Peroxidase secretion from rat lacrimal gland cells in vitro. I. Alpha-adrenergic stimulation in the absence of alpha-adrenoceptors

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.

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.

Signal transduction and control of lacrimal gland protein secretion: a review

Proteins in lacrimal gland fluid are secreted primarily by the acinar cells. Secretory proteins are synthesized in the endoplasmic reticulum, modified in the Golgi apparatus, stored in secretory granules, and released upon a change in the cellular level of second messenger. The second messenger level is controlled by a process termed signal transduction. Agonists, primarily neurotransmitters in the lacrimal gland, bind to receptors in the basolateral membrane of secretory cells. This interaction activates enzymes in the membrane that cause production of second messengers. It has been hypothesized that second messengers stimulate secretion by activating specific protein kinases to phosphorylate proteins important for secretion. In the lacrimal gland, cholinergic agonists stimulate protein secretion. They act by activating phospholipase C to break down phosphatidylinositol bisphosphate into 1,4,5-inositol trisphosphate (1,4,5-IP3) and diacylglycerol (DAG). 1,4,5-IP3 causes release of Ca2+ from intracellular stores. This Ca2+, perhaps in conjunction with calmodulin, activates specific protein kinases that may be involved in secretion. DAG activates protein kinase C which stimulates protein secretion. alpha 1-Adrenergic agonists also stimulate lacrimal gland protein secretion. These agonists use a pathway that is separate from that utilized by cholinergic agonists and vasoactive intestinal peptide (VIP). The specific pathway has not been identified but may be DAG and protein kinase C. VIP, beta-adrenergic agonists, alpha-melanocyte stimulating hormone, and adrenocorticotropic hormone are lacrimal gland secretagogues. They activate adenylate cyclase to produce cAMP. cAMP stimulates protein kinase A, which perhaps causes protein secretion. Thus, three separate cellular pathways stimulate lacrimal gland protein secretion. Cholinergic agonists and VIP also stimulate lacrimal gland fluid secretion, and the same signal transduction pathways utilized by these agonists to stimulate protein secretion are most likely used for electrolyte and water secretion.

Protein secretion in lacrimal gland: alpha 1-beta-adrenergic synergism

We have shown the existence of a homogeneous population of specific binding sites for [3H]prazosin in membranes from rat lacrimal glands. The value of the equilibrium dissociation constant was 0.186 +/- 0.07 nM, and the density of specific binding sites was 20.4 +/- 1 fmol/mg protein. Taking into account the potency sequences for adrenergic agonists and antagonists for competition with these [3H]prazosin binding sites, we identified them as alpha 1-adrenergic receptors. Moreover, we have demonstrated that the stimulation of protein discharge evoked by epinephrine could be partly attributed to the occupation of this alpha-adrenergic receptor subtype. However, the inhibition pattern of the epinephrine effect by a beta-adrenergic antagonist, 1-propranolol, and the characteristics of the secretory response observed when selectively stimulating the alpha 1- and beta-adrenergic receptors, either separately or simultaneously, suggest that 1) a simultaneous activation of both receptors is necessary to produce a maximal secretory response to catecholamines; and 2) a synergism may exist between these two routes of stimulation, leading to an amount of protein discharge higher than that expected in the case of additive effects.

Peroxidase secretion from rat lacrimal gland cells in vitro: II. The role of calcium in stimulus-secretion coupling

The role of calcium in stimulus-secretion coupling in rat lacrimocytes was investigated. Monolayers of lacrimocytes released peroxidase into the incubation medium upon stimulation with calcium ionophore A 23187, L-norepinephrine, L-phenylephrine and tyramine. Our lacrimocyte preparation can be considered free of endogenous catecholamines, because lacrimocytes derived both from normal and from reserpinized rats responded to tyramine with respect to peroxidase secretion. Phenoxybenzamine, cocaine, verapamil and sodiumorthovanadate failed to inhibit the L-phenylephrine-evoked peroxidase secretion. The stimulatory effects of A 23187, L-norepinephrine and L-phenylephrine required extracellular Ca2+ and could be blocked by phentolamine. The action of tyramine was insensitive to all these measures. It is suggested that the stimulus-secretion of peroxidase from lacrimocytes may consist of a Ca2+-dependent phase for stimulants as A 23187, L-norepinephrine and L-phenylephrine and a Ca2+-independent phase for the non-specific tyramine action.