Cyclic GMP potentiates phenylephrine but not cyclic ADP-ribose-evoked calcium release from rat lacrimal acinar cells

Neural regulation of lacrimal gland secretory processes: relevance in dry eye diseases

The lacrimal gland is the major contributor to the aqueous layer of the tear film which consists of water, electrolytes and proteins. The amount and composition of this layer is critical for the health, maintenance, and protection of the cells of the cornea and conjunctiva (the ocular surface). Small changes in the concentration of tear electrolytes have been correlated with dry eye syndrome. While the mechanisms of secretion of water, electrolytes and proteins from the lacrimal gland differ, all three are under tight neural control. This allows for a rapid response to meet the needs of the cells of the ocular surface in response to environmental conditions. The neural response consists of the activation of the afferent sensory nerves in the cornea and conjunctiva to stimulate efferent parasympathetic and sympathetic nerves that innervate the lacrimal gland. Neurotransmitters are released from the stimulated parasympathetic and sympathetic nerves that cause secretion of water, electrolytes, and proteins from the lacrimal gland and onto the ocular surface. This review focuses on the neural regulation of lacrimal gland secretion under normal and dry eye conditions.

Pyridine nucleotides and calcium signalling in arterial smooth muscle: from cell physiology to pharmacology

It is generally accepted that the mobilisation of intracellular Ca2+ stores plays a pivotal role in the regulation of arterial smooth muscle function, paradoxically during both contraction and relaxation. However, the spatiotemporal pattern of different Ca2+ signals that elicit such responses may also contribute to the regulation of, for example, differential gene expression. These findings, among others, demonstrate the importance of discrete spatiotemporal Ca2+ signalling patterns and the mechanisms that underpin them. Of fundamental importance in this respect is the realisation that different Ca2+ storing organelles may be selected by the discrete or coordinated actions of multiple Ca2+ mobilising messengers. When considering such messengers, it is generally accepted that sarcoplasmic reticulum (SR) stores may be mobilised by the ubiquitous messenger inositol 1,4,5 trisphosphate. However, relatively little attention has been paid to the role of Ca2+ mobilising pyridine nucleotides in arterial smooth muscle, namely, cyclic adenosine diphosphate-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). This review will therefore focus on these novel mechanisms of calcium signalling and their likely therapeutic potential.

Nitric oxide and cGMP mediate alpha1D-adrenergic receptor-Stimulated protein secretion and p42/p44 MAPK activation in rat lacrimal gland

PURPOSE

To determine whether alpha(1)-adrenergic receptors use the nitric oxide (NO)/cGMP pathway to stimulate protein secretion by rat lacrimal gland.

METHODS

Identification and cellular location of endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) were determined by Western blot and immunofluorescence techniques, respectively. Rat lacrimal gland acini were isolated by collagenase digestion, and protein secretion stimulated by phenylephrine, an alpha(1)-adrenergic agonist, was measured with a fluorescence assay system. Acini were preincubated with inhibitors for 20 minutes before addition of phenylephrine (10(-4) M). NO and cGMP were measured in response to phenylephrine stimulation. Activation of p42/p44 MAPK was determined by Western blot analysis with an antibody against phosphorylated (active) p42/p44 MAPK.

RESULTS

eNOS and nNOS were both present in lacrimal gland. eNOS appeared to be localized with caveolae, whereas nNOS was present in the nerves surrounding the acini. Inhibition of eNOS with N(G)-nitro-l-arginine methyl ester (l-NAME; 10(-6) M) completely inhibited phenylephrine-stimulated protein secretion, whereas the inactive isomer d-NAME and inhibition of nNOS with S-methyl-l-thiocitrulline did not. Phenylephrine increased NO production in a time- and concentration-dependent manner, but the increase was abolished by the alpha(1D)-adrenergic receptor inhibitor BMY-7378. Inhibition of guanylate cyclase with oxadiazoloquinoxalin (ODQ) also inhibited phenylephrine-induced protein secretion, whereas phenylephrine caused a 2.2-fold increase in cGMP. In addition, preincubation with l-NAME and ODQ inhibited phenylephrine-stimulated p42/p44 MAPK activation.

CONCLUSIONS

alpha(1D)-Adrenergic agonists stimulate eNOS to produce NO, leading to production of cGMP by guanylate cyclase, to transduce the extracellular signal through the cell and stimulate protein secretion in rat lacrimal gland.

Biochemical changes contributing to functional quiescence in lacrimal gland acinar cells after chronic ex vivo exposure to a muscarinic agonist

Profound secretory dysfunction can be associated with relatively modest lymphocytic infiltration of the lacrimal and salivary glands of Sjögren's syndrome (SjS) patients. SjS patients' sera contain autoantibodies to M3 muscarinic acetylcholine receptors (MAChR) that have variously been reported to have agonistic and antagonistic effects. We sought to identify consequences of chronic agonist stimulation by maintaining acinar cells from rabbit lacrimal glands for 20 h in the presence or absence of 10 microM carbachol (CCh). Exposure to CCh diminished the cells' ability to elevate cytosolic Ca2+ and secrete beta-hexosaminidase in response to acute stimulation with 100 microM CCh, but it enhanced their secretory responses to phenylephrine and ionomycin. Secretory vesicles appeared normal by electron microscopy, but confocal fluorescence microscopy revealed depletion of the secretory vesicle membrane marker, rab3D, and decreased ability to recruit secretory transport vesicles in response to acute 100 microM CCh. Additionally, the apical cortical actin cytoskeleton was disrupted and diminished compared to the basal-lateral cortical network. Subcellular fractionation analyses revealed that total membrane phase protein content was increased. The contents of beta-hexosaminidase and MAChR relative to total protein were not significantly altered, and MAChR abundance in the plasma membrane fraction was increased as the result of redistribution from endomembrane pools. However, relative cellular contents of the heterotrimeric guanosine triphosphate (GTP)-binding proteins, Gq and G11, were decreased. Additional biochemical changes included decreased contents of 47 kDa Gs and Gi3, protein kinase Calpha and rab3D and polymeric immunoglobulin (Ig) receptors; internalization of Na,K-ATPase from the plasma membranes to endomembrane compartments and decreased content of beta-hexosaminidase in the lysosomes. The observations demonstrate that chronic exposure to a MAChR agonist induces refractoriness to optimal stimulation, without causing receptor downregulation, by downregulating postreceptor-signalling mediators and effectors. The cells' secretory mechanisms for IgA and electrolytes also appear to be impaired, as does their ability to properly sort proteins to the lysosomes.

Regulatory pathways in lacrimal gland epithelium

Tears are a complex fluid that continuously cover the exposed surface of the eye, namely the cornea and conjunctiva. Tears are secreted in response to the multitude of environmental stresses that can harm the ocular surface such as cold, mechanical stimulation, physical injury, noxious chemicals, as well as infections from various organisms. Tears also provide nutrients and remove waste from cells of the ocular surface. Because of the varied function of tears, tears are complex and are secreted by several different tissues. Tear secretion is under tight neural control allowing tears to respond rapidly to changing environmental conditions. The lacrimal gland is the main contributor to the aqueous portion of the tear film and the regulation of secretion from this gland has been well studied. Despite multiple redundencies in pathways to stimulate secretion from the lacrimal gland, defects can occur resulting in dry eye syndromes. These diseases can have deleterious effects on vision. In this review, we summarize the latest information regarding the regulatory pathways, which control secretion from the lacrimal gland, and their roles in the pathogenesis of dry eye syndromes.

Calcium dependence of exocytosis in lacrimal gland acinar cells

Simultaneous measurements of membrane capacitance and intracellular calcium concentration were used to examine the calcium dependence of exocytosis in single acinar cells from mouse lacrimal gland and to establish the quantitative relation between calcium concentration and rate of exocytosis. Application of adrenergic or muscarinic agonists elevated intracellular calcium and evoked exocytosis, as indicated by an increase in membrane capacitance of single cells. The capacitance response to agonist stimulation was eliminated by internal dialysis with the calcium buffer EGTA, which demonstrated that the increase in intracellular calcium was necessary for agonist-evoked exocytosis. When internal calcium was elevated by application of the calcium ionophore ionomycin, exocytosis was evoked in the absence of agonist stimulation. Thus an increase in intracellular calcium was necessary and sufficient for exocytosis in single acinar cells. The rate of change of membrane capacitance increased as approximately the third power of the calcium concentration, which is similar to the dependence of exocytosis rate on calcium concentration in other secretory cells.

Cyclic ADP-ribose and the regulation of calcium-induced calcium release in eggs and cardiac myocytes

Cyclic ADP-ribose (cADPR) is a cyclic metabolite of NAD+ synthesised in cells and tissues expressing ADP-ribosyl cyclases. Although it was first discovered in sea-urchin egg extracts as a potent calcium mobilizing agent, subsequent studies have indicated that it may have a widespread action in the activation of calcium-release channels in such diverse systems as mammalian neurones, myocytes, blood cells, eggs, and plant microsomes. In this review we focus on recent work suggesting that cADPR enhances the sensitivity of ryanodine-sensitive calcium-release channels (RyRs) to activation by calcium, a phenomenon termed calcium-induced calcium release (CICR). Two roles for cADPR in calcium signaling are discussed. The first is as a classical second messenger where its levels are controlled by extracellular stimuli, and the second mode of cellular regulation is that the levels of intracellular cADPR may set the sensitivity of RyRs to activation by an influx of calcium in excitable cells. These two possible actions of cADPR are illustrated by considering the signal transduction events during the fertilization of the sea-urchin egg and the modulation of CICR during excitation-coupling in isolated guinea-pig ventricular myocytes, respectively.