Ascorbic acid is an endogenous cytosolic inhibitor of ATP-supported rat liver mitochondrial calcium transport

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.

Effect of marginal ascorbic acid deficiency on saliva level of cortisol in the guinea pig

Male guinea pigs subjected to prolonged marginal ascorbic acid deficiency developed moon facies and oedema, features of functional adrenal hypercorticism. Compared with age- and sex-matched controls fed an adequate diet for a similar period, ascorbate deficiency had no effect on submandibular gland weight but elicited a significant (p < 0.005) reduction in stimulated whole-saliva flow rate. Plasma cortisol concentration (nmol/L) was significantly increased (p < 0.005) in the deficient animals (998.21 +/- 57.19 compared to 254.66 +/- 15.62 for the controls). Associated with marked hypercortisolaemia in the deficient animals was a significant (p < 0.01) but less prominent increase in the whole-saliva cortisol level, resulting in a mean saliva/plasma cortisol ratio of 46% for this group compared to 72% for the controls. Increased corticosteroid levels suppress immunological and inflammatory responses, particularly neutrophil function, impair production of some cytokines, inhibit collagen synthesis, and impair wound healing and bone matrix formation. Numerous conditions such as ageing, stress, smoking, ionizing radiation, ingestion of drugs, protein malnutrition, diabetes, and several other pathological states, which are among the risk factors for xerostomia and periodontal/oral mucosal lesions, promote tissue depletion of ascorbate. This study suggests that increased salivary and blood levels of glucocorticoids in these conditions may be important in reducing the ability of the host to mount an effective immune response to oral pathogens.

Ascorbate status and xerostomia

Xerostomia, the subjective feeling of dry mouth, affects millions of people particularly the elderly. It is invariably associated with hypofunction of the salivary glands. The amount, rate of secretion, and composition of saliva are regulated by both sympathetic and parasympathetic receptor systems whose stimulation transmits signals through intracellular messengers (cations, nucleotides, phospholipid derivatives) to structures and enzymes within the cell. Salivary glands express a variety of cell-surface receptors including adrenergic (alpha and beta), muscarinic-cholinergic, substance P, vasoactive intestinal peptide hormone, and ATP receptors. Ascorbate which is present in salivary acinar cells in relatively high concentrations, is closely involved in many cellular functions including the metabolism of pyrimidines, intracellular calcium, the catecholamines and other neurotransmitters which regulate salivary gland exocytosis. Ascorbate-dependent carboxyl-terminal peptide alpha-amidation enzyme similar to the pituitary peptidyl-glycine alpha-amidating monooxygase, is also present in salivary glands. It is therefore not fortuitous that the seemingly unrelated numerous factors like aging, drug ingestion, pregnancy, smoking, ionizing radiation, stress, and various pathological states such as cancer, autoimmune disorders, diabetes mellitus, and hypertension often implicated in the causation of xerostomia, all promote increased tissue requirement for and/or depletion of ascorbate.

Mechanism of inhibition by alloxan of ATP-driven calcium transport by vascular smooth muscle microsomes

The direct in vitro effects of alloxan on the Ca2+ handling by microsomal membranes isolated from dog mesenteric arteries were investigated. Preincubation of the vascular muscle microsomal membranes with alloxan showed a suppressive effect on both binding of Ca2+ (in the absence of ATP) and ATP-driven Ca2+ transport. Such an inhibition was time dependent, dose dependent, and temperature dependent. ATP-driven Ca2+ transport was much more susceptible to the inhibitory action of alloxan than Ca2+ binding under all experimental conditions examined. Alloxan inhibited ATP-driven Ca2+ transport at a comparable level over the entire period of Ca2+ uptake, but had no significant effect on the efflux of Ca2+ from preloaded microsomal membranes. This suggests that alloxan exerts its inhibitory effect on the ATP-driven Ca2+ transport via its action on the Ca-pump protein rather than the membrane permeability to Ca2+. Catalase and mannitol but not superoxide dismutase partially protected against such as inhibition by alloxan. The possible involvement of H2O2 mediating the inhibitory action of alloxan was further supported by the finding of a similar in vitro inhibitory effect of H2O2 on the ATP-driven Ca2+ transport by the vascular smooth muscle microsomes.

Ascorbate regeneration in bovine ocular tissues by NADH-dependent semidehydroascorbate reductase

Despite its fast autoxidation in vitro, ascorbate remains in its reduced form in vivo, indicating a special mechanism may be involved in its regeneration. The presence of an NADH-dependent reductase system, semidehydroascorbate reductase (SDR), for regeneration of ascorbate from its partially oxidized form, semidehydroascorbate (SDA), was demonstrated in bovine ocular tissues after extraction in Triton X-100. Highest SDR activity was detected in retinal extracts in the order of retina greater than pigment epithelium-choroid = ciliary body greater than iris. Minimal or no activity was observed in lens extracts or in aqueous fluid. Freezing and thawing, or boiling, destroyed the NADH-dependent SDR activity. NADH oxidation was significantly reduced (22% of total activity) when assays with retinal extracts were performed at 5 degrees C. Treatment with 4 mM, N-ethylmaleimide reduced the rate of NADH oxidation to 73 or 42% compared with control values with retinal or ciliary body extracts, respectively.

The diffusion of L(+)-ascorbic acid across DPPC vesicle membranes

The transmembrane diffusion of L(+)-ascorbic acid has been studied by means of 1H-n.m.r. spectroscopy using small unilamellar DPPC vesicles as a model system. It is shown that the electro-neutral form of L(+)-ascorbic acid diffuses faster across the membranes than the anionic form by about two orders of magnitude. The diffusion is influenced by a molecular interaction between L(+)-ascorbate and the membrane surface and depends also on the fluidity of the membrane. The calculated permeation coefficients of neutral L(+)-ascorbic acid are between 6 x 10(-10) and 3 x 10(-8) cm s-1 (35-52 degrees C).