AJP - Gastrointestinal and Liver Physiology, Vol 251, Issue 1 84-G89, Copyright © 1986 by American Physiological Society
Choline evokes fluid secretion by perfused rat mandibular gland without desensitization
M. Murakami, I. Novak and J. A. Young
The secretomotor action of choline on salivary secretion has been studied
in the isolated perfused mandibular gland of the rat. Choline made up in
substituted Ringer solutions (Na concentrations of 40, 70, or 100 mM) was
an effective secretomotor agonist in the concentration range of 1-100 mM
and evoked secretory responses comparable with those of acetylcholine
(0.05-1.0 microM) administered at similar Na concentrations. Continuous
infusion of choline, in contrast to acetylcholine, did not lead to a fall
off in the secretory response (desensitization or tachyphylaxis) until the
choline concentration used exceeded approximately 80 mM. The secretomotor
action of choline (46 mM) was blocked completely and reversibly by atropine
(5 microM) but not by propranolol (2.5 microM) and phentolamine (2.5
microM) administered together. When acetylcholine (1.0 or 0.05 microM) was
infused on a background of choline stimulation, a small additional
secretory response was elicited whenever the choline stimulus was
submaximal, and, regardless of the choline concentration employed, addition
of acetylcholine always induced tachyphylaxis. The results suggest that
choline and acetylcholine act on the same population of muscarinic
receptors, but it is unclear why choline does not evoke tachyphylaxis. The
response to choline allows us to exclude a number of the possible causes of
tachyphylaxis that have previously been considered, so that an excessive
buildup of cytosolic free Ca now remains as the most likely cause of the
phenomenon.
