Atrial natriuretic factor intracellular signaling in the rat submandibular gland

cAMP guided his way: a life for G protein-mediated signal transduction and molecular pharmacology-tribute to Karl H. Jakobs

Karl H. Jakobs, former editor-in-chief of Naunyn-Schmiedeberg's Archives of Pharmacology and renowned molecular pharmacologist, passed away in April 2018. In this article, his scientific achievements regarding G protein-mediated signal transduction and regulation of canonical pathways are summarized. Particularly, the discovery of inhibitory G proteins for adenylyl cyclase, methods for the analysis of receptor-G protein interactions, GTP supply by nucleoside diphosphate kinases, mechanisms in phospholipase C and phospholipase D activity regulation, as well as the development of the concept of sphingosine-1-phosphate as extra- and intracellular messenger will presented. His seminal scientific and methodological contributions are put in a general and timely perspective to display and honor his outstanding input to the current knowledge in molecular pharmacology.

Blockade of Multidrug Resistance-Associated Proteins Aggravates Acute Pancreatitis and Blunts Atrial Natriuretic Factor's Beneficial Effect in Rats: Role of MRP4 (ABCC4)

We previously reported that atrial natriuretic factor (ANF) stimulates secretin-evoked cAMP efflux through multidrug resistance-associated protein 4 (MRP4) in the exocrine pancreas. Here we sought to establish in vivo whether this mechanism was involved in acute pancreatitis onset in the rat. Rats pretreated with or without probenecid (MRPs general inhibitor) were infused with secretin alone or with ANF. A set of these animals were given repetitive cerulein injections to induce acute pancreatitis. Plasma amylase and intrapancreatic trypsin activities were measured and histological examination of the pancreas performed. Secretin alone activated trypsinogen but induced no pancreatic histological changes. Blockade by probenecid in secretin-treated rats increased trypsin and also induced vacuolization, a hallmark of acute pancreatitis. ANF prevented the secretin response but in the absence of probenecid. In rats with acute pancreatitis, pretreatment with secretin aggravated the disease, but ANF prevented secretin-induced changes. Blockade of MRPs in rats with acute pancreatitis induced trypsinogen activation and larger cytoplasmic vacuoles as well as larger areas of necrosis and edema that were aggravated by secretin but not prevented by ANF. The temporal resolution of intracellular cAMP levels seems critical in the onset of acute pancreatitis, since secretin-evoked cAMP in a context of MRP inhibition makes the pancreas prone to injury in normal rats and aggravates the onset of acute pancreatitis. Present findings support a protective role for ANF mediated by cAMP extrusion through MRP4 and further suggest that the regulation of MRP4 by ANF would be relevant to maintain pancreatic acinar cell homeostasis.

Endothelins participate in the central and peripheral regulation of submandibular gland secretion in the rat

We previously reported that endothelins (ETs) are involved in the rat central and peripheral regulation of bile secretion. In this study we sought to establish whether ET-1 and ET-3 modulated submandibular gland secretion when locally or centrally applied. Animals were prepared with gland duct cannulation to collect saliva samples and jugular cannulation to administer sialogogues. ETs were given either into the submandibular gland or brain lateral ventricle. Intraglandularly administered ETs failed to elicit salivation per se. However, ET-1, but not ET-3, potentiated both cholinergic- and adrenergic-evoked salivation through ET(A) receptors. ET-1 decreased cAMP content but increased phosphoinositide hydrolysis, whereas ET-3 attenuated both intracellular pathways. The expression of ET(A) and ET(B) receptor mRNAs as well as that of ETs was revealed in the submandibular gland by RT-PCR. Immunohistochemical studies showed that ET(A) receptor staining was localized around the interlobular ducts and acini, compatible with the myoepithelial cells' location, whereas ET(B) receptor staining was restricted to small blood vessels. When applied to the brain, both ETs induced no salivation but enhanced cholinergic- and adrenergic-evoked salivary secretion through parasympathetic pathways. ET-1 response was mediated by brain ET(A) receptors, whereas that of ET-3 was presumably through nonconventional ET receptors. Present findings show that ETs are involved in the brain regulation of cholinergic- and adrenergic-stimulated submandibular gland secretion through the activation of distinct brain ET receptors and parasympathetic pathways. However, when ETs were administered into the gland, only ET-1 enhanced cholinergic and adrenergic salivation likely through myopithelial cell contraction by activating ET(A) receptors coupled to phospholipase C. The presence of ETs and ET receptors suggests the existence of an endothelinergic system in the submandibular gland.

Natriuretic peptides as regulatory mediators of secretory activity in the digestive system

Atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are members of the natriuretic peptide family best known for their role in blood pressure regulation. However, in recent years all the natriuretic peptides and their receptors have been described in the gastrointestinal tract, digestive glands and central nervous system, as well as implicated in the regulation of digestive gland functions. The current review highlights the regulatory role of ANP and CNP in pancreatic and other digestive secretions. ANP and CNP stimulate basal as well as induced pancreatic secretion and modify bicarbonate and chloride secretions. Whereas ANP and CNP exert effects directly on pancreatic cells, CNP also acts through a vago-vagal reflex. At high doses both peptides attenuate pancreatic secretion induced by high doses of secretin through the PLC/PKC pathway. With regards to other digestive secretions, ANP and CNP decrease bile secretion in the rat. ANP does not induce salivation by itself but enhances stimulated salivary secretion and modifies salivary composition in rat parotid as well as submandibular glands. In rat pancreatic, hepatic, parotid and submandibular tissues, the NPR-C receptor mediates mostly peripheral responses whereas NPR-A and NPR-B receptors, which are coupled to guanylate cyclase, likely mediate the central response. In addition, ANP modulates gastric acid secretion via a vagal-dependent mechanism. In the intestine, ANP and CNP decrease water and sodium chloride absorption through an increase in cGMP levels. Overall, these findings indicate that ANP and CNP are members of the large group of regulatory peptides affecting digestive secretions.