Parotid responses to stimulation of the parasympathetic innervation in bursts in weaned lambs.

1. The effects of stimulation of the peripheral end of the parotid nerve in 1 sec bursts at 10 sec intervals has been investigated in weaned Suffolk lambs 3-4 months after birth.2. Stimulation in bursts at either 10 or 20 Hz for 10 min produced a significantly greater flow of parotid saliva, output of Na(+) and K(+) and fall in parotid vascular resistance (p.v.r) than continuous stimulation at the corresponding constant frequency (1 or 2 Hz) for the same period.3. Stimulation at 40 Hz in bursts for the same period produced a significantly greater output of K(+) and fall in p.v.r. than continuous stimulation at 4 Hz, whereas the salivary flow rates and Na(+) outputs were closely similar.4. Comparison of the effects of stimulation of the parotid nerve in bursts over a wide range of frequencies showed that each of these parotid responses was maximal at about 80 Hz.5. Parotid salivary responses to continuous stimulation of the parotid nerve at 20 Hz for 10 min were effectively abolished by prior administration of atropine (> 1.0 mg/kg), whereas the percentage fall in p.v.r. was merely reduced from 80.2+/-3.6 to 33.1+/-8.3% under the same conditions. There was no significant change in the mean arterior-venous difference in the plasma concentration of vasoactive intestinal peptide, substance P or somatostatin, in either the presence or absence of atropine in these experiments.6. The results are discussed in relation to the proposition that the enhancement of parotid responses which is observed during intermittent stimulation of the parasympathetic innervation at relatively high frequencies under experimental conditions also occurs normally in conscious sheep.