Inhibition of muscarinic stimulated phosphoinositide hydrolysis in the rat parotid gland by cAMP

Protein kinase C expression in salivary gland acinar epithelial cells in non-obese diabetic mice, an experimental model for Sjögren's syndrome

We planned to investigate the expression of protein kinase C (PKC) isoforms in acinar epithelial cells of salivary glands in the non-obese diabetic (NOD) mouse to find out if they develop changes of the PKC system like those seen in the human counterpart, i.e. in Sjögren's syndrome. Parotid, submandibular, and sublingual glands from NOD and control BALB/c mice were stained with a panel of monoclonal antibodies directed against conventional (alpha, beta, and gamma), novel (delta, epsilon, and theta), and atypical (lambda and iota) PKC isoforms using the streptavidin/HRP method. Similarly to human labial salivary glands, acinar epithelial cells of the healthy control BALB/c mice contained two of the conventional PKC isoforms, alpha and beta. Acinar and ductal epithelial cells also contained the atypical PKC isoforms lambda and iota. PKC isoforms gamma, delta, epsilon, and theta were not found. NOD mice which displayed focal sialadenitis contained the same conventional and atypical PKC isoforms. The acinar cells in NOD mice, in contrast to the Sjögren's syndrome patients, did not lack PKC alpha or beta. On the contrary, PKC alpha and beta staining was stronger than in the control BALB/c mice. The present results demonstrate that both conventional and atypical PKC isoforms participate in the salivary epithelial cell biology and that there are mouse strain-associated and/or disease state-associated changes in their expression. The lack of PKC alpha and beta isoforms found in Sjögren's syndrome was not reproduced in NOD mice, which discloses one more difference between the human disease and its NOD mouse model.

Early to late sparing of radiation damage to the parotid gland by adrenergic and muscarinic receptor agonists

Damage to salivary glands after radiotherapeutic treatment of head and neck tumours can severely impair the quality of life of the patients. In the current study we have investigated the early-to-late pathogenesis of the parotid gland after radiation. Also the ability to ameliorate the damage using pretreatment with adrenergic or muscarinic receptor agonists is studied. Rats were locally irradiated with or without i.p. pretreatment with phenylephrine (alpha-adrenoceptor agonist, 5 mg kg(-1)), isoproterenol (beta-adrenoceptor agonist, 5 mg kg(-1)), pilocarpine (4 mg kg(-1)), methacholine (3.75 mg kg(-1)) (muscarinic receptor agonists) or methacholine plus phenylephrine. Parotid salivary flow rate, amylase secretion, the number of cells and gland histology were monitored sequentially up to 240 days postirradiation. The effects were described in 4 distinct phases. The first phase (0-10 days) was characterised by a rapid decline in flow rate without changes in amylase secretion or acinar cell number. The second phase (10-60 days) consists of a decrease in amylase secretion and is paralleled by acinar cell loss. Flow rate, amylase secretion and acinar cell numbers do not change in the third phase (60-120 days). The fourth phase (120-240 days) is determined by a further deterioration of gland function but an increase in acinar cell number, albeit with poor tissue morphology. All drug pretreatments used could reduce radiation effects in phase I and II. The protective effects were lost during phase IV, with the exception of methacholine plus phenylephrine pretreatment. The latter combination of drugs ameliorated radiation-damage throughout the entire follow-up time. The data show that combined pre-irradiation stimulation of muscarinic acetylcholine receptors with methacholine plus alpha-adrenoceptors with phenylephrine can reduce both early and late damage, possibly involving the PLC/PIP2 second messenger pathways. This opens perspectives for the development of clinical applicable methods for long-term sparing of parotid glands subjected to radiotherapy of head and neck cancer patients.

Early radiation effects on muscarinic receptor-induced secretory responsiveness of the parotid gland in the freely moving rat

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.

Muscarinic receptor stimulation increases tolerance of rat salivary gland function to radiation damage

PURPOSE

To investigate if muscarinic receptor-stimulated activation of the PLC/PIP2 second messenger pathway prior to irradiation increases the radiotolerance of rat salivary gland.

MATERIALS AND METHODS

Rats were treated with pilocarpine, methacholine, reserpine, methacholine plus reserpine, or atropine prior to irradiation with a single dose of 15 Gy X-rays. Parotid and submandibular/sublingual saliva was collected 4 days before and 1-30 days after irradiation. Lag phase, flow rate, amylase secretion, and salivary sodium and potassium concentration were measured.

RESULTS

Pretreatment with pilocarpine or methacholine resulted in an improvement of all measured functions of both glands. Pretreatment with reserpine had no effect on parotid gland function. Reserpine plus methacholine did not increase parotid gland function when compared with methacholine alone, indicating a purely muscarinic receptor stimulation as the initiator for the induced radioprotection. Pretreatment protective effects on submandibular gland function of reserpine plus methacholine were additive, indicating cooperation of muscarinic and alpha-adrenergic receptors. Atropine pretreatment slightly increased the radiation induced loss of salivary gland function.

CONCLUSIONS

Preirradiation activation of PLC/PIP2 second messenger pathway through stimulation of muscarinic receptors reduces the salivary gland radiosensitivity. The observed protection of salivary gland function may be of a secondary nature, implicating a cell conditioning after receptor stimulation of the PLC/PIP2 pathway.