Read/Search this Article
The FOK rat which has been developed by heat selection and inbreeding, shows a high resistance to heat-exposure as well as to cold-exposure. It secretes a large amount of saliva under a hot environment. To understand how the enhanced salivation is related to the resistance to heat exposure (HE), we investigated at first the saliva secretion of FOK rats under the treatment of adrenergic-, cholinergic blockers as well as cholinergic stimulant, and secondary the immunohistochemical expression of aquaporin-5 (AQP5) and heat shock protein70 (HSP70) in submandibular gland, since aquaporins especially AQP5 is proposed to play a crucical role in water movement through cell membrane and HSP70 is thought to relate stress reaction. By HE, FOK rats secreted a larger amount of saliva than WKAH rats via a moderate cholinergic, a transient α-adrenergic and a strong cholinergic drives depending on the different phase of rectal temperature (Tre). AQP5 immunoreactivity was detected an apical membrane (AM) and lateral membrane (LM) of acinar cells in FOK as well as WKAH rats it was also found in basal membrane (BM) in FOK rats. These immunodensities were significantly higher in FOK rats than those in WKAH rats, suggesting a larger capability of transmembrane water permeability in FOK rats. By HE a slight increase in AQP5 immunoreactivity in the AM and LM of acinar cells and a slight decrease in granular cinvoluted tubule (GCT) was detected in FOK rats, but the immunoreactivity was significantly decrease both in acinar cells and GCT in WKAH rats. Blockage of parasympathetic activity by atropine (1mg/kg) caused inhibition of salivation under HE, but no significant changes in immunoreactivity of AQP5 was found in FOK rats. On the other hand, parasympathetic agonist, pilocarpine (1mg/kg), induced salivation in both strains under normal room temperature. In both strains expression of AQP5 in acinar cells was remained unchanged but decreased in GCT by pilocarpine. The expression of HSP70 was observed with a quite different localization pattern from that of AQP5. In acinar cells, a higher expression of HSP70 was observed in the nucleus than the cytoplasm in both strains, and in the cytoplasm it was higher in FOK than WKAH rats. By HE, the nuclear HSP70 immunoreactivity disappeared completely in WKAH and moderately in FOK rats. This cellular stress response of HSP70 was prevented by atropine treatment in FOK rats. Data suggest that the enhanced salivation of FOK rats under HE is due to a series of events : moderate cholinergic, transient α-adrenerdic, strong cholinergic drives and other factors, and the significant maintenance of acinar AQP5 expression that relates to transmembrane water permeability is deeply involved in thermal salivation and heat resistance.