Frequency Of Sympathetic Stimulation Research Articles

Secretion of IgA by rat parotid and submandibular cells in response to autonomimetic stimulation in vitro

The major antibody in saliva is IgA, which is actively transported by pIgR expressed by parenchymal cells within the salivary glands. The rate of IgA secretion into saliva is regulated by the autonomic nerves supplying the glands in vivo. This study examined the mechanism of increased IgA secretion into saliva with autonomimetic stimulation. In vitro stimulation of IgA secretion from cells prepared by digestion of rat salivary glands found submandibular cell preparations responded to α- and β-adrenergic stimuli whereas the parotid cells responded only to β-adrenergic stimulation, although cells from both glands responded similarly to cholinergic stimulation. The additional responsiveness of submandibular cells to α-adrenergic stimulation probably reflects the presence of granular duct cells (absent in parotid glands) which are known to secrete protein in response to high frequency sympathetic stimulation. The increased secretion of IgA was not dependant upon increased plasma cell activation since isolated salivary gland plasma cells did not respond to agonists. Further evidence for the regulating role of parenchymal cells in IgA secretion into saliva was revealed by analysis of polymeric immunoglobulin receptor (pIgR) levels expressed on cells. Following in vivo nerve stimulation, there was an increased amount of pIgR expressed on the membrane surface. This was functionally demonstrated in vitro by increased uptake of human IgA by acutely prepared rat salivary cells following stimulation by adrenaline, indicating increased mobilisation of pIgR with stimulation. This study confirms that salivary cells increase the delivery of IgA into saliva by a pIgR-mediated mechanism in response to autonomic stimulation.

Sequential secretion of rat submandibular kallikrein and peroxidase during intermittent sympathetic stimulation

Secretion of peroxidase from acinar cells, true tissue kallikrein (rKI) from granular tubules and total protein have been assessed in sequential samples of rat submandibular saliva formed during intermittent periods of sympathetic stimulation in bursts of 50 Hz 1 s every 10 s at 5 V, for 1 or 2 min each with 2 min intervals between periods. This protocol was repeated twice after 1 h rest pauses. The salivary flow remained greater throughout than when using ongoing burst stimulation. Protein secretion was considerable in the first 2 samples (1 min followed by 2 min) reaching the remarkable concentration of 285 ± 14.4 mg/ml, then it gradually decreased with little recovery after 1 h pauses. rKl outputs followed a similar pattern. Peroxidase, however, showed no greater output in initial samples and continued steadily in similar amounts throughout. When intermittent stimulations were used for 1 sequence only (total stimulation time = 9 min) the glands showed 84 ± 2.6% depletion of rK1 compared to the control glands. A correspondingly large depletion of granules occurred from the granular tubules on the stimulated side, which was greater than with ongoing burst stimulation for 1 h (Garrett et al., 1991). Thus, secretion of rK1 from the granules in granular tubules occurs most efficiently with short sharp bursts of high frequency sympathetic stimulation but soon diminishes. In contrast, peroxidase secretion from acinar cells continues steadily and more modestly for long periods of time undiminished.

Time-course and frequency dependence of sympathetic stimulation-evoked inhibition of vagal effects at the sinus node

Vagally-induced chronotropic responses have been shown to be inhibited after the termination of sympathetic stimulation. We sought to characterize the onset and time-course of the sympathetically evoked inhibition of vagal effects by measuring vagally induced chronotropic responses during concomitant sympathetic stimulation. In anesthetized dogs we recorded lead II of the electrocardiogram, arterial pressure and cardiac cycle length. In 7 dogs, the vagi were stimulated for 15 s every minute before, during and after 10-min trains of sympathetic stimulation. The sympathetic stimulation was applied at frequencies of 0.5, 1, 2, 5 and 10 Hz. During the 1, 2 and 5-Hz trains of sympathetic stimulation, the vagally induced changes in cycle length diminished progressively and thus, were less ( P < 0.001) at 3, 5 and 10 min compared with 1 min into the sympathetic stimulation. The magnitude of the attenuation of vagal effects on cycle length depended ( P < 0.001) on the frequency of sympathetic stimulation. To determine the role of alpha- and beta-adrenergic receptors, we measured vagally-induced changes in cycle length during 5-min trains of sympathetic stimulation (1, 2, 5, 10 Hz) in the presence and absence of phentolamine and propanolol ( n = 6). Both before and after combined alpha- and beta-adrenergic receptor blockade, the vagally-induced changes in cycle length decreased ( P < 0.03) progressively during the 1, 2, 5 and 10-Hz trains of sympathetic stimulation and the magnitude of the inhibition depended ( P < 0.002) on the frequency of sympathetic stimulation. These data show that the effects of short trains of vagal stimulation on cardiac cycle length are inhibited progressively during continuous trains of sympathetic stimulation before and after combined alpha- and beta-adrenergic receptor blockade. Thus, substances other than norepinephrine may contribute to the inhibition of cardiac vagal effects that occurs during a continuous train of sympathetic stimulation.

Effects of Varying Frequency of Sympathetic Stimulation on Chloride and Amylase Levels of Saliva Elicited from Rat Parotid Gland with Electrical Stimulation of both Autonomic Nerves

Simultaneous stimulation of the parasympathetic and sympathetic nerves to the parotid gland of rats elicited saliva at a rate dependent on the frequency of sympathetic stimulation when parasympathetic frequency was maintained at 16 Hz. The flow rate was lowest at 2 Hz (sympathetic), moderate at 5 Hz, and highest at 16 Hz. Cl concentration of the saliva evoked with stimulation of both nerves was highest at the highest frequency and flow rate (maintained at the level of 102 mEq/liter, for 35 min) and lowest at 2 Hz (declining from 40 mEq/liter initially to 28 mEq/liter). With sympathetic nerve stimulation alone, Cl concentration ranged from 27 to 58 mEq/liter when frequency was varied from 2 to 16 Hz, and with parasympathetic stimulation alone (16 Hz), it ranged from 132 to 124 mEq/liter. Amylase concentration of sympathetically elicited saliva was, in contrast, highest at 2 Hz (1.5 times the level at 5 Hz, and twice the level at 16 Hz), and nearly 18-38 times that seen with parasympathetic stimulation alone. The same pattern was found when both nerves were stimulated, and at 2 Hz (sympathetic), amylase concentration was 1.6 times the level at 5 Hz and 2.6 times the level at 16 Hz. When the two nerves were simultaneously stimulated, the total amount of amylase secreted over 35 min was twice as high as that observed with sympathetic nerve stimulation alone, at any frequency. The relation of frequency to norepinephrine concentration was examined. There was no consistent difference in norepinephrine concentration related to variation in frequency of sympathetic stimulation. Only when both nerves were stimulated at 16 Hz was there a statistically significant reduction in norepinephrine concentration of 46%. A relation between frequency of sympathetic stimulation, flow rate, amylase concentration, and Cl concentration was established, but these changes could not be directly correlated with quantitative differences in norepinephrine concentration.

Dependence of non-adrenergic inhibition of cardiac vagal action on peak frequency of sympathetic stimulation in the dog.

1. It is known that stimulation of the sympathetic cardioaccelerator nerve is followed by prolonged inhibition of cardiac vagal action. This prolonged inhibitory action of the sympathetic nerve is not blocked by alpha- or beta-adrenoceptor blockade, and is not duplicated by administration of noradrenaline. It has been proposed that it is due to the release of neuropeptide Y (NPY) from the sympathetic nerve terminals (Potter, 1984, 1985). 2. The present experiments examined whether prolonged inhibition of cardiac vagal action could be preferentially produced by sympathetic stimulation of different temporal distribution. The experiments were performed on anaesthetized, vagotomized dogs, with pharmacological beta-adrenoceptor blockade. 3. In six animals intermittent supramaximal sympathetic stimulation at 20 Hz (1/2 s stimulation, 1/2 s off; train duration 2 min; total 1200 stimuli) produced significantly greater inhibition (P less than 0.01) of cardiac vagal action than did continuous stimulation at 5 Hz (stimulus duration 4 min; 1200 stimuli). 4. In another series the same total period of stimulation (2.5 min; 1200 stimuli) was used and it was found that intermittent sympathetic stimulation of 16 Hz (1/2 s stimulation, 1/2 s off) produced significantly greater cardiac vagal inhibition (P less than 0.02) than continuous stimulation at 8 Hz. In this case the mean frequency of stimulation was constant but the higher peak stimulation frequency attenuated cardiac vagal action more effectively.

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.

Attenuation of reactive and active hyperemia by sympathetic stimulation in dog gracilis muscle.

The effects of sympathetic stimulation (SS) on reactive hyperemia (RH) and active hyperemia (AH) were evaluated in dog gracilis muscles. Sympathetic nerves to the muscle vasculature were activated by electrical stimulation of the obturator nerve during neuromuscular blockade. The frequency of stimulation was adjusted to decrease control conductance by 50%. RH responses to 1 and 5 min of arterial occlusion and AH after 1, 4, 7, and 10 s of tetanic contraction (direct muscle stimulation) at 30% maximal tensions were recorded in the absence and presence of SS. RH peak conductance (Cp), recovery half-time (T0.5), and excess flow (EQ) were significantly attenuated by SS at both occlusion durations. The change in conductance (delta C) during RH was not altered by SS, since the absolute reductions in control and peak conductances were not different. The Cp of AH was reduced at each contraction duration while the delta C was reduced only with 1-s contractions. The T0.5 and EQ of AH were not affected by SS. The data demonstrate that low frequency SS limits the degree of vasodilation associated with both muscle ischemia and tetanic contraction. Furthermore, the more pronounced effects of SS on RH suggest that there is greater inhibition of sympathetic vasoconstrictor influences associated with muscle contraction than muscle ischemia possibly due to the production of a substance during contraction, but not ischemia, that antagonizes sympathetic vasoconstrictor mechanisms.

Cardiac and pulmonary norepinephrine release and removal in the dog.

Norepinephrine extraction and spillover rates were determined in the heart and lungs of anesthetized dogs under resting conditions, during sympathetic stimulation, and during epicardial pacing. The fractional extraction of norepinephrine across the coronary and pulmonary vascular beds was measured from the venoarterial difference in tritiated norepinephrine after infusion of a tracer dose to a steady state level. Cardiac extraction averaged 0.299 +/- 0.03 and pulmonary extraction averaged 0.215 +/- 0.014; extraction was unaffected by sympathetic stimulation or pacing. Norepinephrine spillover from sympathetic nerve terminals in the heart and lungs was measured from the venoarterial difference in endogenous norepinephrine and plasma flow after correction for the extraction component. Cardiac norepinephrine spillover increased linearly with increasing frequency of sympathetic stimulation to 7.44 times resting levels at 2 Hz. During pacing, there was no change in cardiac norepinephrine spillover despite marked changes in heart rate. Norepinephrine spillover was demonstrated under resting conditions in the lung and was greater than observed in the heart. Pulmonary norepinephrine spillover increased with sympathetic stimulation to 4.15 times resting levels at 2 Hz. It is possible to separate the contributions of norepinephrine extraction and spillover to measured venoarterial differences of norepinephrine under physiological conditions in the dog.

B-HT 958, a new alpha-adrenoceptor agonist with a high pre/postsynaptic activity ratio.

B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo [5,4-d] azepine) was a potent agonist at presynaptic and a less potent agonist at postsynaptic alpha-adrenoceptor sites. Presynaptically this was shown in pithed rats by the inhibition of tachycardia evoked by sympathetic nerve stimulation (0.2 Hz). The 50% inhibitory dose (ID50) was 0.3 mg/kg i.v. Moreover in isolated perfused cat hearts, the drug inhibited the tachycardia and the outflow of noradrenaline induced by sympathetic nerve impulses. These effects of B-HT 958 were antagonized by phentolamine or yohimbine. At postsynaptic sites high doses of B-HT 958 increased the blood pressure of decentralized rats. The dose which increased pressure by 30 mm Hg (PD 30) was 46.3 mg/kg i.v. This effect was antagonized by rauwolscine 5 mg/kg i.v. After pretreatment with reserpine (7.5 mg/kg i.p., 18 h) B-HT 958 proved much more potent (PD 30 = 0.6 mg/kg i.v.) and its effect was strongly antagonized by yohimbine but hardly by prazosin. The dose of yohimbine which shifted the dose-response curve of B-HT 958 by the factor of 10 (D10) to the right was 1.8 mg/kg iv., the corresponding dose of prazosin was 1,900 mg/kg i.v. (extrapolated). B-HT 958 showed also alpha-adrenoceptor blocking properties. This was demonstrated presynaptically in pithed rats by the drug-induced augmentation of tachycardia elicited by electrical stimulation at high frequency (6.4 Hz). At postsynaptic sites B-HT 958 antagonized the blood pressure increase-caused by B-HT 920 (alpha 2; D10 = 1.1 mg/kg i.v.) but not that caused by methoxamine (alpha1). It is concluded that B-HT 958 is a partial agonist at peripheral alpha 2-adrenoceptors. In doses of about 1 mg/kg and with low frequency sympathetic stimulation (less than 6.4 Hz) it acts presynaptically as agonist; in this dose the drug acts postsynaptically mainly as antagonist.

Hind limb blood flow in the brush possum (Trichosurus vulpecula)

1. In the possum ( T. vulpecula) sympathetic stimulation, the baroreceptor reflex, exogenous noradrenaline and adrenaline (large dose) produce vasoconstriction in hind limb skeletal muscle. These responses are blocked by α-adrenoceptor blocking agents. 2. Exogenous isoprenaline and adrenaline (small dose) produce vasodilatation which is blocked by propranolol. 3. Sympathetic cholinergic vasodilator fibres do not appear to exist in the possum. 4. At supramaximal frequencies of sympathetic stimulation of the hind limb vascular bed, there is less than maximal response.

Microvascular pressures in rat intestinal muscle during direct nerve stimulation

Microvascular pressures are assumed to be lowered by the vasoconstriction caused during increased activity of the sympathetic nervous system. This hypothesis was tested by direct simultaneous measurements of microvascular pressures and inner vessel diameters in rat intestinal muscle before and during sympathetic stimulation. Measurements of inner vessel diameter and pressure were made with a video micrometer and a servo-null pressure transducer, respectively. Direct sympathetic stimulation at 4, 8, and 16 Hz (5–7 V, 2-msec duration) was used to mimic increased sympathetic activity. Stimulation at 8 and 16 Hz caused a reduction in microvascular pressure and inner vessel diameters throughout the microcirculation. The primary site of pressure reduction was in the smallest arterioles during 8-Hz stimulation and in the largest arterioles during 16-Hz stimulation. These data are interpreted to indicate that the major site of microvascular pressure reduction shifted from the small to large arterioles as the frequency of sympathetic stimulation increased.

Tissue Hyperosmolality as a Causal Factor in Vasodilatation Following Sympathetic Stimulation of the Submandibular Gland

In a previous investigation, parasympathetic activation of the submandibular gland in the cat was found to cause a considerable increase of regional tissue osmolality, the degree of which was related to the evoked functional hyperemia; intra-arterial hypertonic infusion to the resting gland producing tissue hyperosmolality of similar magnitudes caused graded and marked dilatations (Lundvall and Holmberg 1974). It was concluded that hyperosmolality contributes significantly to the functional hyperemia response. In the present study evidence is presented to indicate that tissue hyperosmolality is a mediator of the dilatation associated with sympathetic activation as well. An increase of tissue hyperosmolality, as traced in the venous effluent, was found at all frequencies of sympathetic stimulation (2-16 Hz). At high stimulation rates it sometimes exceeded the resting control level by more than 20 mOsm/kg H2O. There was a direct relation between the degree of venous hyperosmolality and the hyperemia response observed immediately after cessation of stimulation. Comparison of the dilator effects evoked by sympathetic stimulation and by hypertonic infusion to the resting gland indicated that tissue hyperosmolality is an important causal factor for the nerve induced dilatation, especially at low and moderate stimulation rates.

Interaction between sympathetic and parasympathetic salivary nerves in anaesthetized dogs

Sympathetic secretory responses from submandibular glands of anaesthetized dogs were studied while the glands were in a state of “resting secretion” elicited by stimulation of the parasympathetic nerve at a low frequency. Vasoconstrictor and myoepithelial effects of sympathetic stimulation had been abolished by injection of the α-adrenoceptor-blocking dihydroergotamine whereas the sympathetic secretory effect, known to be mediated via β-receptors in this gland, was retained. Sympathetic stimulation accelerated the continuous salivary flow and the sympathetic secretory effect thus obtained differed from that usually described in the following respects: the effect appeared with a very short latency; a pronounced effect was obtained even at low frequencies of sympathetic stimulation; the response was well maintained during the stimulation period. Under these conditions, a similar secretory response to sympathetic stimulation was obtained from the parotid gland of the dog, where sympathetic stimulation alone according to the literature has no, or practically no, secretory effect. In both glands, sympathetic stimulation also markedly accelerated a slow flow evoked by parasympathomimetic drugs. The secretory effects of sympathetic stimulation during a “resting secretion” caused by drugs or parasympathetic stimulation could be imitated with isoprenaline and abolished by β-adrenoceptor blocking drugs.

On the sympathetic control of ventricular automaticity: The effects of a reflex increase of sympathetic discharge

The reflex sympathetic control of idioventricular pacemakers in complete atrioventricular (A-V) block was studied in anesthetized dogs. The A-V block was obtained either by ligating the His bundle or by stimulating the vagus. The procedures employed to enhance reflexly the sympathetic discharge were as follows: occlusion of the carotid arteries, administration of nitroglycerine, inhalation of amyl nitrite and stimulation of the central end of the sciatic nerve. These procedures accelerated reflexly the idioventricular pacemakers to a different degree, but the acceleration was rarely as large as that obtained with high frequency sympathetic stimulation. Elimination of one stellate ganglion almost halved the reflex acceleration of ventricular pacemakers, thereby indicating that the acceleration was mainly brought about by increased neural discharge. The reflex enhancement of ventricular pacemakers was not affected by simultaneous vagal stimulation, thus suggesting that the vagal discharge has little influence on ventricular automaticity. By comparing the results obtained with direct sympathetic stimulation and those obtained with a reflex increase of sympathetic activity, it was found that the unilateral sympathetic discharge during the procedures ranged from 1 to 4 impulses/sec.

Influence of the length of the stimulus period and frequency of sympathetic stimulation on the response of the guinea-pig isolated vas deferens to bretylium, guanethidine and amphetamine.

Abstract The height of contraction of guinea-pig isolated vas deferens preparations in response to pre- or postganglionic sympathetic nerve stimulation at various stimulus frequencies was shown to vary with varying length of stimulation period. Short stimulation periods, as used by some workers, produced suboptimal contractions especially at the lower frequencies. It is suggested that variations in the length of the stimulation period could account for some of the contradictory results reported by various workers using this preparation. Thus the effect of low concentrations of dexamphetamine was qualitatively changed as the length of the stimulation period was increased; with short stimulation periods dexamphetamine impaired the responses to sympathetic stimulation but enhanced them if stimulation was continued until contractions had reached the maximum height obtainable. Higher concentrations of dexamphetamine impaired the contractions, the effect being most marked against the higher stimulus frequencies. Using stimulation to maximal effect a qualitative difference in the blocking actions of bretylium and guanethidine was demonstrated. Bretylium reduced the responses to the higher stimulus frequencies to a greater extent than it reduced the responses to the lower frequencies. Guanethidine reduced the responses to all frequencies so that there was a parallel shift of the frequency/response curve to the right. The possible significance of these findings is discussed.