The effect of oestrogens and sympathetic denervation on the response to oxytocin of the blood vessels in the hind limb of the dog

Oxytocin induces anti-catabolic and anabolic effects on protein metabolism in the female rat oxidative skeletal muscle

AIMS

Although it is well established that skeletal muscle contains oxytocin (OT) receptors and OT-knockout mice show premature development of sarcopenia, the role of OT in controlling skeletal muscle mass is still unknown. Therefore, the present work aimed to determine OT's effects on skeletal muscle protein metabolism.

MAIN METHODS

Total proteolysis, proteolytic system activities and protein synthesis were assessed in isolated soleus muscle from prepubertal female rats. Through in vivo experiments, rats received 3-day OT treatment (3UI.kg^-1.day^-1, i.p.) or saline, and muscles were harvested for mass-gain assessment.

KEY FINDINGS

In vitro OT receptor stimulation reduced total proteolysis, specifically through attenuation of the lysosomal and proteasomal proteolytic systems, and in parallel activated the Akt/FoxO1 signaling and suppressed atrogenes (e.g., MuRF-1 and atrogin-1) expression induced by motor denervation. On the other hand, the protein synthesis was not altered by in vitro treatment with the OT receptor-selective agonist. Although short-term OT treatment did not change the atrogene mRNA levels, the protein synthesis was stimulated, resulting in soleus mass gain, probably through an indirect effect.

SIGNIFICANCE

Taken together, these data show for the first time that OT directly inhibits the proteolytic activities of the lysosomal and proteasomal systems in rat oxidative skeletal muscle by suppressing atrogene expression via stimulation of Akt/FoxO signaling. Moreover, the data obtained from in vivo experiments suggest OT's ability to control rat oxidative skeletal muscle mass.

Oxytocin: recent developments

Oxytocin is a neurohypophyseal hormone that is produced centrally by neurons in the paraventricular nucleus and supraoptic nucleus of the hypothalamus. It is released directly into higher brain centres and into the peripheral circulation where it produces a multitude of effects. Classically, oxytocin is known for inducing uterine contractions at parturition and milk ejection during suckling. Oxytocin also acts in a species and gender specific manner as an important neuromodulator. It can affect behaviours associated with stress and anxiety, as well social behaviours including sexual and relationship behaviours, and maternal care. Additionally, oxytocin has been shown to have a variety of physiological roles in peripheral tissues, many of which appear to be modulated largely by locally produced oxytocin, dispelling the notion that oxytocin is a purely neurohypophyseal hormone. Oxytocin levels are altered in several diseases and the use of oxytocin or its antagonists have been identified as a possible clinical intervention in the treatment of mood disorders and pain conditions, some cancers, benign prostatic disease and osteoporosis. Indeed, oxytocin has already been successful in clinical trials to treat autism and schizophrenia. This review will report briefly on the known functions of oxytocin, it will discuss in depth the data from recent clinical trials and highlight future targets for oxytocinergic modulation.

An examination of certain factors which might, or do, affect the vascular response to oxytocin

1. A number of different observations were made on anaesthetized rats and some dogs to try to discover why it is that oxytocin is a vasodilator in dioestrus and anoestrus, and develops constrictor properties in oestrus and following sympathectomy.2. In those circumstances in which oxytocin in small or moderate doses raises blood pressure in the rat, the pressor effect is eliminated if the hormone is given during an infusion of adrenaline. In this respect the rat is closely similar to the dog.3. It was confirmed in anaesthetized oestrous rats that intravenous eserine eliminated the pressor response to oxytocin, and it was found that prostigmine did not reduce or change it. Thus, it may be concluded that the effect of eserine depends on its central sympathetic stimulant action and not on a peripheral effect.4. Rats which had been acutely or chronically adrenalectomized responded normally to oxytocin and both eserine and adrenaline were able to reduce, though not wholly eliminate, the pressor type of response. These results indicate that neither the adrenal medulla nor the cortex is essential for the action of oxytocin, nor for a large part of the action of eserine and adrenaline.5. The plasma concentrations of sodium, potassium and calcium were studied in a number of conditions affecting the response to oxytocin, namely, dioestrus, oestrus, and after administration of oestrogens, progesterone, adrenaline, eserine, hexamethonium, and dihydroergotamine. The cation changes observed could not be correlated with the type of response to oxytocin. The only measure found to affect the response was raising the plasma Na concentration by the infusion of disodium hydrogen phosphate. This reduced the pressor effect of oxytocin seen in oestrous female rats and in oestrogen treated males. The pressor response returned before the plasma sodium had fallen to normal levels.6. The administration of the oxytocin analogue, tyrosine-methyl(2) oxytocin which has a high receptor affinity and a low intrinsic activity, prevented the pressor response to oxytocin of oestrous rats, and the vasodilator response in the hind limb vessels of normal dogs. It is concluded that there is probably a single receptor for oxytocin from which both the constrictor and dilator effects are initiated.7. Oxytocin exerted apparently normal effects on the systemic blood pressure of oestrous or dioestrous rats given the beta-blocking agents pronethalol or propranolol.8. The present results, like previous ones, indicate that adrenaline is the factor linking both the gonadal state and the peripheral sympathetic nervous system with the type of response to oxytocin.9. An incidental observation was that male and female rats show differences in their response to sympathetic blocking agents and to a raised plasma sodium concentration.

Histochemical and ultrastructural study of renal cortical necrosis in rats treated with oestrone + vasopressin, and its prevention with a vasopressin antagonist

Renal cortical necrosis was induced by the administration of vasopressin to oestrogen-pretreated rats. Histochemical (succinic dehydrogenase, trichrome, perjod acid Schiff) and electronmicroscopic methods were applied to examine how the vasopressin antagonist d(CH2)5Tyr(Met)AVP influences the development of this renal cortical necrosis. The experiments revealed that vasopressin did not induce hypoxia or necrosis in the renal tubules if the antagonist was administered simultaneously, even after oestrogen pretreatment. The conclusion is drawn that this pressor antagonist may be of value for the prevention of renal cortical necrosis in rats or in human beings.

The distribution time of intravenously injected raffinose and inulin in intact animals and the effect thereon of sex and infusion of oxytocin

1. In anaesthetized dogs whose renal pedicles were ligated the distribution time for I.V. administered raffinose was noted. In males the plasma concentration of raffinose fell for 7-10 min from the time of injection and in females it fell for 12-15 min; thereafter in both sexes the plasma concentration remained constant. 2. During I.V. infusion of oxytocin 10 mug/kg. min, the distribution time for raffinose was 12-16 min in males and 4-6 min in females. 3. During the period before stabilization of plasma concentration the concentration of raffinose was higher in the femoral artery than in the vein in both sexes. 4. In anaesthetized rats whose renal pedicles were ligated the distribution time for I.V. injected inulin was 7-8 min in males and 5 min in females. 5. Following alpha-adrenoceptor blockade with phentolamine the distribution time in both male and female rats was 14-15 min. 6. The sex differences in distribution time, both in normal and oxytocin-treated dogs, may be related to the state of contraction and differential sensitivity or pre- and post-capillary blood vessels and hence, of capillary pressure.

The effects of oestrogen, ACTH and cortisone administration and hypophysectomy on histological changes induced by unilateral renal pedicle clamping

A study was made in rats of the histological changes occurring in the kidney after temporary application of clamps to the renal pedicle following oestrogen, ACTH and cortisone treatments and removal of the pituitary. It was found that the tubular damage is increased to a small extent by oestrogen administration and to a more significant degree by ACTH and cortisone treatments. In rats hypophysectomized for several weeks the tubular necrosis either did not occur at all or only to an extremely mild degree.

The investigations suggest that the change in sensitivity of the tubular epithelial cells to hypoxia may play a role in the development of the renal cortical necrosis induced by hormone treatment.

Neurohypophysial peptide interaction with magnesium in avian vascular smooth muscle

1. The contractile responses of helically cut vascular strips of chickens to vasoactive agents were studied.2. Large pulmonary arteries were contracted by neurohypophysial peptides, but not by angiotensin, acetylcholine, histamine, 5-hydroxytryptamine, bradykinin or eledoisin. The activity of oxytocin was greater than that of arginine vasopressin.3. Vasodilator effects of oxytocin upon small (200-500 mu diameter) mesenteric and muscular arteries were demonstrable, but inconsistent.4. Magnesium potentiated in a parallel fashion the vasoconstrictor effects of oxytocin and of arginine vasopressin.5. Deamino-oxytocin was potentiated by magnesium. This finding suggests a difference between the peptide-protein interactions of tissue receptors and those of neurophysin.

The effect of oxytocin and adrenaline on blood flow in the hind limb of the dog following chronic lumbar sympathectomy

1. In a number of dogs the last two lumbar ganglia were removed unilaterally. At various dates up to the 56th day after operation the dogs were anaesthetized and the effect of oxytocin and adrenaline on hind-leg blood flow studied. Oxytocin alone reduced leg flow in all dogs after operation.2. Until approximately day 11 after operation oxytocin given during an infusion of adrenaline increased leg blood flow, as it does in normal dogs not given adrenaline infusion. After that date it reduced the flow even during adrenaline infusion. The timing of this change suggests that the normal response to adrenaline depends on the presence of undegenerated nerve fibres.3. In one animal the sympathetic nerves were crushed between the last two lumbar ganglia and beyond the last, and hind-leg blood flow frequently measured by means of venous occlusion plethysmography until day 204, when the animal was anaesthetized and acute observations made. Electrical stimulation of the sympathetic chain above the site of crushing caused a reduction in leg flow, indicating that at least some of the nerve supply had regenerated. However, oxytocin reduced leg flow when used alone and exerted no apparent effect in the presence of adrenaline.4. It is suggested that sympathetic nerves to vascular smooth muscle have a function or functions other than transmitter release and that when crushed nerves regenerate the functions do not recover at the same rate.

EFFECT OF HYPOPHYSECTOMY ON THE DEVELOPMENT OF RENAL CORTICAL NECROSIS INDUCED BY POSTERIOR PITUITARY EXTRACT IN OESTRONE PRETREATED RATS

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