Modulation of serous salivary gland function by the sympathetic nervous system : a biochemical and ultrastructural study with special reference to β-adrenoceptor subtypes

Sammanfattning: The aim of the present investigation was to study the influence of the sympathetic nervous system and of various adrenoceptor agents on enzyme secretion and morphology in rat parotid and guinea-pig submandibular glands. Biochemical methods were combined with electron microscopical techniques.Two different in vitro systems were employed, batch-incubation and microperifusion, to characterize the sympathetically evoked amylase release and its correlation to cyclic AMP. By using various selective β-adrenoceptor agonists and antagonists a dominance of the β1-adrenoceptor over the β2 - in regulating amylase release - was establ ished. Continuous noradrenaline perifusion caused a rapid initial amylase discharge, closely correlated to tissue levels of cyclic AMP; no correlation between the two was observed during the later phase. Prenalterol (a β1-agonist) failed to elevate glandular cyclic AMP. This was in contrast to its potent secretagogic effect. On the other hand, terbutaline (a β2-agonist) was a weak secretagogue but markedly raised the levels of cyclic AMP. Thus, β-adrenoceptor activation may lead to release of large amounts of amylase despite minimal or no increase in cyclic AMP. Moreover, these effects seemed to be dissociated in salivary glands with regard to the β-adrenoceptor subtypes. This was further substantiated by the findings that repeated injections of prenalterol induced qualitative changes in the granule populations, similar to those caused by the non-selective β-agonist isoprenaline. Terbutaline was without effect. However, acinar cells size was increased following both prenalterol and terbutaline treatment. The data suggest that the 3-adrenergic effects on acinar cell size and granule population may be independently regulated.A decreased sympathetic activity of long duration was induced by neonatal or adult extirpation of the superior cervical ganlion on one side. Acinar cell size, as well as granule and amylase content was reduced 9 weeks after neonatal denervation. Ganglionectomy performed in adult animals was without significant effects.The secretory behaviour of neonatally denervated glands was characterized by an increased postjunctional sensitivity to 3-adrenoceptor agonists. Of special interest was the finding that neonatal denervation seemed to transform terbutaline from a partial to a full secretory agonist, thus changing its effects in the direction of those of prenalterol and noradrenaline. Moreover, increased levels of cyclic AMP as well as an enhanced response to DBcAMP were noted in the denervated glands as were intracellular changes. The denervation supersensitivity after neonatal denervation seems to differ from that observed in adult denervated glands. The results of the studies on denervated glands suggest that the sympathetic nervous system plays a fundamental role in the early maturation of the rat parotid gland as well as for the development of the β-adrenoceptor subtypes.