Immunocytochemical localization of salivary peptide P-C in human submandibular gland

Human saliva contains a proline-rich polypeptide, salivary peptide P-C, which potentiates insulin release and reduces glucagon release from perfused rat pancreas to decrease blood glucose level. To elucidate the process of secretion into humoral fluid of this peptide morphologically, we investigated ultrastructural localization of P-C in human submandibular gland by immunogold technique with anti-peptide P-C whose specificity to P-C was confirmed by immunoblotting. The labeling with gold particles which represents the distribution of P-C-like-immunoreactivity (P-C-LI) was detected in the secretory granules and rough endoplasmic reticula of the acinar serous cells and in few mucosa cells. P-C-LI was also observed in the lumen of striated duct but not intracellularly in the ductal cells themselves, indicating that P-C is not probably reabsorbed there. These results suggest that salivary peptide P-C is present in acinar serous cells, is secreted into the oral cavity, and may be reabsorbed through the digestive tract to modulate the blood glucose level after feeding.