Impaired inositol trisphosphate generation in carbachol-stimulated submandibular gland acinar cells from ascorbate deficient guinea pigs

Ascorbic acid induces salivary gland function through TET2/acetylcholine receptor signaling in aging SAMP1/Klotho (-/-) mice

Aging affects salivary gland function and alters saliva production and excretion. This study aimed to investigate whether ascorbic acid can be used to treat salivary gland dysfunction in an extensive aging mouse model of SAMP1/Klotho-/- mice. In our previous study, we found that ascorbic acid biosynthesis was disrupted in the salivary glands of SAMP1/Klotho (-/-) mice subjected to metabolomic profiling analysis. In SAMP1/Klotho -/- mice, daily supplementation with ascorbic acid (100 mg/kg for 18 days) significantly increased saliva secretion compared with the control. The expression of salivary gland functional markers (α-amylase, ZO-1, and Aqua5) is upregulated. Additionally, acetylcholine and/or beta-adrenergic receptors (M1AchR, M3AchR, and Adrb1) were increased by ascorbic acid in the salivary glands of aging mice, and treatment with ascorbic acid upregulated the expression of acetylcholine receptors through the DNA demethylation protein TET2. These results suggest that ascorbic acid could overcome the lack caused by dysfunction of ascorbic acid biosynthesis and induce the recovery of salivary gland function.

Vitamin C Deficiency Reduces Muscarinic Receptor Coronary Artery Vasoconstriction and Plasma Tetrahydrobiopterin Concentration in Guinea Pigs

Vitamin C (vitC) deficiency is associated with increased cardiovascular disease risk, but its specific interplay with arteriolar function is unclear. This study investigates the effect of vitC deficiency in guinea pigs on plasma biopterin status and the vasomotor responses in coronary arteries exposed to vasoconstrictor/-dilator agents. Dunkin Hartley female guinea pigs (n = 32) were randomized to high (1500 mg/kg diet) or low (0 to 50 mg/kg diet) vitC for 10-12 weeks. At euthanasia, coronary artery segments were dissected and mounted in a wire-myograph. Vasomotor responses to potassium, carbachol, sodium nitroprusside (SNP), U46619, sarafotoxin 6c (S6c) and endothelin-1 (ET-1) were recorded. Plasma vitC and tetrahydrobiopterin were measured by HPLC. Plasma vitC status reflected the diets with deficient animals displaying reduced tetrahydrobiopterin. Vasoconstrictor responses to carbachol were significantly decreased in vitC deficient coronary arteries independent of their general vasoconstrictor/vasodilator capacity (p < 0.001). Moreover, in vitC deficient animals, carbachol-induced vasodilator responses correlated with coronary artery diameter (p < 0.001). Inhibition of cyclooxygenases with indomethacin increased carbachol-induced vasoconstriction, suggesting an augmented carbachol-induced release of vasodilator prostanoids. Atropine abolished carbachol-induced vasomotion, supporting a specific muscarinic receptor effect. Arterial responses to SNP, potassium, S6c, U46619 and ET-1 were unaffected by vitC status. The study shows that vitC deficiency decreases tetrahydrobiopterin concentrations and muscarinic receptor mediated contraction in coronary arteries. This attenuated vasoconstrictor response may be linked to altered production of vasoactive arachidonic acid metabolites and reduced muscarinic receptor expression/signaling.

Ascorbate deficiency impairs the muscarinic-cholinergic and ss-adrenergic receptor signaling systems in the guinea pig submandibular salivary gland

Ascorbic acid is preferentially concentrated in the hypothalamus, pituitary and adrenal glands. Its level in the acini of salivary glands is relatively high. We therefore hypothesized that ascorbate may have a role in salivary gland function. Ascorbate-deficient guinea pigs had lower stimulated whole salivary flow rates than well-fed, age-matched controls (P: < 0.005). Total salivary protein concentration was also markedly (P: < 0.005) reduced in the deficient guinea pigs. SDS-PAGE and densitometric quantification of protein bands confirmed significant reduction in specific salivary proteins (e.g., amylase, proline-rich proteins) in the saliva samples of malnourished guinea pigs. Some protein bands not seen in control saliva were detected in the saliva of malnourished guinea pigs. Ascorbate deficiency also produced a significant (P: < 0.005) reduction in the ss-adrenergic receptor density (subtype 1; 95 +/- 19 fmol/mg protein compared with 179 +/- 27 fmol/mg protein for the controls). No significant difference was observed between the two groups with respect to the ss-adrenergic receptor subtype 2. Additionally, ascorbate-deficient guinea pigs had significantly lower muscarinic-cholinergic receptor densities (50 +/- 5 vs. 74 +/- 8 fmol/mg protein for controls). Our data support the conclusion that diminished membrane receptors might impair the capacity of the transmembrane signaling system, resulting in salivary gland hypofunction in ascorbate-deficient guinea pigs. Without implying extrapolation of our findings in experimental animals to humans, it is perhaps relevant that many conditions often associated with salivary gland hypofunction in humans (e.g., smoking or drug ingestion) deplete cellular ascorbate.