SALIVATION TRIGGERED BY PILOCARPINE INVOLVES AQUAPORIN-5 IN NORMAL RATS BUT NOT IN IRRADIATED RATS

Using rats, we examined the muscarinic receptor subtype mediating pilocarpine-induced parotid salivary secretion and the contributions of ion transporter systems (effluxes of K(+) and Cl(-)) and aquaporin-5 (AQP5) translocation to this response in parotid glands in irradiated-induced xerostomia. Salivary secretion was significantly lower in irradiated compared with sham-irradiated (normal) rats. In xerostomia rats, 0.4 and 0.8 mg/kg pilocarpine significantly increased parotid salivary secretion, although the salivary volume was still significantly less than in normal rats after the same dose of pilocarpine. Pirenzepine (1 x 10(-6) to 1 x 10(-1) mol/L), AF-DX 116 (3 x 10(-6) to 3 x 10(-2) mol/L) and N-2-chloroethyl-4-piperidinyl diphenylacetate (4-DAMP; 1 x 10(-8) to 1 x 10(-2) mol/L) dose-dependently displaced radioligand binding to M(1), M(2) and M(3) receptors, respectively, in parotid membranes from both normal and irradiated rats. In each group of rats, 4-DAMP had the highest binding affinity. Pretreatment with 4-DAMP or pirenzepine dose-dependently inhibited pilocarpine-induced parotid secretion in both normal and irradiated rats, with 4-DAMP being markedly more potent than pirenzepine. Normal and irradiated-rat parotid cells did not differ significantly in terms of pilocarpine-induced changes in [Ca(2+)](i), [K(+)](i) and [Cl(-)](i). Pilocarpine markedly increased the amount of AQP5 in the apical plasma membrane of parotid cells isolated from normal but not irradiated rats. Thus, pilocarpine induces parotid salivary secretion mainly via the M(3) receptor subtype in both irradiated and normal rats. The reduction in this pilocarpine-induced secretion seen in irradiated rats is due not to disturbances of intracellular Ca(2+) mobilization or ion transporter systems, but rather to a disturbance of AQP5 translocation, which may be involved in the pathogenesis of X-ray irradiation-induced xerostomia.