Effects of phorbol ester on cholecystokinin octapeptide-evoked exocrine pancreatic secretion in the rat

Nitric oxide and the pancreas: morphological base and role in the control of the exocrine pancreatic secretion

The distribution of nitric oxide synthase in both neuronal and non-neuronal pancreatic tissues and the role of nitric oxide in the control of exocrine pancreatic secretion are reviewed in this article. Earlier reports based on in vivo studies suggested that nitric oxide can affect the secretory activity of the exocrine pancreas through changes in pancreatic blood flow. More recently, the employment of either nitric oxide synthase inhibitors or nitric oxide donors in in vitro preparations has provided evidence that nitric oxide can exert a direct action on this gland independently on its vascular effects. Most research in this area seems to indicate that modulation of exocrine pancreatic function by nitric oxide is exerted via activation of guanylate cyclase and generation of cGMP, although other pathways cannot be excluded. Experiments performed over the last year in our laboratory reveal a novel and interesting mechanism based on the ability of nitric oxide to control the release of endogenous neurotransmitter in the pancreas and, subsequently, the nerve-mediated enzyme secretion.

Adaptation of the exocrine pancreas to dietary fats

This article reviews studies on the adaptation of the exocrine pancreas to dietary fat. We include all the latest information about the mechanisms that underlie the adaptation of the secretory mechanism of the exocrine pancreas to the amount and the type of dietary fat. We review the kinetics of pancreatic adaptation and the mediators of the adaptive response of the pancreas including cellular and molecular mechanisms (modulation of intracellular messengers and gene expression of the different enzymes and secretagogues involved in the adaptation process). At the same time we include our results in this field in dogs and humans.

Cholinergic dependence of pancreatic response to cholecystokinin in rats and guinea pigs

1. Lorglumide and atropine were used to examine the role of cholinergic mechanisms in the pancreatic secretory response to cholecystokinin in two animal species. 2. Anaesthetized rats and guinea pigs with jugular vein and pancreatic cannulae were used and the bile juice was recirculated. In the rat, the treatment with lorglumide (3 mumol/kg) as well as atropine (100 micrograms/kg) did not have effects on basal interdigestive secretion, whereas in guinea pigs only atropine decreased the protein output (41%) and the juice flow (47%) of the basal pancreatic secretion. 3. Infusion of cholecystokinin (150 pmol/kg/hr in the rat and 50 pmol/kg/hr in the guinea pig) induced a marked increase in pancreatic juice flow and protein output compared to saline controls. Pretreatment of both rat and guinea pig with lorglumide resulted in a marked attenuation of the cholecystokinin-evoked secretory response. 4. In the rats, atropine decreased the response to infusion of cholecystokinin octapeptide while this antimuscarinic agent had no effect in the response to cholecystokinin in the guinea pigs. 5. This study supports the concept that the influence of cholinergic system in pancreatic response to cholecystokinin shows interspecific differences.

Inhibitory interactions between stimulus-secretion pathways in the exocrine rat pancreas

In many tissues the cellular responses mediated through different intracellular messenger systems are mutually interactive. In the exocrine pancreas the secretagogues acting via adenosine cyclic monophosphate (cAMP) and those acting via calcium-phosphoinositides can potentiate one another. On the other hand, protein kinase C (PK-C) modulates receptor-induced responses in exocrine pancreatic cells and other cell types. Recording total protein output, monitored on-line at 280 nm, from superfused rat pancreatic segments, we demonstrate that secretin (a cAMP-acting hormone) reduces the efficacy of the calcium-mediated secretagogue cholecystokinin-octapeptide (CCK-8). Likewise, the PK-C activator 12,O,tetradecanoyl phorbol 13 acetate (TPA) reduces both the efficacy of secretin and the potency of cholecystokinin. Thus, the hypothesis of potentiation between different stimulus-secretion coupling mechanisms must be revised, and receptor-activated responses in the exocrine pancreas must be considered a complex model with multiple inhibitory and stimulatory interactions.

Effect of phorbol ester on vagal stimulation and acetylcholine-evoked exocrine pancreatic secretion and cytosolic free calcium in the rat

In anaesthetized rats, interaction between either vagal stimulation or acetylcholine (ACh) and the phorbol ester, 12-0-tetradecanoylphorbol-13-acetate (TPA) and staurosporine on pancreatic juice secretion have been investigated in vivo. In vitro, rat cytosolic free calcium concentration (Ca2+)i in pancreatic acini loaded with the fluorescent dye, Fura-2 acetoxymethyl ester (AM) have been analysed after the same modifications. In vivo, vagotomy caused a marked reduction in the rate of pancreatic juice flow, total protein output and amylase secretion compared to basal secretory parameters. Furthermore, bolus injection of either saline, TPA (10(-8) mol/kg b wt), staurosporine (10(-8) mol/kg b wt) or a combination of TPA and staurosporine (all 10(-8) mol/kg b wt) had no statistically significant effect on pancreatic juice flow, protein output and amylase secretion compared to vagotomy values. Electrical stimulation (E.S.) of vagues nerves resulted in marked and statistically significant (P < 0.05) increases in the rate of pancreatic juice flow, total protein output and amylase secretion in rats injected with either saline or staurosporine, compared to control values. In contrast, E.S. of the vagus nerves failed to enhance all secretory parameters in the presence of either TPA or a combination of TPA and staurosporine. In vitro, on isolated acini, ACh evoked dose dependent increases in (Ca2+)i. Pretreatment these acini with either TPA, staurosporine or a combination of TPA and staurosporine had no significant effect on the ACh-induced (Ca2+)i. These results indicate that TPA can decrease the secretory responses evoked by E.S. of the vagus nerves in the anaesthetized rat. This attenuation is not associated with either protein kinase C inhibition or the mobilization of the second messenger Ca2+ but possibly through activation of protein kinase C by TPA.

Secretagogue-evoked time-course changes on pancreatic juice secretion in the anaesthetized rat

1. In the present time-course study, we have examined the interactions between the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and the synthetic gut hormones, cholecystokinin-octapeptide (CCK-8) and secretin on pancreatic juice secretion in anaesthetized rat. 2. Administration of either TPA (10(-8) mol kg-1 hr-1), secretin (100 pmol kg-1 hr-1) or CCK-8 (150 pmol kg-1 hr-1) in the anaesthetized rat resulted in marked time-course increases in pancreatic juice flow, amylase secretion and total protein output compared to saline controls. The effect of secretin on juice flow was more pronounced and sustained compared to the smaller responses obtained with either CCK-8 or TPA. Similarly, CCK-8 evoked increases in protein output and amylase secretion compared to the responses obtained with either secretin or TPA. 3. Simultaneous infusion of TPA with either CCK-8 or secretin resulted in a marked reduction in pancreatic juice flow, total protein output and amylase secretion compared to the responses obtained with either CCK-8 or secretin alone. 4. Administration of polymyxin B (10(-8) mol kg-1 hr-1), a protein kinase C inhibitor with either TPA and CCK-8 or TPA and secretin caused a partial reduction of the inhibitory effect of TPA on CCK-8 and secretin-evoked secretory responses. 5. The present study further implicates the involvement of protein kinase C in the modulation of CCK-8 and secretin-induced pancreatic juice secretion in the anaesthetized rat.