SUMMARY

The spike activity of 191 antidromically identified paraventricular units was recorded during labour and the oestrous cycle of rats under urethane anaesthesia. Accelerated bursts of neuronal activity, as reported to occur during suckling, were not observed and changes in firing rate were not correlated with individual uterine contractions. There was, however, an increase in the magnitude of the mean spontaneous firing rate of antidromically identified neurosecretory cells during labour, as compared with all stages of the oestrous cycle. This increase was mainly caused by (1) a marked reduction (>25%) in the percentage of totally silent cells and (2) an increase in the magnitude of the mean firing rate of the phasically firing units (1·58–3·44 spikes/s).

It was concluded from these increases in spike activity that the tonic release of neurohypophysial hormones is stimulated during labour, and that this may contribute to the induction and course of labour.

The pituitary contents of oxytocin, vasopressin and α-MSH were measured in fetal and newborn rats to assess possible changes in their release during the process of labour. In the 24 h period during which delivery is likely to occur in the Wistar rat, both the oxytocin and vasopressin content of the fetal pituitary gland increased, whereas α-MSH content remained the same. During and/or just before labour, the oxytocin content was found to decrease by 30%, indicating an enhanced fetal release of the hormone at this stage. It was concluded that the expulsion of each fetus did not provide an extra stimulus for release of oxytocin by the fetus.

In addition, if the fetus remained in the uterus after decapitation of the mother, the oxytocin content of the fetal pituitary gland decreased a further 30%. Neither vasopressin nor 30%. content was altered by the process of labour or by the fetus remaining in the uterus after decapitation of the mother. The levels of vasopressin and α-MSH were, however, 20 times higher than the oxytocin content in the fetus and the newborn, which might have obscured the demonstration of a relatively small change in levels of these two hormones.

SUMMARY

Labour was studied in 69 primiparous and multiparous rats by continuous observation and by the recording of intra-uterine activity. The effect of electrical stimulation of the neurohypophysis with stimulation parameters selected to create a pulsatile release of oxytocin was investigated. Stimulation was applied to the neurohypophysis through chronically implanted electrodes every 5 min, in 45 min sessions, from noon on Day 21 of gestation and at 3 h intervals thereafter.

Electrical stimulation successfully promoted (or induced) the onset and facilitated the course of labour. Stimulation at 12.00 h on Day 21, or at a subsequent stimulation session 3, 6, 9 or more hours later, promoted an immediate increase in the frequency and amplitude of uterine contractions. Overt signs of abdominal straining followed within 5–30 min and the first pup was delivered shortly thereafter. These 'induced' deliveries were almost identical to those displayed by control rats; labour continued to completion despite the termination of the stimulation session after 45 min. By contrast, one third of the stimulated animals displayed an interrupted pattern of labour in which events virtually ceased for 30–60 min when stimulation was terminated. Stimulation, however, only advanced labour by 1–2 h in relation to control animals; this was not statistically significant.

Stimulation accelerated the delivery of the first 5 pups in each litter. In both stimulated and control animals, the birth intervals declined over these first few deliveries to reach the lowest values of 5–6 min throughout the remainder of labour. The most common litter size was 12 pups.

The distribution of labour on Day 21 and 22 was bimodal. Seventy per cent of the animals gave birth between 12.00 and 18.00 h on Day 21, a few gave birth during the following night and the remainder formed a second peak on Day 22. All litters of less than 6 pups were born during this later period.

The implications of these results in the context of spontaneous labour are discussed. We conclude that endogenous oxytocin (with perhaps other neurohypophysial hormones) released in pulses of 1–3 mu. every 5 min can promote a pattern of labour on Day 21 of gestation that is almost indistinguishable from that which occurs naturally.

Gonadotrophins including LH have been suggested to play an important role in the etiology of epithelial ovarian cancers. The goal of the present study was to obtain more insight in the mechanism of gonadotrophin action on ovarian surface epithelium (OSE) cells. As the Fas system is known to be a major player in the regulation of the process of apoptosis in the ovary, we investigated whether LH interfered with Fas-induced apoptosis in the human OSE cancer cell lines HEY and Caov-3. Activation of Fas receptor by an agonistic anti-Fas receptor antibody induced apoptosis, as was evaluated by caspase-3 activation, poly(ADP-ribose) polymerase fragmentation, phosphatidylserine externalization and morphological changes characteristic of apoptosis. Co-treatment with LH reduced the number of apoptotic cells following activation of Fas in a transient manner, while LH by itself did not affect apoptosis or cell proliferation. The anti-apoptotic effect of LH could be mimicked by the membrane-permeable cAMP analog 8-(4-chlorophenylthio) cAMP (8-CPT-cAMP), and blocked by H89, a specific inhibitor of protein kinase A (PKA). In conclusion, these findings suggest that LH protects HEY cells against Fas-induced apoptosis through a signaling cascade involving PKA. Although it is plausible that in vivo LH might also enhance OSE tumor growth through inhibition of apoptosis, further research is necessary to confirm this hypothesis.

Abstract

TRH-like immunoreactivity (TRH-LI) was estimated in methanolic extracts of rat tissues and blood by RIA using antiserum 4319, which binds most peptides with the structure pGlu-X-ProNH₂, or antiserum 8880, which is specific for TRH (pGlu-His-ProNH₂). TRH-LI (determined with antiserum 4319) and TRH (determined with antiserum 8880) contents were 8 and 8 ng/g in brain, 216 and 222 ng/g in hypothalamus, 6·5 and 6 ng/g in pancreas, 163 and 116 ng/g in male pituitary, 105 and 77 ng/g in female pituitary, 1 and 0·1 ng/g in salivary gland, 61 and 42 ng/g in thyroid, 12 and 3 ng/g in adrenal, 3 and 0·3 ng/g in prostate, and 11 and 0·8 ng/g in ovary respectively. Blood TRH-LI (antiserum 4319) and TRH (antiserum 8880) levels were 31 and 18 pg/ml in male rats, and 23 and 10 pg/ml in female rats respectively. Unextracted serum obtained from blood kept for at least 1 h at room temperature no longer contained authentic TRH but still contained TRH-LI (males 20·3 ± 3·1, females 15·9 ± 3·0 pg/ml; means ± s.e.m.). Isocratic reverse-phase HPLC showed that TRH-LI in serum is largely pGlu-Glu-ProNH₂ (<EEP-NH₂), a peptide previously found in prostate and anterior pituitary.

In urine, TRH-LI (antiserum 4319) and TRH (antiserum 8880) levels were 3·21 ± 0·35 and 0·32 ± 0·04 ng/ml in male rats and 3·75 ± 0·22 and 0·37 ± 0·04 ng/ml in female rats respectively (means ± s.e.m.). Anion-exchange chromatography on QAE-Sephadex showed that urine of normally fed rats contains both basic/neutral TRH-LI (b/nTRH-LI) and acidic TRH-LI (aTRH-LI) in a ratio of ≈ 40:60, and further analysis by HPLC indicated that aTRH-LI represents <EEP-NH₂. Analysis of food extracts and urine from fasted rats demonstrated that b/nTRH-LI is derived from food particles spilled by the rats during urine collection, while aTRH-LI is endogenously produced. While urinary aTRH-LI levels were higher in female than in male rats (2·99 ± 0·41 vs 2·04 ± 0·20 ng/ml), the daily urinary excretion was similar in both sexes (females 15·6 ± 1·4, males 19·5 ± 2·0 ng/day). Intravenously injected <EEP-NH₂ disappeared from serum with a half-life of ≈ 1 h, and was recovered unchanged and quantitatively in urine. In contrast, when <EEP-NH₂ was administered with food, only ≈ 0·5% was recovered in urine. The urinary clearance rate of serum TRH-LI amounted to 0·52 ± 0·10 ml/min in males and 0·34 ± 0·05 ml/min in females.

In view of the presence of <EEP-NH₂ in the anterior pituitary gland, and the regulation of its content in parallel with gonadotrophins, we examined the possibility that serum <EEP-NH₂ is of pituitary origin and correlates with gonadotrophin secretion. However, treatments that alter pituitary <EEP-NH₂ content and gonadotrophin release had no effect on serum TRH-LI or urinary aTRH-LI.

In conclusion, the TRH-like peptide <EEP-NH₂ is present in rat serum and is excreted into the urine. Moreover, <EEP-NH₂ in serum and urine is not derived from rat food and is probably not of pituitary origin.

Journal of Endocrinology (1997) 153, 411–421

Neurohypophysial hormones are thought to be involved in alterations in fluid balance during pregnancy and delivery. In the course of normal pregnancy intravascular volume is increased whereas sodium restriction is thought to reduce plasma volume and cardiac output. In the present study, we measured the effect of long-term severe sodium restriction on vasopressin (AVP) and oxytocin (OT) levels during normal pregnancy and after delivery.

Fifty-nine healthy nulliparous women were randomized either for a low sodium diet (20 mmol sodium daily) or for a normal diet from week 12 of pregnancy onwards. Circulating plasma levels and urinary excretion of AVP and OT, their neurophysins (Np-AVP and Np-OT) and AVP bound to platelets were determined at regular intervals during pregnancy and after delivery. After completion of the study, women on a sodium-restricted diet were compared with control women on a normal diet using repeated measurement ANOVA with adjustment for potentially confounding variables.

After randomization, a reduction in urinary sodium excretion of, on average, 40–82% was found. In general, no effect of sodium restriction could be demonstrated on the various parameters (0·53<P<0·98) with the exception of a significantly lower 24-h urinary AVP excretion by non-smokers with sodium restriction compared with non-smokers having a normal diet (P=0·018) For all parameters, clear changes were found in the course of pregnancy and puerperium (P<0·0001 to P<0·005). Platelet-bound AVP decreased and Np-OT increased during pregnancy. After birth, free plasma AVP, plateletbound AVP, OT, osmolality, sodium and potassium increased, while Np-AVP and Np-OT decreased.

Although elevated Np-AVP and Np-OT levels during pregnancy seem to indicate increased release of neurohypophysial hormones, pregnancy up to 36 weeks of gestation is accompanied by low circulating AVP and OT levels.

Long-term severe sodium restriction diminishes urinary AVP excretion in (non-smoking) pregnant women, without changing circulating levels of AVP and OT, despite the known reduction in circulating volume. The reduced circulating (platelet-bound) AVP levels during pregnancy, whether or not in combination with severe sodium restriction, support the absence of significant non-osmotic stimulation of AVP during pregnancy.

Journal of Endocrinology (1997) 152, 345–354