Complementary action of substance P and vasoactive intestinal peptide on the rat parotid secretion

Augmentation of the rat parotid salivary secretion to intravenous injections of substance P (SP) occurred when SP was combined with vasoactive intestinal peptide (VIP), or stimulation of the auriculo-temporal nerve in the presence of atropine and the adrenergic blockers, dihydroergotamine and propranolol. The largest increase was obtained when SP (0.5 micrograms kg-1) was used together with subthreshold doses of VIP (84% at 0.05 micrograms kg-1 and 105% at 0.5 micrograms kg-1) and low frequency stimulation (92% at 2 Hz and 97% at 5 Hz), which did not produce any salivary secretion by itself. There was no facilitated secretion when VIP and nerve stimulation were combined. Amylase output was much larger (250-500%) when SP was combined with nerve stimulation (0.5-5 Hz) or VIP (0.005-5 micrograms kg-1) than when SP was used alone. Similar results were obtained in rats where the auriculo-temporal nerve was stimulated during the early phase (24-90 h) of Wallerian degeneration, when the nerve-induced responses were seemingly completely blocked. Our results are consistent with the hypothesis that both VIP and SP contribute to the atropine-resistant parotid secretion, and that they have a complementary role in the rat parotid exocrine function.     

Comparison of the vasodilatory effects of vasoactive intestinal polypeptide (VIP) and peptide-HI (PHI) in the rabbit and the cat

In rabbits, intravenous infusion of increasing doses of peptide-HI (PHI), with direct measurement of the uveal blood flow from a cannulated vortex vein, caused a dose-dependent decrease in the uveal vascular resistance. The maximal effect, a 50% decrease, was achieved with about 60 pmol kg-1 min-1. This is similar to what has previously been reported for vasoactive intestinal polypeptide (VIP). Determination of local blood flows, with radioactive microspheres, showed that i.v. infusion of VIP or PHI (60 pmol kg-1 min-1) caused about the same increase in the choroidal blood flow, while the local blood flow in the anterior uvea was unaffected by both peptides. The most marked effect of VIP was observed in glandular tissues, such as the pancreas, submandibular, parotid and thyroid glands. The pancreas was the only one of these tissues in which PHI caused an increased blood flow. In cats, i.v. infusion of VIP (30 pmol kg-1 min-1) during 5-7 min caused a markedly increased blood flow in several tissues. The vasodilation in glandular tissues was even more marked than in rabbits; 3-15 times the normal compared with two to five times the normal in rabbits. The choroidal blood flow was however significantly decreased and the local flow in the anterior uvea tended also to be reduced. Intravenous infusion of either a higher dose of VIP (60 pmol kg-1 min-1) or of PHI (1800 pmol kg-1 min-1) during 2 min had no effect on the uveal vascular resistance. This dose of PHI had only minor effects on local blood flows in other tissues. The results of the present study indicate that porcine VIP is a more potent vasodilator than porcine PHI in most tissues of both the cat and the rabbit, but that there may be exceptions such as the rabbit uvea. The difference in potency may also vary considerably between the species.     

Pituitary adenylate cyclase activating peptide (PACAP) in salivary glands of the rat: origin,and secretory and vascular effects

Pituitary adenylate cyclase activating peptide (PACAP)-38. injected Lv. to the anaesthetized rat. evoked secretion of saliva from the three major salivary glands. the submandibular glands responding with the greatest and the sublingual glands with the smallest volumes. The parotid saliva was rich in amylase and protein. In vitro. pieces of parotid and submandibular gland tissues released K⁺ and protein in response to PACAP-38. with atropine and adrenoceptor antagonists present. The blood flow in the submandibular gland increased in response to PACAP-38. despite a marked fall in mean aortic blood pressure. PACAP is a vasoactive intestinal peptide (VIP)-like neuropeptide. A comparison between the two peptides showed PACAP-38 to be more effective than VIP with respect to vascular responses and less or equi-effective with VIP with respect to the secretory responses. thus suggesting the involvement of PACAP type I and type II receptors. respectively PACAP-38 and -27 were present in the parotid gland as judged by radioimmunoassay. the concentration of the former being about twice that of the latter. Parasympathetic denervation. by cutting the auricula-temporal nerve. reduced the total parotid gland contents of PACAP-38 and -27 by 23 and 44%. respectively (compared with a previously demonstrated 95% reduction of VIP). Sympathetic de nervation. section of the facial nerve or treatment with the sensory neurotoxin capsaicin did not affect the content of PACAP. The difference in efficacy between PACAP and VIP in the vascular and secretory responses as well as the difference in localization suggest that the two peptides play different physiological roles in the salivary glands.     

Secretin potentiates cholinergically induced glucagon secretion in the mouse

Glucagon secretion is stimulated by cholinergic activation, and it is known that the polypeptides VIP (vasoactive intestinal polypeptide) and GIP (gastric inhibitory polypeptide) both potentiate this cholinergically induced glucagon secretion. In this study, we investigated whether secretin, which shows structural similarities to both VIP and GIP, affects basal and cholinergically induced glucagon secretion in the mouse. Secretin was injected i.v. to mice at dose levels varying from 0.53 to 17 nmol kg-1, and plasma samples were taken at 2, 6 and 10 min following injection. It was found that secretin in this wide dose range did not affect basal glucagon concentrations. When the cholinergic agonist carbachol was injected i.v. at 0.16 mumol kg-1, plasma glucagon levels were elevated; at 2 min at 0.84 +/- 0.04 ng ml-1 compared to 0.31 +/- 0.02 ng ml-1 in controls (P less than 0.001). A combination of carbachol and secretin (4.25 nmol kg-1) enhanced plasma glucagon levels to 1.22 +/- 0.07 ng ml-1. Thus, secretin potentiated carbachol-induced glucagon secretion by 70% (P less than 0.001). Concomitantly, plasma glucose levels were elevated: 10.8 +/- 0.4 mmol l-1, compared to 9.2 +/- 0.4 mmol l-1 in controls (P less than 0.001). We conclude that secretin, while being without effect on basal glucagon secretion, markedly potentiates cholinergically induced glucagon secretion in the mouse, resulting in increased plasma glucose levels.     

Effects of intravenous calcitonin gene-related peptide (CGRP) and substance P on the blood-aqueous barrier in the rabbit

The irritation response of the rabbit eye to trigeminal nerve stimulation, which includes a breakdown of the blood-aqueous barrier (BAB), seems to be due to the release of substance P (SP) and calcitonin gene-related peptide (CGRP). In order to assess the relative importance of these two peptides for the barrier effect, and the role of arachidonic acid metabolites (AAM) in the response, we have studied the effects of intravenous injections of the peptides on the permeability of the blood vessels of the anterior uvea and the BAB using labelled albumins. At a dose of 120 pmol kg-1 there was marked leakage of labelled albumin into the aqueous humour in animals under pentobarbital anaesthesia. The leakage was enhanced by sympathotomy. In conscious animals 5 pmol kg-1 CGRP caused enhanced leakage from the blood vessels of the ciliary processes in those pre-treated with biperiden in order to abolish the cholinergic vasoconstrictor tone in the anterior uvea. 24 and 120 pmol kg-1 CGRP caused marked leakage of albumin and a breakdown of the epithelial part of the BAB. These effects were not modified by biperiden pre-treatment, but markedly reduced by pre-treatment with indomethacin. The protecting effect of indomethacin was lost when biperiden was given as well. SP did not cause a leakage with 5 nmol kg-1 and only moderate leakage with 25 nmol kg-1. This effect was abolished by pre-treatment with indomethacin but not if indomethacin was combined with biperiden.(ABSTRACT TRUNCATED AT 250 WORDS)     

Atropine treatment induced cholinergic supersensitivity at receptor and second messenger levels in the rat salivary gland

Physiological, biochemical and morphological correlates of chronic treatment of rats with the classical muscarinic antagonist atropine for 14 days (20 mg kg-1 day-1 s.c.) were studied in submandibular salivary glands. The amount of saliva collected from submandibular glands following a single injection of isoproterenol (30 mg kg-1 i.p.) was significantly larger and had higher protein concentration in rats treated with atropine than in saline-treated animals. In the glands of atropine-treated rats a conspicuous increase in the amount of rough endoplasmic reticulum (RER) along with a decrease in the mucous volume was observed in the acinus when examined by light microscopy. Several biochemical changes were observed in an enriched plasma membrane fraction from the submandibular gland of the atropine-treated rats: (1) an increase in the number of muscarinic antagonist binding sites (31 + 3.4%), (2) a decrease in the specific activity of basal adenylate cyclase, (3) a significantly lower Vmax of the adenylate cyclase in the presence of GTP (10 microM) and varying concentrations of Mg2+ (0-22.5 mM) with no apparent change in affinity of the enzyme for Mg2+ but (4) higher magnitude of stimulation in the presence of GTP (100 microM), vasoactive intestinal polypeptide (5 microM), isoproterenol (100 microM), NaF (10 microM) and forskolin (10 microM). There was however no change in the density of beta-adrenergic receptors upon atropine treatment. In tissue slices from the submandibular glands of atropine-treated rats we found lower basal cAMP levels (decrease 29 +/- 6.9%) and no significant change in the phosphatidylinositol breakdown stimulated by carbachol (10(-6) to 10(-4) M). It appears that chronic blockade of an inhibitory muscarinic input to the adenylate cyclase system is compensated by lowered adenylate cyclase activity. Phosphoinositide metabolism is not subject to the same adaptation, suggesting that cAMP may be the pivotal second messenger in the supersensitive salivary response.     

Neuropeptide Y antagonises secretagogue evoked chloride transport in rat jejunal epithelium

Neuropeptide Y (NPY) inhibits electrogenic Cl secretion in rat jejunal epithelium under voltage clamp conditions. This effect is dependent upon endogenous eicosanoid formation since it is blocked by the cyclooxygenase inhibitor, piroxicam, which itself has an inhibitory action upon chloride secretion. A number of chloride secretagogues have been examined for their ability to restore the antisecretory effects of NPY. Data presented here shows that NPY responsiveness is restored, in piroxicam pretreated tissues, by vasoactive intestinal polypeptide (VIP), forskolin, prostaglandin E₂ (PGE₂) isobutyl-1-methyl-xanthine (IBMX) and dibutyryl cAMP added prior to the neuropeptide. While all these agents cause chloride secretion by elevating intracellular cAMP, NPY is also effective in inhibiting the secretory effects of carbachol (CCh) and substance P (SP), agents believed to act by raising intracellular calcium (Ca_i). Although there is evidence that NPY can inhibit adenylate cyclase, its ability to attenuate chloride secretion brought about by secretagogues acting through both adenylate cyclase and calcium mechanisms, implies that NPY has either a more general fundamental mechanism or has multiple interactions with different second messenger systems.This work was supported by the Medical Research Council     

Effects of angiotensin II on blood flow in rat submandibular gland

The effect of angiotensin II (Ang II) was studied on blood flow in the submandibular gland and tongue in male rats. Blood flow changes were determined with laser Doppler flowmetry and Ang II was infused into the common carotid artery before and after i.v. doses (18 nmol kg-1) of the angiotensin II antagonist saralasin. Angiotensin II (10-60 pmol min-1) dose-dependently increased blood pressure and tongue blood flow, whereas glandular blood flow decreased at all of the doses used. After saralasin administration the angiotensin II effects on blood pressure, tongue and glandular blood flow were significantly diminished (glandular blood flow reduction was diminished from 29%-3%, P less than 0.005, n = 9). However, the responsiveness of these 3 parameters to local infusions with noradrenaline (0.75-3.0 pmol min-1) was unaffected by saralasin. The dose of saralasin used in the present study did not affect any of the parameters on it's own. Our results show that vascular receptors sensitive to angiotensin II operate in the submandibular gland but not in the tongue.     

Effects of intravenous calcitonin gene-related peptide (CGRP) on local blood flow in the cat

The effects of i.v. calcitonin gene-related peptide (CGRP) on regional blood flow, vascular resistance, heart rate and cardiac output in cats were studied using the microsphere method. Three series of experiments were performed. In the first there was no pre-treatment of the animals. In the second the animals were pre-treated with indomethacin (5 mg kg-1) in order to prevent the formation of prostaglandins. In the third series the ganglionic blocking agent hexamethonium bromide (80 mg kg-1) was given in order to prevent autonomic reflexes. CGRP was given as an i.v. infusion. Two doses were tested in each series. Animals with no pre-treatment and those pre-treated with indomethacin received first 60 pmol kg-1 infused over a 5-min period and then 180 pmol kg-1 infused over a 3-min period. Animals under ganglionic blockade received the same dose as the total infusions in the other series (240 pmol kg-1) infused over 4 min and a second infusion of 1.2 nmol kg-1 over 5 min. Vasodilatory effects were observed in most of the tissues tested, but there were marked differences in sensitivity. The most sensitive tissues seemed to be the lacrimal, submandibular and parotid glands, the nictitating membrane, the tongue and the gallbladder. The patterns of sensitivity were similar in all the series, indicating that neither prostaglandins nor autonomic reflexes were involved in the effects. In experiments with animals under ganglionic blockade, an increase in blood pressure, concomitant with an increase in heart rate, was seen 5 min after the infusions ended. Thus, it is likely that the peptide exerts a direct positive chronotropic effect on the feline heart.     

Complementary role of vasoactive intestinal polypeptide (VIP) and acetylcholine for cat submandibular gland blood flow and secretion I. VIP release

The effects of parasympathetic and sympathetic nerve stimulation on VIP release in relation to blood flow and secretion were studied in the cat submandibular salivary gland. Parasympathetic nerve stimulation caused a marked VIP overflow (over thousand fold increase in VIP output) into the venous effluent from the gland which was simultaneous with profuse salivation and an about 10–15 fold increase in blood flow. The VIP output was dependent on the stimulation frequency, the duration of the stimulation period as well as the glandular blood flow. At 15 Hz maximal VIP output (about 4 fmol per impulse) was obtained after about 5 min of stimulation simultaneously with the maximum of the maintained phase of vasodilation. About 50 pmol VIP was recovered in the venous effluent from the gland during 1 h of maximal nerve stimulation. The VIP output after 1 h, was only about 20% of maximal, however, suggesting that the storage reserves and/or resupply of VIP might be running out. Under physiological conditions (frequencies ≤ 6 Hz) it was estimated that the axonal transport mechanism should be sufficient for replacement of VIP. At lower frequencies (2 and 6 Hz) the VIP output was parallel to vasodilation and secretion, while at a high frequency (15 Hz) a much more pronounced VIP output was seen. The increased overflow at 15 Hz may either be due to an actual increase in release or to a saturation of local VIP inactivating mechanisms. When stimulated simultaneously, the parasympathetic vasodilator mechanism seemed much more potent than sympathetic vasoconstriction. Since VIP may be present in cholinergic neurons, data from the literature concerning acetylcholine release are discussed in relation to the observed VIP output.     

Galanin: effects on basal and stimulated insulin and glucagon secretion in the mouse

Galanin was recently demonstrated to be a neuropeptide in intrapancreatic nerves. In this study, the effects of galanin on basal and stimulated insulin and glucagon secretion in the mouse were investigated. Galanin, injected intravenously at dose levels ranging from 0.53 to 8.5 nmol kg-1, markedly lowered basal plasma insulin levels and transiently increased basal plasma glucagon levels. Furthermore, galanin induced hyperglycaemia: plasma glucose levels were 11 +/- 0.2 mmol l-1 2 min after injection of galanin (4.25 nmol kg-1) compared with 9.3 +/- 0.3 mmol-1 in controls (P less than 0.001). Galanin also impaired the plasma insulin response to either glucose or the cholinergic agonist carbachol. Thus, galanin (4.25 nmol kg-1) inhibited the plasma insulin response to glucose by 65% (P less than 0.001), and that to carbachol by 85% (P less than 0.001). Moreover, glucose abolished the galanin-induced plasma glucagon response. Also, galanin and carbachol exerted additive stimulatory effects on glucagon levels. It is concluded from this study in mice that galanin inhibits basal and stimulated insulin secretion, stimulates glucagon secretion, and induces hyperglycaemia. It is suggested that the intrapancreatic neuropeptide galanin is of importance in the regulation of both insulin and glucagon secretion.     

Calcitonin gene-related peptide in rat salivary glands: neuronal localization, depletion upon nerve stimulation, and effects on salivation in relation to substance P

Calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibres occurred predominantly around blood vessels and large ducts and, to a minor extent, around acini and small ducts in the parotid, sublingual and submaxillary glands of the rat. Double immunostaining showed most of the CGRP-containing nerve fibres to contain substance P. However, the vast majority of substance P-immunoreactive periacinar nerve fibres in the parotid and submandibular glands lacked CGRP. After parasympathetic denervation of the parotid gland by section of the auriculotemporal nerve these periacinar substance P-immunoreactive nerve fibres disappeared almost completely, whereas the number of substance P/CGRP-immunoreactive nerve fibres seemed unchanged. After this operation the total amount of substance P in the parotid gland was reduced by about 90% as judged by radioimmunoassay; in denervation experiments the facial nerve was found to contribute to the residual substance P content. In contrast, the contribution of the auriculotemporal nerve to the CGRP content of the gland was small; the reduction in CGRP after section of the nerve was 20%. The facial nerve and the dorsal root nerves (C3 and C4) contributed to the CGRP content with about 50%. The source of the remaining 30% of the parotid gland CGRP is unknown. It is not the sympathetic nerve: sympathetic denervation resulted in a marked increase in CGRP, regardless of whether the auriculotemporal nerve was intact or not. Upon long-lasting electrical stimulation of the auriculotemporal nerve at a high frequency the parotid gland content of CGRP was gradually reduced, indicating depletion of this peptide in response to nerve stimulation. Intravenous injections of CGRP evoked no salivary flow; however, a release of amylase was revealed. Also, when CGRP was tested on isolated parotid gland lobules amylase was released into the medium. When, in vivo, CGRP was injected in combination with substance P, the substance P-evoked flow of parotid and submaxillary saliva was markedly enhanced. In addition, CGRP enhanced the in vivo secretory response to parasympathomimetics and to vasoactive intestinal peptide. The localization of CGRP-containing nerve fibres suggests that CGRP is involved in the regulation of secretion and blood flow of salivary glands. CGRP may interact positively with acetylcholine and certain nonclassical transmitters, and it may be involved (together with other neuropeptides) in the atropine-resistant parasympathetic secretion occurring in the glands under study.     

Beta-adrenergic receptors and salivary gland secretion during aging.

Beta-adrenergic signal transduction is primarily responsible for the control of the protein secretions by salivary cells. To examine the relationship between beta-adrenergic signal transduction and beta-adrenergic agonist-stimulated salivary secretion, we simultaneously assessed beta-adrenergic receptor number and pilocarpine-isoproterenol-stimulated salivary flow and secreted proteins in parotid and submandibular glands from 3-, 12- and 24-month-old female NNIA F-344 rats. There were no age-related changes in the density of beta-adrenergic receptors in the parotid gland or in the submandibular gland. In the parotid gland there was a significant increase in saliva flow rate in the oldest age group and no changes in the amount of total proteins secreted over 30 min. However, when normalized to gland weight, flow rate was unchanged and the amount of total secreted proteins decreased with age. In the submandibular gland there were age-related increases in both absolute volume and total secreted protein, but when normalized to gland weight there were no longer changes with age. Changes in flow rate were paralleled by reciprocal changes in protein secretory function such that changes in the salivary protein concentrations for the most part were unchanged with age for both the parotid and the submandibular gland. These parameters were compared to our previous data on adenylate cyclase activity, and collectively, these data suggest that in the submandibular gland salivary secretory function does not correlate with changes in beta-adrenergic receptor density or isoproterenol-stimulated adenylate cyclase activity.     

Lowering of interstitial fluid pressure in rat trachea after substance P alone and in combination with calcitonin gene-related peptide

Neurogenic inflammation is mediated following a release of sensory neuropeptides including calcitonin gene-related peptide (CGRP) and substance P (SP). The release of peptides can be mediated chemically with capsaicin, or electrically by stimulation of the vagal nerve, both inducing vasodilation, plasma protein extravasation and lowering of interstitial fluid pressure (Pif) which will contribute to the enhancement of oedema formation. AIM: Lowering of Pif has previously been demonstrated following intravenous (i.v.) treatment with CGRP, but it was not possible to demonstrate that SP had this effect under the same condition. METHODS: Micropuncture measurements of Pif in the submucosa, without opening of the trachea, was conducted on rats anaesthetized with pentobarbital sodium (50 mg kg-1) and cardiac arrest was induced with i.v. KCl. RESULTS: Pif in vehicle-treated animal averaged -1.7 +/- 0.4 (SD) mmHg (n = 9). Intravenous injection of SP induced significant lowering of Pif compared with control, both at low dose (0.47 nmol kg-1 body weight) with 1 min distribution time (P < 0.007, -4.2 +/- 2.3 mmHg) and at high dose with seconds of distribution time (P < 0.03, -4.2 +/- 1.6 mmHg). The same response was observed after treatment with SP co-injected with CGRP. CONCLUSIONS: Substance P alone or in combination with CGRP is able to induce a rapid lowering of Pif showing that this peptide is a potent agent in increasing the hydrostatic driving pressure initially transporting fluid into the tissue during an acute inflammatory reaction.     

Acinar degranulation in the rat parotid gland induced by neuropeptides

Vasoactive intestinal peptide (VIP, 4 microg kg(-1) min(-1)), substance P (3 microg kg(-1) min(-1)) and neurokinin A (2.5 microg kg(-1) min(-1)) were infused intravenously for 30 min in anaesthetized rats and the effects of these peptides on the parotid gland were examined. VIP reduced the numerical density of parotid acinar secretory granules, storing proteins, by 29 % and the glandular amylase activity by 33 %. Substance P reduced the number of secretory granules by 18 % but the amylase activity was unchanged. These results make VIP and substance P likely contributors to the parasympathetic non-adrenergic, non-cholinergic (NANC)-evoked parotid acinar degranulation. Neurokinin A, on the other hand, caused no reduction in granular number but reduced the glandular amylase activity by 19 %, indicating vesicular protein secretion.     

Comparison of vasoactive intestinal peptide-mediated protein phosphorylation in human lymphoblasts and colonic epithelial cells

Vasoactive intestinal peptide (VIP) induces phosphorylation of a basic 38,000 mol. wt protein in a human lymphoblastic cell line (Molt 4b) and a human colon carcinoma cell line (HT29). In both cell types, VIP interacts with specific high affinity receptors to activate adenylate cyclase and cAMP-dependent protein kinase. The two cell types appear to express homologous receptors with similar affinity and specificity for VIP, but the colonic epithelial cells express a greater number of receptors. HT29 colonic cells also exhibit a greater stimulation of adenylate cyclase and a higher phosphorylation index for the 38,000 mol. wt protein in response to VIP. This 38,000 mol. wt protein, which is phosphorylated in the presence of VIP, appears to be identical in both cell lines; it is phosphorylated in both lymphoblasts and colonic epithelial cells in the presence of forskolin, but not in the presence of phorbol 12-myristate 13-acetate. Phosphorylation of this 38,000 mol. wt protein may be an important step in VIP regulation of water and electrolyte secretion from colonic epithelial cells, and in VIP regulation of immunoglobulin and lymphokine secretion from lymphocytes.     

Forskolin activation of adenylate cyclase in rat myocardium with age: effects of guanine nucleotide analogs

Beta-adrenergic and post-receptor activation of adenylate cyclase decreases with age in the rat myocardium. Forskolin-stimulated adenylate cyclase activity decreases with age to the same extent as NaF or isoproterenol stimulation, suggesting that a loss of catalytic unit activity accounts for the loss of activity with age. However, recent evidence suggests that there are both a guanine nucleotide-dependent and independent component of forskolin activation. We assessed the possible role of each of these components with age by measuring forskolin dose-response stimulation of adenylate cyclase activity in the presence and absence of guanosine 5'-O-(2-thiodiphosphate) (GDP-beta S) and beta,gamma-imidoguanine 5'-triphosphate (Gpp(NH)p) in myocardial membranes from F-344 rats of 3, 12, and 24 months of age. Maximal forskolin stimulation of adenylate cyclase was least in the 24- (121 +/- 21 pmol cAMP/min/mg) as compared with either the 12- (212 +/- 29) or 3- (190 +/- 19) month-old rats. The presence of GDP-beta S had no effect on either the EC50 or maximum activity, and the age-related decline persisted. Gpp(NH)p plus isoproterenol enhanced forskolin activation but the effect was additive and not synergistic. There was no effect on the EC50, and the maximum activity was least in the 24-month-old rats. These data reconfirm the loss of catalytic unit activity with age and indicate that the reduced activity with age is independent of the presence or absence of activated G-protein.     

The inhibition of salivary secretion by histamine H2-antagonists—a study on the cat submandibular gland

The aim of the present work was to establish whether the secretory process can be influenced by histamine H₂-receptor antagonists, burimamide and metiamide. These drugs were applied intravenously and the secretion was evoked by the electrical stimulation of the chorda tympani nerve or by carbachol (i.v.). In addition to the measurements of the flow of saliva, the blood flow through the gland was measured in some experiments. Both H₂-antagonists significantly reduced the rate of salivary secretion induced by the chorda tympani stimulation. The experiments with burimamide did not permit the calculation of dose-response relationship. From the experiments with metiamide the ED₅₀ was 4.6 μmol/kg and E_max was 30% reduction of secretion. The secretory response to carbachol was not diminished by burimamide. In addition to the effect of metiamide on salivation, the reduction of the blood flow through the gland was observed: the effect on the blood flow was significantly smaller, and slower in onset, than the effect on salivation. These results support the hypothesis that H₂-receptors are involved in the process of salivary secretion. Histamine effects on glandular elements seem to be more significant than its effect on the blood vessels.     

Vasoactive intestinal polypeptide (VIP): effects in the eye and on regional blood flows

The effect of vasoactive intestinal polypeptide (VIP) on regional blood flows was studied with labeled microspheres in albino rabbits. Intravenous injection of 500 ng VIP/kg b.w. during 100 s did not change the arterial blood pressure significantly, but caused a rise in intraocular pressure (IOP) and an increase in the choroidal blood flow by 35%, while the blood flow through the anterior uvea was unaffected. The most pronounced vasodilation was observed in the pancreas, the thyroid gland and the parotid gland. In these tissues local blood flow increased by more than 100%. Other tissues, in which this dose of VIP produced vasodilation, were the submandibular gland, the eyelids, the nictitating membrane, the choroid plexus and the heart muscle. Ganglionic or muscarinic blockade had little or no effect on the VIP-induced vasodilation in most of the tissues. Intracameral injection of VIP (1 microgram) produced vasodilation in the iris and the ciliary body, but did not affect IOP. VIP had no apparent effect on the pupil size or the blood-aqueous barrier. In experiments with direct blood flow determination from an opened vortex vein intravenous infusion of VIP, 100 ng X kg-1 X min-1 b.w., during five minutes reduced the uveal vascular resistance by about 50%. This study shows that VIP is a potent vasodilator in many tissues at doses hardly affecting the arterial blood pressure and supports the suggestion, that VIP is responsible for the non-cholinergic vasodilation in the eye caused by facial nerve stimulation.     

Somatostatin inhibits prolactin secretion by multiple mechanisms involving a site of action distal to increased cyclic adenosine 3',5'-monophosphate and elevated cytosolic Ca2+ in rat lactotrophs

The release of prolactin (PRL) from a clonal cell-line of anterior pituitary cells (GH4C1) was inhibited by somatostatin (SRIH) in a dose-dependent manner (ED50 nM). The inhibition (20% of control levels) was detectable within 50 s and maximal within 90 s. Thyroliberin (TRH) enhancement of PRL secretion was biphasic. SRIH inhibited both phases equally. Ionomycin in combination with the phorbol ester, TPA, mimics the TRH-elicited PRL release, and SRIH partly inhibited this effect. SRIH had no effect on TRH-stimulated formation of inositol trisphosphate, and only small effects on TRH-activated adenylate cyclase. Vasoactive intestinal peptide (VIP) and forskolin stimulated cAMP formation and PRL release potently. SRIH inhibited both effects of VIP and forskolin, and there was a close correlation between the inhibition of PRL secretion and cAMP accumulation. 8-Bromo-cAMP enhanced PRL release, an effect that was also partly reduced by SRIH. The Ca2+ channel activator, BAY-K-8644 and high extracellular K+ increased PRL release, and SRIH caused a partial reduction in the release response to both secretagogues. SRIH lowered [Ca2+]i, and markedly reduced the rise in [Ca2+]i elicited by TRH, VIP and K+. SRIH did not influence the Ca2+ spikes recorded in Na+-free solution, and had no effect on the TRH-induced membrane potential changes. Our results demonstrate that SRIH may inhibit PRL release from GH4C1 cells by (1) inhibiting hormone-sensitive adenylate cyclase, (2) blocking the effect of cAMP and (3) lowering [Ca2+]i. None of these effects is, however, sufficient to explain all the effects of SRIH, suggesting that SRIH also exerts a major action at a step subsequent to cAMP accumulation and [Ca2+]i elevation. Since the GH4C1 cells possess one single class of binding sites, this implies that the same SRIH receptor is coupled to several cellular signalling systems.