Adenosine A2 receptor presence and synergy with cholinergic stimulation in rabbit lacrimal gland

The Relationship Between Caffeine Intake and Dry Eye Disease

PURPOSE

The aim of this study was to determine the association between caffeine intake and dry eye disease (DED) in the large, population-based LifeLines cohort in the Netherlands.

METHODS

DED was cross-sectionally assessed in 85,302 participants (59% female participants) using the Women's Health Study dry eye questionnaire. Dietary caffeine was calculated from the intake of coffee, tea, cola, and energy drinks. Logistic regression was used to investigate the relationship between DED and caffeine, correcting for demographic variables, smoking status, alcohol intake, and 48 comorbidities of DED.

RESULTS

The mean (SD; range) age of participants was 50.7 years (12.4; 18-96), and 50,339 (59%) were female. The mean (SD) caffeine intake was 285 (182) mg/d. After correcting for demographics, body mass index, smoking status, and alcohol intake, higher caffeine intake was associated with a decreased risk of Women's Health Study-defined DED [odds ratio (OR) 0.971 per 100 mg/d, 95% CI, 0.956-0.986, P < 0.0005]. When additionally adjusting for medical comorbidities, no significant effect was observed (OR 0.985, 95% CI, 0.969-1.001, P = 0.06). Caffeine's effect on DED was similar in male and female participants and independent of sleep quality and stress at work. Decaffeinated coffee intake was significantly associated with an increased risk of DED, when adjusted for caffeinated coffee, demographics, alcohol intake, smoking status, and comorbidities (OR 1.046 per cup/d, 95% CI, 1.010-1.084, P = 0.01). None of the beverages were significantly associated with the risk of DED, when correcting for intake of the other caffeinated beverages, demographics, smoking status, alcohol intake, and all comorbidities.

CONCLUSIONS

Dietary caffeine intake does not seem to be a risk factor for DED in the general population.

Novel Insights Into Muscarinic and Purinergic Responses in Primary Cultures of Rat Lacrimal Gland Myoepithelial Cells

Purpose

The functional characteristics of receptors that regulate lacrimal gland myoepithelial cells are still somewhat unclear. To date, mainly muscarinic receptors have been of interest; however, further knowledge is needed regarding their expression and functional roles. For this purpose, primary cultures of rat lacrimal gland myoepithelial cells were established and examined functionally.

Methods

Rat lacrimal glands were excised, minced, and further digested, yielding mixtures of cells that were seeded in culturing flasks. After 4-6 weeks, primary monocultures of myoepithelial cells were established, verified by immunocytochemistry. The cells were stained for all muscarinic receptor subtypes (M1–M5) and examined functionally regarding intracellular [Ca²⁺] responses upon activation of muscarinic receptors. For methodological verification, purinergic functional responses were also studied.

Results

Expression of muscarinic receptor subtypes M2-M5 was detected, whereas expression of muscarinic M1 receptors could not be shown. Activation of muscarinic receptors by the non-selective muscarinic agonist methacholine (3 × 10⁻¹¹–10⁻³ M) did not cause a significant increase in intracellular [Ca²⁺]. However, activation of purinergic receptors by the non-selective purinergic agonist ATP (10⁻⁸–10⁻³ M) caused a concentration-dependent increase in intracellular [Ca²⁺] that could be blocked by the P2 antagonists PPADS and suramin.

Conclusions

Primary cultures of rat lacrimal gland myoepithelial cells were established that displayed a heterogeneous expression of muscarinic receptors. Purinergic functional responses demonstrated a viable cell population. Upon treatment with methacholine, no significant increase in intracellular [Ca²⁺] could be detected, indicating that cholinergic activation of myoepithelial cells occurs via other intracellular messengers or is dependent on interaction with other cell types.

Signaling Pathways of Purinergic Receptors and Their Interactions with Cholinergic and Adrenergic Pathways in the Lacrimal Gland

PURPOSE

Purinergic receptors play a key role in the function of the lacrimal gland (LG) as P1 purinergic receptors A₁, A_2A, and A_2B, P2X_1-7 receptors, and many of the P2Y receptors are expressed.

METHODS

This review examines the current knowledge of purinergic receptors in the LG as well as the signaling pathways activated by these receptors.

RESULTS

These receptors are expressed on the acinar, ductal, and myoepithelial cells. Considerable crosstalk exists between the pathways activated by P2X₇ receptors with those activated by M₃ muscarinic or α_1D adrenergic receptors. The mechanism of the crosstalk between P2X₇ and M₃ muscarinic receptors differs from that of the crosstalk between P2X₇ and α_1D adrenergic receptors.

CONCLUSIONS

Understanding purinergic receptors and how they modulate protein secretion could play a key role in normal and pathological responses of the LG.

p38 Mitogen-activated protein kinase modulates exocrine secretion in rabbit lacrimal gland

The lacrimal gland (LG) is an exocrine gland important for secretion of the tear film. The kinase p38 has important signal transduction functions, e.g. in gene transcription, but has previously not been known to modulate exocrine secretion. The aim of the current study was to investigate the role of p38 in carbachol (Cch)-induced LG secretion in LG acinar cells in vitro. Western blotting was used to determine the phosphorylation status of p38 and p42/44 and determine expression of p38 isoforms. To determine the effect of p38 inhibition on LG secretion, PD 169316, a general p38 inhibitor, and SB 239063, an inhibitor of p38α and β, were added to the cells prior to secretion measurements. The results revealed activation of p38 mediated by Cch stimulation and inhibition of Cch-induced secretion as a result of p38 inhibition. The inhibition was observed with PD 169316 isoforms, but not with SB 239063. The p38δ isoform was shown to have robust expression both by Western blotting of acinar cells and immunofluorescence of the whole gland. In conclusion, p38 activation mediates secretion in cholinergic stimulation of rabbit LG cells.