The action of neurotransmitter hormones and analogues and cyclic nucleotides and theophylline on electrolyte transport by the excretory duct of the rabbit mandibular gland

Ion Transport and Water Movement

Secretion of water and electrolytes in salivary glands occurs by a dual process involving the formation of a plasma-like, isotonic primary-secretion in salivary acini and its subsequent modification in salivary-ducts by the removal and addition of specific ions. The mechanisms underlying the formation of primary acinar secretion have been investigated with a number of experimental approaches such as electrophysiology, the measurement of ion transport in gland fragments and dispersed acinar cells, and the evaluation of the ionic requirements for secretion in isolated, perfused gland preparations. The accumulated evidence suggests that salivary secretion is formed by a complex interaction between passive and active ion movements across acinar cell membranes, resulting in the trans-acinar movement of CI and Na+ and, by the osmotic gradient which develops, of water. A major consequence of stimulation is the release of K + through Ca++ -and voltage-sensitive channels and its subsequent recycling back into the cells by ouabain- and furosemide-sensitive transport systems. This results in NaCl uptake across the basolateral cell membrane and the subsequent efflux of CI through luminal membrane channels, which also appear to be sensitive to cellular Ca + +. The rates of these various ion movements appear to be, therefore, closely linked and interdependent. Ductal modification of the primary secretion has been studied in microperfused duct preparations. The evidence likewise indicates that it involves interactions between complex conductance pathways in the luminal cell membrane and a Na, K pump present in the basolateral cell membrane and that it is under autonomic and hormonal control. Activation of ductal transport mechanisms results in NaCl reabsorption and KHCO3 secretion. Final saliva thus differs from primary secretion in electrolyte composition and, because water permeability is low in the duct epithelium, becomes hypotonic. Alterations in fluid and electrolyte secretion such as those observed in disease can result, therefore, from disturbances in one or more of these complex transport processes in acinar or duct cells.

Modulation of Ca2+ mobilization by protein kinase C in the submandibular duct cell line A253

The expression of protein kinase C (PKC) isoforms and the modulation of Ca2+ mobilization by PKC were investigated in the human submandibular duct cell line A253. Three new PKC (nPKC) isoforms (delta, epsilon, and theta) and one atypical PKC (aPKC) isoform (lambda) are expressed in this cell line. No classical PKC (cPKC) isoforms were present. The effects of the PKC activator phorbol 12-myristate-13-acetate (PMA) and of the PKC inhibitors calphostin C (CC) and bisindolymaleimide I (BSM) on inositol 1,4,5-trisphosphate (IP3) and Ca2+ responses to ATP and to thapsigargin (TG) were investigated. Pre-exposure to PMA inhibited IP3 formation, Ca2+ release and Ca2+ influx in response to ATP. Pre-exposure to CC or BSM slightly enhanced IP3 formation but inhibited the Ca2+ release and the Ca2+ influx induced by ATP. In contrast, pre-exposure to PMA did not modify the Ca2+ release induced by TG, but reduced the influx of Ca2+ seen in the presence of this Ca2+-ATPase inhibitor. These results suggest that PKC modulates elements of the IP3/Ca2+ signal transduction pathway in A253 cells by (1) inhibiting phosphatidylinositol turnover and altering the sensitivity of the Ca2+ channels to IP3, (2) altering the activity, the sensitivity to inhibitors, or the distribution of the TG-sensitive Ca2+ ATPase, and (3) modulating Ca2+ entry pathways.

Elevation of intracellular cAMP by noradrenaline and vasoactive intestinal peptide in striated ducts isolated from the rabbit mandibular salivary gland

Salivary gland intralobular ducts are responsible for the modification of the electrolyte composition of the primary fluid secreted by the acini. However, the intracellular messengers that regulate this and other intralobular duct cell processes have not been fully characterized. To investigate the possibility that cAMP-mobilizing agonists may be involved, intralobular (striated) ducts were isolated from the rabbit mandibular salivary gland by tissue dissociation and microdissection and maintained in tissue culture overnight. Individual duct fragments were stimulated with the secretory agonists noradrenaline, vasoactive intestinal peptide (VIP) and substance P and their cAMP content measured by acetylated radioimmunoassay. Both noradrenaline and VIP elevated intracellular cAMP content concentration dependently, but substance P did not. The response to noradrenaline was blocked by the beta-adrenoceptor antagonist propranolol, but not by the alpha-adrenoceptor antagonist prazosin. Application of the VIP analogue [D-p-Cl-Phe6, Leu17]-VIP decreased the VIP-induced cAMP response. These results demonstrate that striated intralobular duct cells possess beta-adrenoceptors and peptidergic receptors that are coupled to adenylate cyclase and activated by noradrenaline and VIP, respectively. By elevating ductal cAMP content, these agonists may regulate both the electrolyte content of the primary saliva and the secretion of protein(s) from the ducts.

Secretion by the mandibular gland of the red kangaroo (Macropus rufus) during isoprenaline infusion

Intracarotid infusion of isoprenaline, either alone or in combination with acetylcholine infusion was used to stimulate salivation by the mandibular glands of anaesthetized red kangaroos. Isoprenaline alone (0.20-1.25 eta mol.kg-1.min-1) elicited flow rates ranging from 0.014 to 0.239 ml.min-1 (1.21-28.1 microliters.g gland-1.min-1). Salivary concentrations of sodium, chloride, phosphate and urea were negatively correlated with flow, whereas potassium, calcium, magnesium, hydrogen ion, bi-carbonate, protein, and osmolality were poorly correlated with flow. Relative to cholinergic saliva produced at equivalent flow rates, isoprenaline-evoked saliva had higher osmolality, saliva/plasma urea ratios and concentrations of protein, potassium, magnesium, bicarbonate, and phosphate, but lower sodium, chloride and hydrogen ion levels. At a steady salivary flow (0.5 ml.min-1), superim-position of isoprenaline infusion (0.15 eta mol.kg-1.min-1) on a pre-existing acetylcholine infusion reduced the rate of acetylcholine administration necessary to maintain flow, increased osmolality and the concentrations of protein, urea, potassium, calcium, magnesium, bicarbonate and phosphate and decreased sodium, chloride and hydrogen ion in the saliva. Salivary amylase activity was low and highly variable and the amylase activity/protein ratio fell substantially during isoprenaline stimulation. These results support the conclusion that the enzyme is of extrinsic origin. The response of the kangaroo mandibular gland to isoprenaline stimulation was very similar to that reported for rat mandibular gland, suggesting that the same ion transport phenomena underlie mandibular secretion in both species and probably in therian mammals generally.

The effect of beta-sympathomimetic stimulation on parotid salivation in the red kangaroo (Macropus rufus)

Salivation was stimulated by intracarotid isoprenaline infusion given alone or combined with acetylcholine. By itself, isoprenaline (0.12-1.2 nmol kg-1 min-1) stimulated flow rates of 0.037-0.233 ml min-1 (2.77-10.5 microliters/g gland per min). Salivary Na, Cl, PO4 and total solute concentrations were positively correlated with flow; K, Mg and urea were negatively correlated with flow; and Ca, H+, HCO3, protein and amylase activity were not correlated with flow. Relative to cholinergic saliva, isoprenaline-evoked saliva had higher levels of amylase activity, urea, protein, K, Mg, H+, PO4 and Cl but lower osmolality, Na, Ca and HCO3. At a steady flow (1 ml min-1), isoprenaline infusion (0.3 nmol kg-1 min-1) superimposed on a pre-existing acetylcholine infusion increased salivary amylase activity, protein, urea, K, Mg, Cl and PO4, reduced HCO3 and did not alter Na, Ca, H+ and osmolality. Superimposition of isoprenaline infusion (0.5 nmol kg-1 min-1) on a low-level acetylcholine infusion increased flow rate by 400-900%. Excretion rates of K, Mg, Cl and PO4 were higher and Ca lower than predicted for saliva secreted at equivalent flows during acetylcholine stimulation. Na, H+ and HCO3 were as predicted for the same flow rate under cholinergic stimulation. The simplest coherent interpretation of these data is that isoprenaline affects transport of protein and ions at the end organs, but has little effect on the resting transport characteristics of the striated and excretory ducts of the kangaroo parotid, in accord with the known nerve distribution of this gland.

Effects of acetylcholine, isoprenaline and forskolin on electrolyte and protein composition of rabbit mandibular saliva

1. The major purpose of this study was to investigate cellular regulation of the ductal transport processes in salivary glands which act to modify the electrolyte composition of primary saliva and cause it to become hypotonic. This was achieved using an isolated mandibular gland preparation by observing the effect of different stimuli on the electrolyte composition of saliva secreted at the same flow rate, on the assumption that these stimuli do not influence primary saliva composition. The effects of the same stimuli on the volume of primary fluid secretion and on protein secretion were also observed. Proteins were measured in total and as individual components after their separation by high-performance liquid chromatography. 2. Acetylcholine was used as a 'Ca2+-mobilizing' agonist (i.e. one which both elevates intracellular Ca2+ concentration and activates protein kinase C). Isoprenaline was initially used to elevate intracellular cyclic AMP concentration but was subsequently abandoned in favour of forskolin. 3. Acetylcholine was a very potent stimulus of primary fluid secretion. By contrast, isoprenaline and forskolin were essentially without effect, even when superimposed on acetylcholine stimulation. 4. As judged by saliva electrolyte composition, increasing the concentration of acetylcholine enhanced ductal absorption of Na+ and Cl- and secretion of K+ (and presumably HCO3-). Forskolin had the opposite effect: when superimposed on submaximal acetylcholine stimulation it caused saliva concentrations of Na+ and Cl- to remain high and K+ low (i.e. it inhibited ductal transport processes). The inhibitory effect of forskolin on ductal transport could be overcome by increasing the concentration of acetylcholine, and vice versa. 5. Acetylcholine, isoprenaline and forskolin each increased salivary protein secretion, although the kinetics of secretion differed. The spectrum of proteins secreted in response to the three stimuli was the same. The relative proportions of the individual proteins was influenced by the strength of stimulation (i.e. the proportions at high total protein output differed from those at low total protein output) but not apparently by the nature of the stimulus. 6. Thus, the three major secretory processes in the rabbit mandibular salivary gland respond differently to the two major signal transduction mechanisms. For primary fluid secretion, Ca2+ is stimulatory and cyclic AMP almost without effect; for ductal transport, Ca2+ is stimulatory and cyclic AMP inhibitory; and for protein secretion both Ca2+ and cyclic AMP are stimulatory.

Ion transport and water movement

Secretion of water and electrolytes in salivary glands occurs by a dual process involving the formation of a plasma-like, isotonic primary secretion in salivary acini and its subsequent modification in salivary ducts by the removal and addition of specific ions. The mechanisms underlying the formation of primary acinar secretion have been investigated with a number of experimental approaches such as electrophysiology, the measurement of ion transport in gland fragments and dispersed acinar cells, and the evaluation of the ionic requirements for secretion in isolated, perfused gland preparations. The accumulated evidence suggests that salivary secretion is formed by a complex interaction between passive and active ion movements across acinar cell membranes, resulting in the trans-acinar movement of Cl- and Na+ and, by the osmotic gradient which develops, of water. A major consequence of stimulation is the release of K+ through Ca++- and voltage-sensitive channels and its subsequent recycling back into the cells by ouabain- and furosemide-sensitive transport systems. This results in NaCl uptake across the basolateral cell membrane and the subsequent efflux of Cl through luminal membrane channels, which also appear to be sensitive to cellular Ca++. The rates of these various ion movements appear to be, therefore, closely linked and interdependent. Ductal modification of the primary secretion has been studied in microperfused duct preparations. The evidence likewise indicates that it involves interactions between complex conductance pathways in the luminal cell membrane and a Na, K pump present in the basolateral cell membrane and that it is under autonomic and hormonal control. Activation of ductal transport mechanisms results in NaCl reabsorption and KHCO3 secretion. Final saliva thus differs from primary secretion in electrolyte composition and, because water permeability is low in the duct epithelium, becomes hypotonic. Alterations in fluid and electrolyte secretion such as those observed in disease can result, therefore, from disturbances in one or more of these complex transport processes in acinar or duct cells.

Effects of cyclic nucleotide derivatives on acetylcholine-induced secretion from isolated, perfused rat submandibular salivary glands

The effects of dibutyryl cAMP, 8-bromo-cGMP, 3-isobutyl-1-methyl xanthine (MIX) and isoproterenol were examined. When administered simultaneously with acetylcholine (10(-6) M) in concentrations of 10(-5) M, both isoproterenol and cAMP reduced fluid secretion (16 and 31 per cent, respectively) and enhanced the decay in rate of flow with time, while cGMP and MIX increased salivary volumes (25 and 19 per cent, respectively). Isoproterenol-decreased Na and Cl and increased K and residual anion concentrations. These effects, except for increased K, were also observed with MIX, cAMP and cGMP did not significantly affect cation concentrations but reduced Cl and increased residual anion, although to a lesser extent than isoproterenol or MIX. The cyclic nucleotides did not affect the flow of saliva when added to the perfusate for 40 min after 40 min of acetylcholine stimulation. Prior exposure to cGMP increased the volume of saliva secreted after re-exposure to acetylcholine alone and both cGMP and cAMP increased the Na and reduced the K concentration of the stimulated secretion. These results suggest that cyclic nucleotides are involved in salivary fluid and electrolyte secretion and can modulate the effects of cholinergic stimuli. cGMP may be involved mainly in fluid secretion and, as salivary fluid originates almost exclusively in salivary acini, it may exert its action in acinar cells and activate specific components of the mechanism of primary secretion. cAMP may be primarily involved in transductal electrolyte transport, and may also inhibit certain parts of the fluid secretory mechanism, or reduce the effectiveness of cholinergic stimuli in activating them.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of amiloride on electrolyte transport parameters of the main duct of the rabbit mandibular salivary gland

We have studied the response of the rabbit mandibular main duct perfused in vitro to luminally administered amiloride. The half-maximal inhibitory concentrations (KI) when the duct was bathed in Cl solutions were: for net Na+ transport, 3 X 10(-6) mol l-1; for transepithelial potential difference, 6 X 10(-6) mol l-1; and for transepithelial conductance, 3 X 10(-7) mol l-1. Substitution of the impermeant SO2-(4) anion for Cl- changed the KI for conductance to 3 X 10(-6) mol l-1. Within Cl- -containing media, the time course of the amiloride effect on potential difference showed an early rapid fall of 10 mV with a half-time 2 s, followed by a slower depolarization of 9 mV, and the conductance change followed the slower component of the potential change. In SO2-(4)-containing media, the potential difference and conductance changes followed time courses similar to one another. Finally, experiments on the effect of serosal applications of ouabain revealed that, although, in general, ouabain reduced resistance, it caused an increase in resistance in those ducts where the initial resistance was low. We conclude that: i) luminal Na+ transport occurs via amiloride-sensitive, conductive Na+ channels; ii) the Cl- conductance is the major determinant of transepithelial conductance; iii) the first phase of the potential response is due to blocking of the Na+ conductive channels, whilst the slow phase reflects secondary inhibition of an electrogenic Na+ pump; and iv) duct resistance changes are secondary to alterations in intracellular Cl- concentration.

The concentration of sodium, potassium and chloride in rabbit submandibular saliva during postnatal development

Saliva was collected from the cannulated ducts of submandibular glands of rabbits at various ages from birth to adulthood and analysed for sodium, potassium and chloride. Saliva was obtained at all ages by electrical stimulation of the parasympathetic innervation to the gland. On the day of birth (day 1), salivary sodium was 108.5 +/- 3.4 mM (n = 8) and chloride was 89.9 +/- 3.6 mM (n = 9) falling to adult-like concentrations of 5.7 +/- 0.6 mM (n = 13) and 16.5 +/- 1.2 mM (n = 9) respectively by day 10. On Day 1, salivary potassium was 23.9 +/- 1.8 mM (n = 7) and rose progressively to 76.8 +/- 2.5 mM (n = 8) on day 17. Adult potassium concentration was 30-40 mM. A wide range of flows was obtained at all ages and at each age sodium and chloride concentrations were approximately constant at all flows. Therefore the changes in electrolyte concentration cannot be explained by differences in flow. The results suggest a maturation of ductal transport of sodium, potassium and chloride ions concurrent with the structural maturation of striated duct cells.

Na+ and K+ concentration of rat parotid saliva. Comparison of carbachol and auriculo-temporal stimulation

The effects of carbachol and auriculo-temporal stimulation on the Na+ and K+ concentrations of rat parotid saliva have been compared. The main duct perfused in situ, does not transport Na+ or K+ and is water impermeable. The Na+ concentration of secretion evoked by either stimulus is flow dependent, increasing with increasing flow rate and plateauing at near plasma Na+ levels. At low flow rate the carbachol evoked secretion has a higher Na+ concentration. This is not due to the release of catecholamines since neither sympathectomy nor adrenoceptors block altered the nature of the secretion. The K+ concentration, whilst flow dependent, decreasing with increasing flow rate, was the same for both stimuli.

The action of physalaemin on electrolyte excretion by the mandibular and sublingual salivary glands of the rat

The effect of physalaemin, an undecapeptide belonging to a family known collectively as the tachykinins, on water and electrolyte excretion of the mandibular and sublingual salivary glands of the rat has been investigated and compared to that of acetylcholine. Drugs were administered intravenously or by close-arterial infusion. Physalaemin is a powerful stimulant of fluid secretion by both glands although less potent than acetylcholine. The Na and K excretion patterns in physalaemin-evoked saliva resembled, but were by no means identical to those evoked by acetylcholine and other parasympathomimetic drugs: saliva evoked by physalaemin was considerably poorer in Na and K at all secretory rates. The HCO3 excretion curves, on the other hand, seemed to be identical to those evoked by parasympathomimetic drugs. From an analysis of the Na and K excretion patterns, it can be concluded, both for the mandibular and the sublingual glands, that physalaemin stimulated Na reabsorption and K secretion across the gland duct epitheluim, whereas acetylcholine has the opposite effect. These findings agree nicely with what has previously been demonstrated in vitro in the isolated perfused main excretory duct of the rat mandibular gland.

Modification of salivary duct electrolyte transport in rat and rabbit by physalaemin, VIP, GIP and other enterohormones

The effects of various polypeptide enterohormones and the tachykinin secretogogue, physalaemin, on electrolyte transport by the main excretory duct of the mandibular gland of the rabbit were studied in vitro. Vasoactive intestinal peptide (VIP, 2 X 10(-11) mol 1(-1)) and gastric inhibitory polypeptide (GIP, 10(-11) mol 1(-1)) reduced nett Na+ movement from lumen to interstitium and VIP also reduced the transepithelial potential difference; the effective concentrations of the two hormones lay within the range of normal plasma concentrations. Gastrin (5 x 10(-7) mol 1(-1)) and synthetic secretin (2 x 10(-7) mol 1(-1)) had similar effects but only at concentrations well above the normal plasma levels. Caerulein, an analogue of the octapeptide of cholecystokinin, had no effect on duct function even at a concentration of 10(-6) mol 1(-1). The potent salivary secretogogue, physalaemin (4 x 10(-8) mol 1(-1)), which is an analogue of Substance P, a putative mammalian enterohormone and neurotransmitter substance, caused a marked increase in ductal Na transport (in rat as well as rabbit). It is concluded that VIP and GIP would normally play a role in determining salivary electrolyte composition and it is postulated that their action may be antagonized by a tachykinin such as Substance P.

[The effect of sympathomimetric drugs on exocrine pancreatic function. Studies in the isolated cat pancreas (author's transl)]

This study reports on effects of beta-sympathomimetic drugs on the exocrine function of the isolated cat pancreas. The injection of these drugs resulted in an increase of pancreatic protein and enzyme secretion, the excretion of calcium into the juice and the rate of volume output were not changed. The enzyme secretion was inhibited by different adrenolytic drugs (phenoxybenzamine, practolol, butoxamine), by tetracain, hemicholinium, and atropine. The pancreatic function of cats pretreated by reserpine or 6-hydroxydopamine was not altered. It is concluded, that beta-sympathomimetic drugs influence the pancreatic protein and enzyme secretion in a cholinergic way. These results are consistent with those of other investigators showing a suppressibility of the isoproterenol-induced pancreatic amylase secretion by atropine.

Inhibition of sodium transport by angiotensin II in the main duct of the rabbit mandibular gland isolated and perfused in vitro

The effect of angiotensin II on nett electrolyte transport by the main duct of the rabbit mandibular gland was investigated in vitro using a perfused duct preparation bathed in a Haemaccel -nutrient salt solution. In a bath concentration of 4 X 10(-10) M, angiotensin reduced nett absorption of Na+ and Cl- by about 8% and depolarized the transepithelial electrical potential difference (P.D.) by about 13%; the drug had no effect on ductal transport of K+ and HCO-3. In both lower (4 X 10(-11) M) and higher (4 X 10(-9) M) concentrations, angiotensin had qualitatively similar effects. After exposure to the hormone for about 30 min, Na+ transport and P.D. became unstable and gradually fell away towards zero. It is concluded that angiotensin in physiological concentrations has a specific inhibitory effect on Na+ absorption by salivary duct cells which could arise either from a change in the Na+ pump rate or from a conductance change in the apical or basal membrane of the epithelial cell.