Although the salivary glands have a low rate of cell turnover, they are relatively radiosensitive. To study the possible mechanism behind this inherent radiosensitivity, a rat model was developed in which saliva can be collected after local irradiation of the parotid gland without the use of anesthetics or stressful handling. Saliva secretion was induced by the partial muscarinic receptor agonist pilocarpine (0.03-3 mg/kg) with or without pretreatment with the beta-adrenoceptor antagonist propranolol (2.5 mg/kg), or the full muscarinic receptor agonist methacholine (0.16-16 mg/min), and measured during 5 min per drug dose before and 1, 3, 6 and 10 days after irradiation. The maximal secretory response induced by pilocarpine plus propranolol was increased compared to that with pilocarpine alone but did not reach the level of methacholine-induced secretion, which was about five times higher. One day after irradiation a decrease in maximal pilocarpine-induced secretion was observed (-22%) using the same dose of pilocarpine that induces 50% of the maximal response (ED(50)), in both the absence and presence of propranolol, indicating that the receptor-drug interaction was not affected by the radiation at this time. The secretory response to methacholine 1 day after irradiation, however, was normal. At day 3 after irradiation, the maximal methacholine-induced secretion was also affected, whereas pilocarpine (+/-propranolol)-induced maximal secretion decreased further. At day 6 after irradiation, maximal secretory responses had declined to approximately 50% regardless of the agonist used, whereas ED(50) values were still unaffected. No net acinar cell loss was observed within the first 10 days after irradiation, and this therefore could not account for the loss in function. The results indicate that radiation does not affect cell number or receptor-drug interaction, but rather signal transduction, which eventually leads to the impaired response. We hypothesize that the early radiation effect, within 3 days, may be membrane damage affecting the receptor-G-protein signaltransfer. Later critical damage, however, is probably of a different nature and may be located in the second-messenger signal transduction pathway downstream from the G protein, not necessarily involving cellular membranes.

To investigate whether secretory granules play a role in the radiosensitivity of the salivary glands of rats, parotid acinar cells, submandibular acinar cells and/or submandibular granular convoluted tubule (GCT) cells were degranulated prior to irradiation. Degranulation of GCT cells was obtained by pretreatment with phenylephrine (5 mg/kg, t = -60 min) and methacholine (3.75 mg/kg, t = -120 min). Degranulation of acinar cells was attained by pretreatment with isoproterenol (5 mg/kg, t = -90 min). Combinations of pretreatments were also tested. Irradiation was performed with a single dose of 15 Gy of X rays. Samples of parotid and submandibular/sublingual saliva were collected 4 days prior to and 1, 3, 6, 10 and 30 days after irradiation. Pretreatment with phenylephrine, isoproterenol and methacholine plus phenylephrine resulted in less radiation damage to parotid gland function as indicated by the lag phase and flow rate. Since the pretreatment with phenylephrine and phenylephrine plus methacholine did not degranulate parotid gland acinar cells, the observed protective effect on this gland cannot be explained by the "degranulation concept." Furthermore, salivary gland function was significantly greater 3 days after irradiation as a result of pretreatment with phenylephrine and phenylephrine plus methacholine compared to rats given only radiation. This may indicate recovery from damage rather than a reduced amount of initial damage. The sparing was most obvious for the later effects (6-30 days). Submandibular/sublingual gland function was improved significantly after pretreatment with methacholine plus phenylephrine, although no increase in degranulation of GCT cells was observed compared to pretreatment with phenylephrine alone, again not favoring the degranulation concept. The results indicate that the secretory granules do not play the often-assumed important role in the radiosensitivity of the salivary gland. The mechanism underlying the observed improvement of salivary gland function may involve second messenger-induced increases in proliferation of salivary gland cells resulting in recovery of tissue after the irradiation.