Effects of Protein Deficiency and Diet Consistency on the Parotid Gland and Parotid Saliva of Rats

Protein deficiency results in an increased susceptibility to dental caries, suggesting that oral host-defense properties are compromised. An important component of oral host defense is salivary gland function, which is affected by both protein deficiency and diet consistency. This study describes the effects on rat parotid gland growth and secretory function induced by feeding rats diets of normal (20%) or moderately low (7%) protein content, provided in either a powdered or solid form. In addition, since protein deficiency may result in a secondary zinc deficiency which, in turn, may affect salivary gland function, the effects of these diets on liver zinc concentration were also measured. From 22 to 47 days of age, rats (18/group) were fed the following diets: normal protein, powdered; normal protein, solid; low protein, powdered; and low protein, solid. With each diet consistency, liver zinc was higher for the normal protein group. Within each protein level, liver zinc was higher for the solid diet. This latter observation suggests that food mastication and the resultant stimulation of salivary gland function may also play a role in zinc metabolism. With the normal-protein diet, parotid gland weight was higher for the solid diet; with the low-protein diet, parotid gland weight was similar for both consistencies and did not differ from that of the group fed the "normal protein, powdered" diet. For both consistencies, parotid saliva protein concentration was greater for malnourished rats. Parotid saliva protein composition, measured by quantitative scans of SDS-PAGE gels, showed that basic proline-rich proteins were increased several-fold in both groups of malnourished rats, while proteins identified as deoxyribonuclease, parotid secretory protein, acidic epididymal glycoprotein, and common salivary protein 1b were reduced. Within each dietary protein level, only minor differences in protein composition were found between the solid and powdered consistencies. These studies show that protein deficiency has a marked effect on the protein composition of rat parotid saliva.