The pharmacology of salivary myoepithelial cells in dogs

A case of acute postoperative transitory sialadenosis of the submandibular glands in a healthy dog

A 1-year-old healthy female cross-breed dog, weighing 4.5 kg, was scheduled for elective neutering. Fentanyl (5 µg/kg) and propofol (4 mg/kg) were administered intravenously (IV) to induce anesthesia, which was maintained with isoflurane and a constant fentanyl infusion rate (10 µg/kg/hr). During the recovery from the anesthesia, the presence of bilateral dense submandibular masses was recognized, as was the excessive secretion of saliva. An ultrasound examination was performed and revealed bilateral abnormally-diffused enlargement of the submandibular salivary glands. A cytology examination was conducted, and no signs of abnormality were found. The size of the swellings subsequently diminished, completely subsiding after 2 hr, as did the hyper-salivation. To the authors' knowledge, this represents the first case report of an acute transient swelling of submandibular glands after general anesthesia in a dog.

Autonomic nervous control of myoepithelial cells and secretion in submandibular gland of anaesthetized dogs

In dog submandibular gland, the activity of myoepithelial cells was assessed by simultaneous measurement of intraductal pressure (P(du)) and subcapsular pressure (P(ca)) using catheter-tip pressure transducers; their resting values were 2.5 +/- 0.21 and 3.0 +/- 0.19 mmHg, respectively (n = 40). Retrograde infusion of saliva (collected from preceding parasympathetic nerve stimulation) increased P(du) (coefficient of 50 mmHg ml(-1) for rates < 1 ml min(-1) and 85 mmHg ml(-1) for higher rates) and P(ca) (coefficient of 0.47 mmHg ml(-1) for all rates). Blood flow changes did not affect P(du) but increased P(ca) (coefficient of 0.04 mmHg ml(-1)). Parasympathetic nerve stimulation increased P(du) but decreased P(ca) abruptly; the response threshold was 0.1 Hz, with maximal responses at 16 Hz. The coefficients for P(du) and P(ca) on salivary secretion to parasympathetic nerve stimulation in glands with spontaneous blood flow (5.3 x 10(-3) and 4.87 x 10(-2) ml min(-1) g(-1) mmHg(-1)) were close to their values in glands with constant-flow vascular perfusion (4.9 x 10(-3) and 3.68 x 10(-2) ml min(-1) g(-1) mmHg(-1)). The finding that P(ca) fell despite concomitant increased blood flow suggests contraction of myoepithelial cells. Additional ductal occlusion further increased P(du) and enhanced the fall in P(ca), suggesting that the myoepithelial cells can contract when distended. Atropine blocked salivary secretion and responses of P(du) and P(ca) to parasympathetic nerve stimulation. ACh elicited responses similar to that of parasympathetic nerve stimulation. VIP caused very scanty salivary secretion and gradual slight increases in P(du) and P(ca); the change in P(ca) was abolished in glands with constant-flow vascular perfusion. Hence, contraction of myoepithelial cells to parasympathetic nerve stimulation is via muscarinic receptors. Sympathetic nerve stimulation increased P(du) and decreased P(ca) abruptly; the response threshold was 0.1 Hz, with maximal responses at 16 Hz. The coefficients for P(du) and P(ca) on salivary secretion to sympathetic nerve stimulation in glands with spontaneous blood flow (3.0 x 10(-3) and 3.2 x 10(-3) ml min(-1) g(-1) mmHg(-1)) were similar to their values in glands with constant-flow vascular perfusion (3.2 x 10(-3) and 3.1 x 10(-3) ml min(-1) g(-1) mmHg(-1)). The finding that P(ca) fell even in glands with constant-flow vascular perfusion suggests contraction of myoepithelial cells. Superimposed sympathetic nerve stimulation immediately enhanced the pressure changes and secretory response to parasympathetic nerve stimulation, indicating that the two autonomic nerves act synergistically to evoke myoepithelial cell contraction. Phentolamine and prazosin but not propranolol and yohimbine blocked the sympathetic enhancement. The finding that phenylephrine, but not clonidine and isoproterenol, abruptly decreased P(ca) in glands with constant-flow vascular perfusion suggests that the sympathetic activation of myoepithelial cells is via the alpha(1)-adrenoceptors.

Nerve interactions in salivary glands

In the salivary reflex, not only secretory cells are activated, but also myo-epithelial cells are contracted to support these cells and promote the flow of saliva, and blood vessels dilate to meet the increased demands of the tissues. The various effector cells often receive nerves from both parts of the autonomic system, and interactions may occur when the nerves act on the same type of effector, or on different types of effectors. While in an experiment electrical stimulation of the sympathetic trunk may decrease a parasympathetic salivary flow by causing marked vasoconstriction, this does not occur in the salivary reflex, since the vasoconstrictors do not take part. On the contrary, the normal sympathetic vasoconstrictor tone of the resting gland is easily overcome by activity in parasympathetic vasodilator nerves when secretion starts. Pronounced synergism can be demonstrated between sympathetic and parasympathetic secretory nerves. In dogs, for instance, in which sympathetic secretion is beta-adrenoceptor-mediated, this is marked in the case of fluid secretion. In rats and rabbits, in which beta-receptors elicit secretion of amylase, the potentiating interaction among the nerves is striking when amylase secretion is considered. Even the random release of acetylcholine from the post-ganglionic parasympathetic axons, by itself insufficient to evoke secretion, can increase the sympathetic effects. Motor nerves interact with secretory nerves by causing myo-epithelial contraction, mechanically promoting secretion. Interactions between the nerves in their long-term regulatory function on the sensitivity of the acinar secretory and myo-epithelial cells can also be demonstrated.

Functional and structural studies concerning the control of activity in zygomatic glands of cats

The zygomatic gland of the cat consists of at least two parts, separated by a septum, and each drains by a separate duct opening on a mucosal ridge postero-medial to the parotid orifice. It is composed on thin-walled ducts and tubulo-acini, containing mainly mucous cells, but scattered cap cells are also present. In cats under chloralose anaesthesia there is a spontaneous flow of extremely viscous saliva and often, in addition, there is a reflexly elicited component to the secretion. In fact, in contrast to the other major salivary glands, the zygomatic gland is easily made to secrete reflexly even in deep] anaesthesia, e.g. by pinching ipsilateral parts of the tongue, by stimulation of the oesophagus mimicking swallowing, or by afferent excitation of ipsilateral lingual, glossopharyngeal or vagal nerves. The efferent link of the reflex arc is contained in the buccal branch of the mandibular nerve. Section of this nerve abolishes these reflexes, and two weeks later a great loss of acetylcholinesterase positive nerves can be demonstrated in the gland. Efferent stimulation of the buccal nerve evokes a lively secretion that is not affected by hexamethonium but is abolished by atropine. Histochemically, adrenergic nerves are also found surrounding the acini and these nerves disappear after excision of the superior cervical ganglion. Electrical stimulation of the cervical sympathetic trunk causes some secretion, mainly by way of beta 1-adrenoreceptors. Myoepithelial cells are present around the tubulo-acini, and indications of the effect caused by their contraction on the flow from the glands have been observed. Such activity can be induced reflexly and it is then abolished by cutting the buccal nerve or by injecting atropine.

Supporting effects of myoepithelial cells in submandibular glands of dogs when acting against increased intraluminal pressure

1. In dogs under chloralose-urethane anaesthesia secretion from the submandibular gland was recorded with the outflow at gland level and at heights of up to 50 cm above the gland.2. With the outflow level increased, secretion elicited by sympathetic nerve stimulation was far better maintained before than after injection of the alpha-adrenoceptor blocking agent dihydroergotamine.3. When the outflow level was raised while no secretion occurred, fluid flowed into the gland. Part of this amount was returned on lowering the outflow to gland level. This volume was assumed to have been accommodated in the distended luminal system, whereas some fluid was obviously lost into the gland tissues.4. Both these fractions of the fluid flowing into the gland when the outflow level was high could be reduced by injecting the alpha-receptor agonist phenylephrine. Bradykinin, which like phenylephrine is known to contract salivary myoepithelial cells, had the same effect on the two inflow volumes.5. It is concluded that myoepithelial contraction reduced the distensibility of the luminal system and in addition supported the acini, thereby diminishing backflow into the glandular tissues and enabling the gland to discharge saliva against a high pressure.6. Morphologically it was found that in resting glands PAS-positive saliva was displaced from the ductal system when the outflow cannula was raised, but it was preserved in the lumina under similar conditions when the myoepithelial cells were being stimulated by phenylephrine or bradykinin.7. Although sympathetic secretion could be maintained against a head of pressure, so long as it was accompanied by myoepithelial contraction, the increased force caused by the secretion led to disruptive damage of striated ducts which are the first part of the luminal system not supported by myoepithelial cells.8. The morphological findings reinforce the belief that contraction of myoepithelial cells gives active support to the underlying parenchyma.

Myoepithelial activity and the appearances of sialograms: a possible role in the formation of sialectasis

In experimental studies on dogs it has been shown that the sialographic filling of the ductal system may be severely restricted if the myoepithelial cells are contracting and it seems likely that myoepithelial activity in man will also influence sialographic appearances. It has also been found that when secretory forces were accompanied by myoepithelial contraction against a sialographic pressure the ersuing damage to striated ducts produced appearances of sialectasis. IT IS WONDERED WHETHER SIMILAR EVENTS MAY SOMETIMES BE RESPONSIBLE FOR SIALECTATIC APPEARANCES IN MAN.

Secretory and motor effects in the submaxillary gland of the rat on intraarterial administration of some polypeptides and autonomic drugs

Bradykinin, oxytocin, physalaemin and some autonomic drugs were injected in to the common carotid artery. Physalaemin evoked secretion and a pressure rise in the submaxillary duct. A duct pressure rise could be elicited by bradykinin which did not evoke secretion. Autonomic blocking agents did not diminish secretion evoked by physalaemin and did not change pressure responses elicited by bradykinin or physalaemin. Neither secretion, nor duct pressure changes could be recorded after administration of oxytocin. In agreement with previous experiments secretion evoked by autonomic drugs was found to be mediated via cholinergic, alpha- and beta-adrenergic receptors, while motor effects were due to activation of cholinergic and alpha-adrenergic receptors.

Contraction of myoepithelial cells in secreting submandibular glands of dogs

In slowly secreting submandibular glands of dogs, a sympathetic effect on myoepithelial cells was demonstrated as an acceleration of the flow, followed by retardation, on short-lasting sympathetic stimulation, particularly after beta-adrenoceptor-blocking drugs.

Influence of autonomic nerves and drugs on myoepithelial cells in parotid glands of cat

When a single electrical shock is applied to either the parasympathetic or to the sympathetic nerves a pressure rise is seen in the parotid duct of the cat, probably due to myoepithelial cell activation. No secretion occurred on single impulse stimulation. When repetitive stimulation was used both divisions of the autonomic nervous system could evoke secretion. The motor response to sympathetic stimulation was mediated via alpha-receptors while secretion mainly was mediated via beta-receptors.

Spontaneous salivation in the rabbit submandibular gland

1. Salivation has been studied in the submandibular gland of the rabbit. A very slow spontaneous salivation took place when all possibility of nerve influence had been excluded. Salivation was not due to ultrafiltration.2. The ;spontaneous' saliva had a mean K concentration of 148 mM and Na concentration of 46 mM. With increasing salivation rate produced by parasympathetic nerve stimulation, K concentration fell to a plateau level of about 30 mM whilst Na concentration fell rapidly to reach the low value of 3 mM, then began to rise again at the higher flow rates.3. Ligation of the submandibular duct produced a reversal of the ion concentrations in spontaneous saliva. By 4 days K concentration was lower and that of Na higher than control values until by 2 weeks the effect was maximal with mean concentrations of 25 mM for K and 153 mM for Na.4. Ouabain increased the spontaneous salivation rate and ethacrynic acid slowed or prevented it altogether. On the basis of the known sites of action of these drugs it is postulated that two pumps are involved in the regulation of spontaneous salivation. There appears to be basal activity of an acinar mechanism pumping NaCl into the lumen, taking water with it. This pump is activated directly or indirectly by the intracellular Na concentration which itself is controlled by an Na-K exchange pump.5. Excitation of the sympathetic trunk produced a small, though definite, increase in salivation rate. There was evidence that myoepithelial cells might also be involved in the sympathetic response and that they were activated by alpha receptor stimulation. Salivation evoked by sympathetic nerve stimulation would seem to be a response to beta receptor stimulation, but the possibility that activation of both alpha and beta receptors was required could not be excluded entirely.

A tion of drugs on denervated myoepithelial cells of salivary glands

1. The pressure in the ducts of the two submaxillary glands was recorded in anaesthetized dogs in which either the superior cervical ganglion had been removed or the chorda-lingual nerve had been cut on one side, 17-30 days earlier. Noradrenaline, adrenaline, phenylephrine, acetylcholine and methacholine were injected to produce pressure rises attributed to contraction of myoepithelial cells.2. After sympathectomy increased pressure responses were obtained with all the drugs, but particularly with noradrenaline. Cocaine increased the effect of noradrenaline on the normal gland but slightly less than sympathectomy.3. Parasympathetic decentralization increased the pressure effects of acetylcholine and methacholine, and in most cases also increased those of the sympathomimetic drugs.4. It is concluded that the myoepithelial cells of this gland normally receive motor impulses from both divisions of the autonomic nervous system, and that many, if not all, of the cells are innervated by both. Disconnecting the gland from the central nervous system by either pathway causes supersensitivity of the classical post-junctional type, and sympathetic ganglionectomy causes in addition a pre-junctional sensitization.

On the function of myoepithelial cells in salivary glands

1. In dogs under chloralose-urethane anaesthesia the submaxillary duct was cannulated and connected either to an open outflow system, in which saliva displaced water, or to a closed pressure recording system.2. The secretory cells, which are supplied with beta-adrenoreceptors, were stimulated with isoprenaline, adrenaline or sympathetic nerve stimulation. Myoepithelial cells, supplied with alpha-receptors, were activated with phenylephrine, adrenaline or sympathetic stimulation, and also with bradykinin. To abolish alpha-receptor stimulating effects of adrenaline and sympathetic nerve stimulation, dihydroergotamine, phenoxybenzamine and phentolamine were used.3. When the secretory cells were activated alone, saliva flowed from the salivary duct, but the flow started late and only a moderately high pressure could be produced in the closed system. Saliva appeared much earlier in the duct when secretion was combined with myoepithelial contraction, and a much higher pressure could be built up in the duct system.4. It is concluded that rapid emptying of saliva in the mouth and a maintained flow at a high rate of the viscous saliva is promoted by contractions of the myoepithelial cells.

The secretory nerves of the parotid gland of the dog

1. Secretory nerve fibres to the parotid gland of the dog have been found in the adventitia of the internal maxillary artery. In dogs under chloralose anaesthesia the course of these fibres from the mandibular nerve to the gland was determined by electrical stimulation at different sites. Secretion induced by these nerves was abolished by atropine but not by hexamethonium.2. In dogs in light barbiturate anaesthesia parotid secretion was evoked reflexly by pouring citric acid on the tongues. After division of the auriculo-temporal nerve and the nerves on the artery this secretion disappeared in about half the experiments and was greatly reduced in the others.3. The maximal secretory rate which could be obtained reflexly from the normally innervated parotid gland was first estimated in light barbiturate anaesthesia. Chloralose was then given and the auriculo-temporal nerve and the nerves on the artery were electrically stimulated together; in about half the experiments this could produce secretion at a rate as high as that obtained reflexly.4. By separate and simultaneous stimulation of different sets of nerve fibres convergence of the secretory fibres to the glandular cells could be demonstrated.5. It is concluded that a large number of post-ganglionic, cholinergic secretory fibres to the canine parotid gland run on the internal maxillary artery. In some dogs a small number of fibres seem to have escaped detection. Possibly they may reach the gland by way of the facial nerve.

Action of kinins on salivary myoepithelial cells

1. Bradykinin and kallidin were injected close-arterially into the submaxillary glands of dogs and the effects on the pressure in the salivary ducts were recorded.2. Both kinins in very low dosage (threshold about 0.001 mug/kg) caused a pressure rise in the salivary duct without eliciting any salivary secretion, indicating that they produced contraction of the salivary myoepithelial cells.3. This effect was obtained even after administration of atropine, dihydroergotamine and propranolol.