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Locally-produced oxytocin appears to play a role in both labor initiation and regulation. Vasopressin can act on both uterine muscle cell oxytocin receptors and vasopressin receptors and may play a role in regulating labor. Researchers studied uterine muscle cell samples obtained during cesarean sections at term and preterm, during labor and in the absence of labor. Samples were assayed for the number of oxytocin and vasopressin receptors and for their relation to the activities of oxytocin, vasopressin, and an oxytocin inhibitor. Women at term average higher numbers of oxytocin receptors, and in both cases, the number of receptors decreased as labor progressed. This suggests the initiation of labor does not depend on an increase in the number of receptors but an increase in amount of oxytocin. An oxytocin antagonist blocked both vasopressin and oxytocin receptors, but the degree of contraction inhibition by an oxytocin antagonist corresponded to the number of oxytocin receptors and not vasopressin receptors. This suggests that oxytocin alone is responsible for contractions.

A polyphosphoinositide phospholipase C has been identified in highly purified plasma membranes from shoots and roots of wheat seedlings. The enzyme preferentially hydrolysed phosphatidylinositol 4‐phosphate and phosphatidylinositol 4,5‐bisphosphate and had a different phosphoinositide substrate profile from soluble phospholipase C. The enzyme activity was lower in plasma membranes isolated from light‐grown shoots than from dark‐grown ones, whereas no differences in activity between plasma membranes from light‐ and dark‐grown roots were seen. Maximum activity of the membrane‐bound enzyme was observed around pH 6. It was activated by micromolar concentrations of Ca2+, but not by GTP or GTP analogues. The enzyme may participate in signal transduction over the plant plasma membrane.

A polyphosphoinositide phospholipase C has been identified in highly purified plasma membranes from shoots and roots of wheat seedlings. The enzyme preferentially hydrolysed phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate and had a different phosphoinositide substrate profile from soluble phospholipase C. The enzyme activity was lower in plasma membranes isolated from light-grown shoots than from dark-grown ones, whereas no differences in activity between plasma membranes from light- and dark-grown roots were seen. Maximum activity of the membrane-bound enzyme was observed around pH 6. It was activated by micromolar concentrations of Ca 2+, but not by GTP or GTP analogues. The enzyme may participate in signal transduction over the plant plasma membrane.

Phospholipase C activity against phosphoinositides in isolated rat liver plasma membranes has been examined using exogenous substrates. The enzyme hydrolyzed phosphatidylinositol 4,5-bisphosphate 30–40-times faster than phosphatidylinositol 4-monophosphate, while phosphatidylinositol was not a substrate. Maximum activity was observed with 1.1 mM phosphatidylinositol 4,5-bisphosphate at pH 5.0. The enzyme was stimulated by micromolar concentrations of Ca 2+. The GTP analogue guanylyl (β,γ-methylene)diphosphonate enhanced phospholipase C activity at and above 0.3 μM Ca 2+, but was inhibitory at 0.1 μM Ca 2+. This supports the suggestion that plasma membrane phospholipase C is regulated by guanine nucleotide-binding protein, but indicates a regulatory mechanism different from that of other enzymes regulated by such proteins.

Abstract. A sensitive and specific radioimmunoassay for 1-deamino-8-d-arginine vasopressin (DDAVP) was developed. Plasma concentrations of DDAVP after intragastric administration were determined in non-fasted rats and a dose-related increase in plasma levels was obtained. The immunoidentity of plasma DDAVP and standard was established by high performance liquid chromatography (HPLC). Correlation of biological and immunological responses of DDAVP was assessed using the antidiuretic effect in the Brattleboro (diabetes insipidus) rat.

Abstract. The pharmacokinetics in the human of 1-deamino-2-D-Tyr(OEt)-4-Thr-8-Orn-vasotocin (dE-TVT), was studied after iv and intranasal administration in 11 subjects at 12 experiments each route. The plasma concentration of the analogue was analysed by means of an arginine vasopressin antibody, which cross-reacted with dE-TVT to 4.7%. When given intravenously as bolus injection (10 nmol/kg/body weight), the total body clearance amounted to 0.623 ± 0.099 (sem) 1/h kg and the half-life to 16.2 ± 2.4 min. After intranasal administration (100 nmol/kg/body weight), the bioavailability was 10.5 ± 2.9%. Peak concentrations in plasma appeared 2–8 min after iv and 10–45 min after intranasal administration. At the end of an observation period of 2 h measurable amounts in plasma were still found in one of the iv and seven of the intranasal experiments. It is concluded that the moderately long half-life is suitable for the treatment of hospitalized patients in premature labour where promising results with intravenous infusion (50 μg/min) of dE-TVT have been obtained. It is still uncertain whether or not the absorption of dE-TVT is sufficient for intranasal administration to out-patients with uterine hyperactivity in late pregnancy and to patients with primary dysmenorrhoea, where significant relief of symptoms were seen after iv administration (10 μg/kg body weight).

The effects of treatment with synthetic oxytocin on the urinary and pituitary gonadotrophins in male rabbits were evaluated. Oxytocin was injected twice (2 × 1 IU per kg body weight) every third day for a period of 18 days. The amounts of total urinary gonadotrophins were significantly increased for 1–3 days after each day of oxytocin injection. The total pituitary gonadotrophins and FSH were reduced to about 20% of the control values, 2 hours after the last of the oxytocin injections.

Oxytocin was administered (2 × 1 IU per kg body weight every third day for 22 days) to sexually mature male rabbits. The semen was collected from one group and the sperm output was estimated for various periods of time before, during and after the injections. A significant (P< 0.001) increase in the sperm output was observed, beginning 3–4 weeks after the start of treatment with oxytocin. The effect persisted for about 7 weeks and was followed by a period with a significant decrease (P< 0.05) in the sperm output.Another group of animals received similar injections of oxytocin, but these animals were killed on the 18th day after the injections. The effects of the hormone were evaluated by a quantitative analysis of the spermatogenesis. Following the oxytocin injections the number of type B spermatogonia, primary spermatocytes and spermatids increased (P< 0.001), but the Sertoli cells decreased in number (P< 0.05). A small but significant increase of the diameter of the seminiferous tubules (P< 0.01) was noted in the animals treated with oxytocin.Evidence was obtained that oxytocin was more potent in individual animals with a subnormal testicular function.

The effects of single intravenous injections of synthetic oxytocin and lysine-vasopressin on the contractile activity of the distal part of the ductus epididymidis, the proximal and distal parts of the vas deferens and the vesicula seminalis were studied in seventy-two anaesthetized rabbits. The muscular contractions were registered as changes in the intraluminal pressure of the organs, and the amplitude, frequency, duration and tonus were measured.An injection of 20 m-i.u. of oxytocin/kg body weight caused a small but significant increase of the amplitude of the contractions in the ductus epididymidis and the proximal part of the vas deferens. An injection of 100 m-i.u. of oxytocin increased the tone in the proximal part of the vas deferens but decreased this parameter in the distal part of the organ. The infusion of 20 m-i.u. of vasopressin enhanced the frequency, decreased the duration and elevated the muscular tone in the ductus epididymidis and increased the frequency and the tone in the proximal part of the vas deferens. The infusion of 100 m-i.u. of vasopressin resulted in increased amplitudes in both these regions. A gradual increase in the effects of oxytocin and vasopressin towards the testis was observed, indicating a polarity in response to the hormones of the male reproductive ducts.The results support the theory that neurohypophysial hormones may regulate sperm transport in the male through their effects on the contractile elements of the genital organs, possibly being complementary in their actions.