Localization and secretion of epidermal growth factor in the parotid gland and its intragastric kinetics in sheep

Ruminants secrete a large quantity of saliva that is rich in electrolytes; however, it remains unclear whether their parotid saliva contains epidermal growth factor (EGF). The present study was set up to examine the distribution of EGF and transforming growth factor-α (TGF-α) in the ovine parotid and submandibular glands and the salivary secretion of EGF-like binding activity (EGF-LBA) as the sum of EGF and TGF-α in conscious sheep. We also measured changes in the intragastric concentration of EGF-LBA in the ovine rumen and abomasum, and examined the effect of bilateral diversion of parotid saliva on intragastric EGF-LBA concentration in sheep. Both the ovine parotid and, to a lesser extent, the submandibular glands contained EGF-LBA. Immunohistochemical study showed that EGF and TGF-α-immunoreactivities were localized in the ductal epithelium in both glands. Transcriptional expression of EGF and TGF-α mRNA was demonstrated in both glands by reverse transcription–polymerase chain reaction (RT–PCR). In conscious sheep, the parotid gland continuously secreted EGF-LBA in the saliva before feeding, and the secretion of parotid EGF-LBA was markedly increased during feeding. After diversion of the parotid saliva for 1 week, EGF-LBA concentration in the ruminal fluid, but not in the abomasal fluid, decreased in the postprandial period, indicating that parotid EGF-LBA is a primary source of EGF-LBA for the rumen fluid during the postprandial period in sheep. Moreover, RT–PCR detected the expression of TGF-α mRNA in the rumen and abomasum and that of EGF in the abomasum, implying that these stomachs possibly supply, in part, EGF-LBA to the luminal fluid.