Influence of magnesium on salivary gland secretion: physiological and pathophysiological studies

Abstract

The divalent abundant cation Magnesium (Mg2+) has been known to play an important regulatory role in the stimulus-secretion coupling events in a number of epithelial secretory cells including the exocrine pancreas, lachrymal and the parietal cells. Since the salivary glands acinar cells have a similar structure and function as the exocrine pancreas, the stomach and the lachrymal gland, it was decided to investigate specifically the effect of perturbation of extracellular magnesium ([Mg2+]0) on both basal and secretagogue-evoked amylase secretion and total protein output and intracellular free calcium concentrations ([Ca2+]i) in the rat submandibular and parotid glands using spectrofluorimetry and spectroscopic techniques. The results have shown that both zero and elevated (5 and 10 mM) [Mg2+]0 can significantly (P<0.01) inhibit basal amylase or protein output compared to normal (1.1 mM) [Mg2+]0 Either acetylcholine (ACh), noradrenaline (NA) or Phenylephrine (Phe) can elicit marked increases in amylase secretion and total protein output from the isolated parotid and submandibular gland segments respectively in normal [Mg2+]0. In contrast, in the presence of either zero or elevated (5 and 10 mM) [Mg2+]0 the ACh, NA and Phe-evoked amylase secretion and total protein output were markedly attenuated when compared to normal (1.1 mM) Mg2+]0. The inhibitory effect of zero [Mg2+]0 was much more pronounced when compared to 10 mM [Mg2+]0. A perturbation of [Mg2+]0 had little or no effect on the Electrical Field Stimulation (EFS) -evoked amylase responses in the parotid gland. Isoprenaline-evoked amylase secretion revealed a different pattern of secretion in the presence of [Mg2+]0 perturbation. In Fura-2 loaded salivary acinar cells a perturbation of [Mg2+]0 had no significant effect on basal [Ca2+]i in the parotid gland but elevated (5 and 10 mM) [Mg2+]0 attenuated [Ca2+]i in acinar cells taken from submandibular gland. In acinar cells from both glands ACh (105 M), evoked a marked increase in [Ca2+]i above basal level in [Mg2+]0. The response comprises of an initial rise (peak response) followed by a plateau phase (plateau response) before declining back and stabilising a little above control level. In the presence of either zero or elevated (5 and 10 mM) [Mg2+]0 the ACh evoked increases (both peak and plateau phases) in [Ca2+]i was significantly (P<0.05) attenuated compared to normal (1.1 mM) [Mg2+]0. In a nominally free Ca2+ medium containing 1mM Ethyl Glycol Tetracetic Acid (EGTA), ACh evoked only the initial Ca2+ peak, which was sensitive to Mg2+ perturbation. Reperfusion of acinar cells with normal Ca2+ medium resulted in marked increases in [Ca2+]i, which was also sensitive to perturbation of [Mg2+]0. These results indicate that Mg2+ is regulating cellular Ca2+ homeostasis. In Magfura 2-loaded parotid acinar cells an increasing perturbation of [Mg2+]0 resulted in a gradual increase in intracellular free Mg2+ concentrations ([Mg2+])i. Stimulation of the cells with ACh resulted in a significant (P<0.01) decrease in [Mg2+]i. Treatment of Magftira-2 loaded parotid acinar cells with either zero extracellular sodium ([Na+]0) (substituting it with N-Methyl-D-Glucamine (NMDG)), dinitrophenol (DNP) bumetanide, amiloride, quinidine or lidocaine resulted in significant (P<0.01) increases in [Mg2+]i. In the presence of these inhibitors ACh still stimulated a reduction in {Mg2+]i. Taken together, these studies on animal model have demonstrated marked interactions between Ca2+ and Mg2+ signalling during the stimulus-secretion coupling process in the salivary glands.
The study also employs male and female human subjects to investigate the rate of salivary secretion and the quality of saliva during normal healthy and such pathophysiolgical conditions such as ageing, diabetes and surgical procedures. The results have demonstrated that either the ageing process or type I and type II diabetes are associated with significant (P<0.01) decreases in salivary secretion. However, both ageing and type I (but not type II diabetes) are associated with increased levels of protein in the saliva. Both ageing and diabetes are associated with significant (P<0.05) increases in Ca2+ levels and significant decreases in Mg2+ and Zn2+ levels in saliva compared to age-matched controls. Ageing and diabetes are also associated with reduced levels of K in the saliva. Although type II diabetic patients presented the same type of salivary changes these were less intense when compared to type I diabetic. Surgical procedures had little or no effect on the various salivary parameters measured except for small increase in resting salivary output and protein concentration. These results indicate that both ageing and diabetes can affect markedly the function of the salivary glands leading to decreased secretion in saliva and to a variation in its composition.