In a previous study we have shown that the rat lacrimal gland acinar granules change from serous to seromucous and subsequently to mucous in nature during ageing. Moreover, these changes are associated with decreased tear secretion in both quantity and composition (Draper et al. 1999). Since the parotid salivary gland is similar in structure and function to the lacrimal gland, then it is possible that the ageing process may also affect the structure and function of the parotid gland, which may lead to such complications as dry and burning mouth syndromes. The aim of this study was to investigate the morphology and α-amylase secretion in parotid glands obtained from young and ageing male Wistar rats. Four age groups of 2- to 3-month (n = 4), 12-month (n = 4), 16-month (n = 4) and 20-month-old (n = 4) rats were humanely killed and used for the morphological study using established methods (Draper et al. 1999). Acini counts were performed using light microscopy at X 40 magnification and a total of ten random fields were taken from four slides using four animals for each age group. For α-amylase measurement, acini were isolated from 2- to 6-month and 12-month-old parotid glands, and incubated for 30 min at 37 °C in a shaking water bath with different concentrations of acetylcholine (ACh, 10^-9 M to 10^-4 M), noradrenaline (NA, 10^-9 M to 10^-4 M), or isoprenaline (ISOP, 10^-9 M to 10^-4 M). α-Amylase release was measured using the Phadebas blue starch method (Ceska et al. 1969).

Light microscopic investigations into age-related changes in the isolated rat parotid gland showed marked differences in the organisation and distribution of parotid acini within the gland. Typically, in young (2Ð3 months as controls) parotid glands, the acini were visible in a compact arrangement, with defined parotid ducts, and with little or no connective tissue and lipid infiltration. At 12 months no striking age-related morphological change was noticed compared with 2- to 3-month-old glands. In contrast, glands from 16- and 20-month-old animals showed a spacious acini distribution, and enlarged parotid ducts with numerous and thick connective tissue septa which were localised around and within the parotid ducts. Lipid accumulation was also apparent in 16- and 20-month-old glands when compared with control. Acini counts indicated a significant (Student’s unpaired t test; P < 0.01) decrease in the mean acini distribution, when compared with control mean acini count (mean ± S.E.M.; 286 ± 16) (n = 4). Acini counts for 12, 16 and 20 months were 208 ± 8.08 (n = 4), 167.77 ± 7.82 (n = 4) and 138 ± 11.4 (n = 4), respectively. Basal amylase release from parotid acini of 2- to 6-month and 12-month-old glands was 6.76 ± 1.06 (n = 8) and 4.98 ± 0.88 % of total (n = 8), respectively. Incubation of acini from 2- to 6-month-old gland (n = 8) with ACh, NA or ISOP (all 10^-9Ð10^-4 M) resulted in marked dose dependent increases in amylase release with maximal effect occurring at 10^-5 M. In contrast, parotid acini from 12-month-old glands released significantly (P < 0.05) less amylase during stimulation with different concentrations of ACh, NA or ISOP (n = 8 for each). The results indicate that ageing can lead to marked morphological changes and a decrease in amylase release from the rat parotid gland.

All procedures accord with current UK legislation.