Early Radiation Effects on Muscarinic Receptor-Induced Secretory Responsiveness of the Parotid Gland in the Freely Moving Rat

Abstract

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.

Defects in muscarinic receptor-coupled signal transduction in isolated parotid gland cells after in vivo irradiation: evidence for a non-DNA target of radiation

“…These data, however, were based on essentially qualitative studies showing radiation-induced apoptosis in monkey salivary glands. Our more quantitative analysis in rat parotid glands did not detect apoptosis (Paardekooper et al, 1998) or cell loss (Coppes et al, 2000 to an extent that could explain the large drop in parotid gland function after irradiation. Therefore, the classical concept of DNA being the primary target was abandoned and, based on in vivo secretion studies, a defect in receptor-effector signal transduction of water secretion was proposed as a novel target, at least in rats (Coppes et al, 2000.…”

“…In rats, however, a 50% drop in secretory capability 3 days after irradiation was associated with less than 3% of apoptosis activity (Paardekooper et al, 1998). Moreover, accurate cell counting demonstrated no significant cell loss within 10 days after irradiation (Coppes et al, 2000), implying that disappearance by cell death pathways other than classical apoptotic routes also cannot explain the acute loss in function of the salivary glands after radiation. Thus, loss in gland function must be due to radiation-induced cellular dysfunction rather than cell loss and/or impaired cell renewal.…”

“…An increase in intracellular free Ca 2 þ (Ca 2 þ i ) concentration is the predominant mechanism triggering the fluid secretion from acinar cells (Melvin, 1999), whereas PKC activates the secretion of low amounts of proteins in the saliva (Tojyo et al, 1992). Changes herein may, however, be difficult to show in vivo due to the relatively low amount induced through simulation of this pathway (Coppes et al, 2000). The bulk of protein secretion is mediated through stimulation of b-adrenoceptors and subsequent cAMP-induced exocytosis .…”

Évaluation quantitative de la secrétion et du pouvoir tampon de la salive au cours de la radiothérapie des cancers des voies aéro-digestives supérieures au Sénégal

“…Les récepteurs muscariniques responsables de la libération salivaire ne sont plus activés, induisant la réduction précoce du flux salivaire. Il n'y a pas de perte cellulaire dans les 10 premiers jours [12]. Si l'irradiation est arrêtée à ce stade, une récupération fonctionnelle est possible, le flux reste stable et s'améliore même légèrement à la fin du traitement [12].…”

“…Il n'y a pas de perte cellulaire dans les 10 premiers jours [12]. Si l'irradiation est arrêtée à ce stade, une récupération fonctionnelle est possible, le flux reste stable et s'améliore même légèrement à la fin du traitement [12]. Néanmoins, à partir du 10 ème jours, Tableau III.…”