PG which inhibits LES tone. The possibility that this may be PGI₂ is discussed.     

Effects of prostanoids on the rat's myometrium in vitro during pregnancy.

The objectives of this study were to determine the effects of pregnancy in the rat on the contractile response of the myometrium in vitro to a number of prostanoids. Longitudinally and circularly oriented strips were studied separately. Responses to PG (prostaglandin)F2 alpha, PDG2, the PGI2-mimetic iloprost, and the thromboxane (Tx) A2-mimetic U-46619 were investigated on Days 10, 15, 18, 20, 21, and 22 of pregnancy. Responses were prostanoid-dependent, and differences between longitudinal and circular strips were small. PGF2 alpha and PGD2 produced similar patterns, with a high potency but low maximal response on Day 10; thereafter potency fell to a minimum value on Day 18 and then gradually increased until Day 22, when it was still lower than at Day 10. In contrast, for PGE2 there were no changes in potency over the period of study (longitudinal muscle) or a slight increase between Days 15 and 21 (circular muscle). Both iloprost and U-46619 maintained a low potency throughout pregnancy. We conclude that the pregnant rat's myometrium is probably not a target for PGI2 or TxA2 and that the difference patterns of responses to PGE2 and PGF2 alpha during pregnancy support the hypotheses that these prostanoids act at different sites within the myometrium.     

Determination of eltanolone in human plasma by high-performance liquid chromatography

A high-performance liquid chromatographic method for the determination of eltanolone in plasma has been developed. Plasma samples containing eltanolone were diluted with acetonitrile to precipitate plasma proteins, and derivatized with 2,4-dinitrophenylhydrazine before direct injection onto a C₁₈ column. The mobile phase was acetonitrile–water (70:30, v/v) containing 0.1% trifluoroacetic acid and detection was by UV absorbance at 367 nm. The quantitation limit was 0.020 μg/ml. The method has proven to be rapid, precise and sensitive in the range of concentrations found during and following intravenous anaesthesia.     

Localization of the oxytocin receptor in the plasma membrane of rat myometrium.

The distribution of [3H]oxytocin binding sites among various subcellular fractions of rat myometrium paralleled the distribution of 5'-nucleotidase, a plasma membrane marker enzyme, but not of NADPH-cytochrome c reductase or succinate-cytochrome c reductase, which are endoplasmic reticulum and mitochondrial marker enzymes respectively. [3H]Oxytocin binding to the most enriched plasma membrane fraction showed the degree of selectivity with respect to hormone analogues that is expected for the oxytocin receptor. The binding of oxytocin to this fraction showed an apparent Kd of 1.98 X 10(-9) M and a capacity of 1.28 pmol mg-1. It is concluded that the oxytocin receptor is located on the plasma membrane of the smooth muscle cells of the rat uterus.     

Determination of remifentanil in human blood by capillary gas chromatography with nitrogen-selective detection

A validated method for the determination of remifentanil in human blood, applicable to all therapeutic concentrations, using capillary GC with nitrogen-specific detection and fentanyl as the internal standard has been developed. Citrated whole blood samples were extracted into 1-chlorobutane following precipitation of proteins with methanol. The drugs were back extracted into 10 mM HCl and re-extracted into methanol-1-chlorobutane. The extracts were reconstituted in methanol and injected onto a 25-m BPX-5 column. The lower limit of quantitation was 0.2 ng/ml with within- and between-day coefficients of variation of less than 15%.     

The sensitivity of the longitudinal and circular muscle layers of the rat's myometrium to oxytocin in vitro during pregnancy

To test the hypothesis that binding site regulation is the primary process controlling the responsiveness of the rat's myometrium to oxytocin during pregnancy, I have studied the effects of oxytocin on longitudinal and circular strips of myometrium in vitro throughout pregnancy. Longitudinal muscle was as sensitive on day 10 of pregnancy (EC50 = 1.6 nM) as it was at term (EC50 = 1.3 nM) and there was no significant change in the mean maximal force developed in response to the hormone (2.1 +/- 0.9 vs. 1.5 +/- 0.3 N cm-2). Circular muscle on the other hand was essentially refractory to the hormone until day 21 of pregnancy at which time its sensitivity and the maximum response were similar to those of longitudinal muscle. These results indicated that regulation of oxytocin sensitivity in the two muscle layers was temporally different, and they suggested different mechanisms. The effect of oxytocin on longitudinal muscle was not compatible with the hypothesis that changes in binding site number regulate the responsiveness of the tissue, whereas the effect on circular muscle was.     

Characterization of putative beta-adrenoceptors in the myometrium of the pregnant ewe: correlation between the binding of [3H] dihydroalprenolol and the inhibition of myometrial contractility in vitro

beta-Adrenoceptors in the myometrium of the pregnant ewe were studied both functionally and by radioligand binding techniques using [3H] dihydroalprenolol (DHA). Spontaneous contractile activity in vitro was inhibited by beta-adrenoceptor agonists in a stereoselective manner; the order of potency suggested a beta 2-adrenoceptor was involved. The effects of salbutamol were antagonized competitively by propranolol but the antagonism demonstrated by atenolol and ICI 118,551 (beta 1- and beta 2-selective, respectively) was complex. DHA binding was saturable, rapidly reversible, stereoselective and appeared to occur to a single class of noninteracting sites with a Kd of 4.1 +/- 0.3 nM and a maximal capacity of 0.8 +/- 0.05 pmol/mg protein. Agonists demonstrated the same order of potency in competition for ligand binding sites as in inhibition of contractile activity. These data suggested that the DHA binding sites were part of the beta-adrenoceptor. Strong agonists occupied only 0.1% of all receptors to produce the full response. Competition experiments with antagonists produced complex curves which could be resolved into two components comprising approximately 70% and 30% of the total number of sites. It was suggested that these sites represented beta 2- and beta 1-adrenoceptors, respectively. The possibility of regulation of the relative numbers of beta 2- and beta 1-adrenoceptors in the myometrium was discussed.