Effects of sympathetic nerve stimulation in the presence of a slow parasympathetic secretion in the parotid and submaxillary glands of the rabbit

Response of the parotid gland of red kangaroos, Macropus rufus, to phenylephrine stimulation

Intracarotid infusions of l-phenylephrine at 1.0 or 10 nmol kg(-1) min(-1) were accompanied by increases in salivary amylase activity, protein, potassium, magnesium and chloride relative to cholinergically-stimulated saliva. Intravenous infusions of phenylephrine at the same dose rates had a lesser effect on salivary composition particularly protein. Propranolol administered with phenylephrine via the carotid artery, at an antagonist/agonist ratio of 10:1, was much more effective in blocking the phenylephrine-induced changes in salivary composition than equimolar infusion of phentolamine with phenylephrine. It was concluded that alpha(1)-adrenoreceptors were not present in functionally significant numbers in the gland and that the effect of phenylephrine on the kangaroo parotid was mediated by beta-adrenoreceptors. As the phenylephrine dose rates in the kangaroos were comparable with those used to determine alpha-adrenergic responses of eutherian salivary glands and as phentolamine appeared to have minor beta-sympathomimetic activity, at least one subtype of beta-adrenoreceptors in macropods may not be identical to its eutherian counterpart.

Sympathetic and parasympathetic interaction in vascular and secretory control of salivary glands in anaesthetised dogs

The present study was undertaken to examine sympathetic-parasympathetic interactions in the regulation of salivary gland function, with special reference to the possible role of the sympathetic cotransmitter neuropeptide Y (NPY). In dogs anaesthetised with pentobarbitone, electrical stimulation of the parasympathetic nerve to the submandibular gland evoked an increase in glandular blood flow and salivary secretion. Sympathetic nerve stimulation evoked a significant prolonged attenuation of vasodilator and secretory responses to subsequent parasympathetic stimulation. This attenuation was not significantly altered by alpha- and beta-adrenoceptor blockade. Systemic administration of the sympathetic cotransmitter, NPY, mimicked the effect of the sympathetic stimulation by significantly attenuating vasodilatation and salivary secretion. The NPY Y1 receptor agonist, [Leu31, Pro34]NPY and the specific NPY Y2 receptor agonist N-acetyl[Leu28, Leu31]NPY 24-36 both significantly attenuated the vasodilatation and salivary secretion evoked by stimulation of the parasympathetic nerve. The NPY Y1 receptor antagonist, GR231118 significantly antagonised the attenuation of vasodilatation caused by both sympathetic stimulation and the NPY Y1 receptor agonist. GR231118 also inhibited the pressor response of NPY. Intra-arterial injection of methacholine and stimulation of the parasympathetic nerve both caused local vasodilatation in the gland which was significantly attenuated by pretreatment with sympathetic stimulation or the NPY Y1 agonist. The NPY Y2-specific agonist did not attenuate methacholine-induced vasodilatation but did attenuate vasodilatation evoked by parasympathetic stimulation. The results indicate that NPY as a sympathetic cotransmitter may have a role in the regulation of vascular secretory function of salivary glands.

Response of the parotid gland of the brushtail possum, Trichosurus vulpecula, to adrenergic stimulation

Adrenergic stimulation of parotid secretion was investigated in anaesthetised brushtail possums to ascertain fluid secretion rates and salivary composition. Because neither alpha- nor beta-adrenergic stimulation evoked saliva output, infusion of the adrenergic agonists was superimposed on a pre-existing bethanechol-stimulated flow. Isoprenaline infusion (2.4 nmol min-1) increased salivary amylase activity, [protein]; [HCO3]; [PO4] and [Ca], and amylase/Ca and protein/Ca ratios; reduced [Cl]; [K] and osmolality; but did not alter H+ activity; [urea]; [Na]; [Mg]; amylase/protein or saliva/plasma urea ratios. These data are consistent with isoprenaline stimulating acinar secretion of protein, Ca and PO4 but not the ion transport necessary for primary fluid formation at the endpieces and modifying transport of monovalent ions in the excurrent ducts. Consequently, the possum parotid has beta-adrenergic receptors in both the endpieces and excurrent ducts. Phenylephrine infusions at 2.4 and 24 nmol min-1 were without effect whereas phenylephrine at 240 nmol min-1 caused changes in salivary composition which paralleled those for isoprenaline administration but were generally of lesser magnitude. Thus, the possum parotid has few or no alpha-adrenergic receptors and the salivary response elicited was the result of cross-reaction of phenylephrine with beta-adrenergic receptors.

Structural and functional studies of the effects of sympathetic nerve stimulation on rabbit submandibular salivary glands

Continuous sympathetic stimulation at 8-10 Hz caused intense vasoconstriction in the gland, so stimulation was generally given in an interrupted pattern to minimize this detrimental effect on secretion. Only a small increase in fluid secretion occurred; it became thick and tended to block the cannula; therefore in later experiments the main duct was not cannulated. After sympathetic stimulation there was substantial degranulation of acinar cells. However, as this was accompanied by little movement of water, the secreted mucosubstance distended the ductal lumina. The granular tubule cells were unchanged by sympathetic stimulation. Use of selective blocking agents revealed that the sympathetically-evoked secretion of acinar mucin was mediated mainly via beta-adrenoreceptor activation. As stimulation of the sympathetic nerves alone caused little additional formation of fluid, the effects of superimposing continuous low frequency sympathetic stimulation onto a background of low parasympathetic secretion were compared with similar parasympathetic stimulation alone of the contralateral gland. These double nerve stimulations did not augment the volume of fluid secreted, or cause morphological changes additional to those from parasympathetic stimulation alone. Nevertheless, it is likely that, under natural reflex conditions, sympathetic impulses can increase the amount of acinar mucosubstance secreted.

Effect of cold exposure on the amylase activity of submaxillary gland of rats

Cold exposure of rats for 3 h (6 +/- 2 degrees C) caused an increase in amylase activity of the submaxillary gland. This effect was not observed in other salivary glands i.e. parotid and sublingual or in the pancreas. The increase of amylase activity during cold exposure was completely abolished by the beta-receptor antagonist, propranolol, and the alpha-receptor antagonist, phenoxybenzamine, reduced the effect. Administration of actinomycin D to the cold-exposed rats produced a tremendous increase of enzyme activity instead of abolition of the increase as had been expected.

Structural and functional studies of the effects of parasympathetic nerve stimulation on rabbit submandibular salivary glands

The effects were assessed by combining physiological experiments with morphological examination, using light and electron microscopy. Continuous, parasympathetic nerve stimulation, at frequencies varying from 1 to 10 Hz, caused a copious flow of saliva. Both acinar and granular tubule cells showed extensive degranulation, but the effects on tubule cells were the more dramatic. After stimulation, there was a marked loss of acidic mucosubstances from the acinar cells and an almost complete loss of neutral mucosubstances from the granular tubule cells; this was particularly evident with stimulation at higher frequencies (6-10 Hz).

Sympathetic stimulation of amylase secretion during a parasympathetic background activity in the rat parotid gland

The amylase secretion in vivo was examined in the rat parotid gland. A comparison was made between individual stimulation of sympathetic and parasympathetic nerves and simultaneous activation of both nerves. When sympathetic stimulation was superimposed on a parasympathetic background activity, amylase secretion was elicited at a frequency far below the threshold for secretion of fluid, and increased in a frequency-dependent way. This augmented amylase secretion, seen when the two nerves were activated at the same time, gave an amylase output which far exceeded the sum obtained at individual nerve stimulation. The sympathetic amylase secretion obtained in a background of parasympathetic activity seemed to be dependent entirely on beta 1-adrenoceptors. When high sympathetic stimulation frequency (3-10 Hz) was used a reduction in salivary flow was seen, which was accompanied by a reduced amylase output. This effect was counteracted by alpha-adrenoceptor blockade. Isoprenaline, injected intravenously during an ongoing parasympathetic activity, was found to evoke an augmented amylase secretion in a similar way as sympathetic nerve stimulation.

Effects of some alpha-adrenoceptor stimulating and blocking agents on the salivary amylase secretion in the rabbit

In rabbits under urethane anaesthesia parotid secretion of fluid and amylase in response to electrical stimulation of the sympathetic and parasympathetic nerves was measured before and after injections of various agents acting on alpha-adrenoceptors. Amylase secretion in response to sympathetic nerve stimulation at 0.5 and 1 Hz was markedly reduced by clonidine, 0.5-30 micrograms/kg, in a dose related manner. The effect was not due to an altered responsiveness of the gland, since isoprenaline still caused a large release of amylase. Phenylephrine, 10 micrograms/kg, and prazosine, 300 micrograms/kg, had no effect on the sympathetically evoked amylase secretion. Yohimbine in a dose of 0.5 mg/kg increased the amylase output in response to sympathetic nerve stimulation at 0.5 Hz by 70%, while the response at 1 Hz, which is close to maximum for the gland, was not significantly increased. The fluid and amylase secretion produced by parasympathetic nerve stimulation at 1.5, 5.0 and 10 Hz remained unchanged after clonidine, 1.0-30 micrograms/kg, or yohimbine 0.5 mg/kg. In rabbits provided with chronic parotid fistulae fluid and amylase secretion were studied after injections of clonidine, 30 micrograms/kg, and yohimbine, 1 mg/kg. In the conscious animal clonidine reduced not only amylase but also fluid secretion, by around 50 and 30%, respectively, indicating an effect on the activity in both the sympathetic and parasympathetic secretory nerves. Yohimbine increased the output of amylase during feeding, seen as an increased mean output of amylase due to an increased concentration of amylase in the saliva, while fluid secretion remained unchanged. The various experiments suggest that amylase secretion in response to sympathetic activation may be influenced by prejunctional control of transmitter release via alpha-2-adrenoceptors, and that this control may be of physiological significance. Parasympathetically evoked secretion does not seem to be under the influence of a similar control.

Studies on the autonomic nervous system in borderline hypertension

Parameters of the autonomic nervous system were studied in normotensive subjects (NT; standing blood pressure (BP) less than or equal to 125/85 mmHg) and in subjects with borderline hypertension (BHT; 140/90 less than or equal to standing BP less than 160/100 mmHg). No differences in plasma noradrenaline and adrenaline levels were found between NT and BHT subjects, neither at rest nor during exercise at 75% of maximum work capacity. The dose of noradrenaline required to increase systolic BP by 10 mmHg was significantly higher in NT than in BHT subjects (5.13 +/- 0.42 vs 3.50 +/- 0.57 microgram . min -1). No difference between NT and BHT subjects was found in the dose of isoprenaline required to increase heart rate by 20 beats . min -1 (1.21 +/- 0.12 vs 1.09 +/- 0.11 microgram . min -1), suggesting decreased parasympathetic activity in the former group. The enhanced pressor effect of noradrenaline, together with the decreased parasympathetic activity, could explain the elevated blood pressure and heart rate in subjects with borderline hypertension.

Blood flow and secretion in the submaxillary gland of the rabbit during stimulation of the autonomic nerves

The excitation of the chorda-tympani nerve caused a marked increase in the blood flow through the gland and in secretion of saliva; both phenomena had a threshold frequency of around 0.3 Hz and reached a maximum at around 20 Hz, where a 6-fold increase in the blood flow and a salivary secretion of at the most 300 microliters/min were seen. Sympathetic excitation severely retarded the blood flow, which almost ceased at frequencies as low as 3-5 Hz; mostly single shocks of sympathetic stimulation reduced the blood flow through the gland. An on-going parasympathetic activation did not alter the effects of sympathetic stimulation on the blood flow and the parasympathetically produced salivary secretion almost stopped. The experiments support previous results, which have suggested electrical excitation of the sympathetic trunk at or above 2 Hz to cause a vasoconstriction which adversely affects the fluid secretion in the rabbit salivary glands.

Amylase secretion in response to activation of different autonomic receptors in the rabbit parotid gland

The contribution by different autonomic receptors to the amylase secretion from the parotid gland of the anaesthetized rabbit was studied as the response to various parasympathomimetic and sympathomimetic drugs. Amylase secretion by infusions of pilocarpine and parasympathetic nerve stimulation was low, but regularly higher in response to pilocarpine than to parasympathetic nerve stimulation. These effects were reduced to the same level by beta-adrenoceptor block indicating the presence and for pilocarpine also the release of catecholamines, probably from the adrenals. Isoprenaline injections produced a high amylase secretion, that was blocked by atenolol, indicating that predominantly beta 1-adrenoceptors were activated. Phenylephrine was without amylase secretory effects. By accepting isoprenaline maximum as maximum for sympathetically produced amylase secretion, a theoretical frequency-response relationship for amylase secretion by sympathetic nerve stimulation could be calculated: ED50 was 0.9 Hz. The results indicate that under experimental conditions in vivo there are certain differences between the rat and the rabbit parotid glands in the autonomic control not only of fluid, but particularly of amylase secretion.

Effects of carbachol on isoprenaline evoked amylase release from the rabbit parotid gland in vitro

Amylase secretion from dispersed lobules of the parotid gland of the rabbit was studied in response to isoprenaline (10(-8)-10(-5) M) and to carbachol (10(-8)-10(-5) M). The effects of each agent were investigated along and in combination at certain concentrations. Isoprenaline produced a dose-related amylase secretion with an average maximum of 688 units/100 mg, at 10(-5) M. The amylase secretion produced by submaximal concentrations of isoprenaline could be further increased by carbachol, already in low concentrations (10(-8)-10(-7) M), that were subthreshold for amylase secretion. This potentiating effect was seen not only as a larger secretion but also as a greater depletion of amylase from the tissue. In contrast to what is known from experiments in vivo, carbachol in a high concentration (10(-5) M), by activating muscarinic receptors only, released amylase to the same extent as that released by isoprenaline. This concentration of carbachol regularly decreased the amylase secretion evoked by isoprenaline, and may be regarded as unphysiologically high. The increase in isoprenaline evoked amylase secretion, brought about by carbachol in lower concentrations, may be due to an improved transport of amylase, because of secretion of fluid, but could also be caused by augmentation of the beta-adrenoceptor mediated effects.

A morphometric analysis of the autonomic innervation of cat tracheal glands

Quantitative methods have been used to examine the distribution of adrenergic and cholinergic axon varicosities to serous and mucous cells of cat tracheal glands. Cats were injected with 5-hydroxydopamine (5-OHDA) prior to removal of the trachea in order to fill adrenergic vesicles with intensely osmiophilic material. This facilitated the identification of adrenergic varicosities and provided a means by which they could be easily discriminated from cholinergic varicosities. Of all varicosities identified within 10 micrometer of the glands, 90% were cholinergic and 10% were adrenergic. Most of these varicosities were present in interstitial bundles located between acini. A small number of cholinergic varicosities, however, were located within 20 nm of acinar cells, forming close contact junctions. No differential innervation of serous and mucous cells was observed.