Parotid gland secretion of the aging rat

Saliva secretion stimulated by alpha adrenergic beta adrenergic and muscarinic cholinergic neurotransmitters was evaluated in young adult and old rats. Parotid saliva was collected following methoxamin (METH) (6 mg/kg), isoproterenal (ISO) (30 mg/kg), or pilocarpin (PILO) (7.5 mg/kg) as secretory stimuli. The flow rate and the concentrations of protein, Na+ and K+ were measured. Saliva flow rate was 40%–50% lower (P < 0.01) in old rats following METH and PILO, but there was no difference following ISO stimulation. Saliva of old rats stimulated by METH or PILO contained about 30%–40% more (P < 0.01) protein. Comparable protein levels were in saliva stimulated by ISO. No changes were found in Na+ and K+ concentrations in all saliva samples. These results suggest that the age-related alteration in flow rate is not a general phenomenon, but rather specific and limited to the high flow rate stimulants (alpha-adrenergic and muscarinic-cholinergic). Transport alteration from the circulatory system into the parotid gland may be the underlying mechanism.