Parotid secretion patterns during meals and their relationships to the tonicity of body fluids and to gastrin and pancreatic polypeptide in sheep.

The flow pattern of unilateral parotid saliva in sheep was compared when a total of 800 g lucerne (Medicago sativa) hay was offered as one, two, four or eight distinct meals. These patterns were related to changes in the tonicity of rumen fluid and plasma and to plasma concentrations of gastrin and pancreatic polypeptide. Sheep having ad lib access to hay overnight were offered fresh hay from 08.00 to 09.00 hours and were then given one, two, four, or eight meals of fresh hay according to a schedule such that the mean deprivation period was 6.5 h for each meal frequency-size. Neither the peak in saliva flow rate nor the time of this peak differed among the different meal sizes. The flow rate decreased rapidly after reaching a maximum at 3.2 min into the meal. After 7 min of eating, the tonicity of plasma and rumen fluid had increased by only 2.2 and 8.2 mosmol/kg respectively. These increases would not cause the rapid decline in parotid flow observed after 3.2 min of eating. There was no postprandial change in the concentration of gastrin in jugular plasma. However, it did increase significantly (P = 0.0043) from 16 to 4 min before eating commenced. There was a postprandial peak in plasma pancreatic polypeptide concentration after 4.5 min of eating. However, the parotid flow rate remained low after the concentration of this peptide returned to prefeeding levels. The rapid decrease in parotid secretion rate observed early in the meal may be due to subsiding central excitation rather than to an inhibitory factor limiting production.