Am. Beal, MECHANISMS OF FLUID AND ION SECRETION HY THE PAROTID-GLAND OF THE KANGAROO, MACROPUS-RUFUS, ASSESSED BY ADMINISTRATION OF TRANSPORT-INHIBITING DRUGS, Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology, 165(5), 1995, pp. 396-405
Possible mechanisms of primary fluid formation by macropodine parotid
glands were investigated in anaesthetized red kangaroos using ion tran
sport inhibitors. Carotid plasma amiloride Concentrations of 0.05-0.5
mmol . l(-1) progressively reduced a stable acetylcholine-evoked half-
maximal flow rate of 2.0 +/- 0.04 to 0.22 +/- 0.024 mi min(-1) (mean /- SEM). Concurrently, saliva bicarbonate concentration and secretion
fell (135 +/- 1.6 to 67 +/- 1.7 mmol . l(-1) and 272 +/- 7.6 to 15 +/-
2.6 mu mol . min(-1), respectively); [phosphate], [chloride] and [sod
ium] rose and [potassium] and osmolality were unaltered. High-rate cho
linergic stimulation did not increase saliva flow beyond 11 +/- 1.0% o
f that for equivalent pre-amiloride stimulation. Equipotent levels of
amiloride and methazolamide given concurrently were no more effective
at blocking flow and bicarbonate secretion than when given separately.
Furosemide (up to 2 mmol . l(-1)), bumetanide (up to 0.2 mmol . l(-1)
) and ethacrynate (1 mmol . l(-1)) in carotid plasma had no effect on
salivary flow or ion concentrations. During methazolamide blockade, fu
rosemide did not curtail the concurrent increase in salivary [chloride
]. Chlorothiazide at 0.25-1.0 mmol . l(-1) caused progressive depressi
on of saliva flow and [bicarbonate], and elevation of [chloride]. 4-ac
etamido-4'-isothiocyanatostilbene-2, phonic acid at 0.1 mmol . l(-1) w
as without effect, whereas at 0.5 mmol . l(-1) it stimulated fluid sec
retion and increased saliva [protein], [sodium], [potassium], [bicarbo
nate] and osmolality. Concurrently, mean arterial blood pressure and p
ulse pressure fell and heart rate, haematocrit and carotid artery plas
ma flow rose. These responses were absent if saliva flow was kept cons
tant by reduction in cholinergic stimulation during acetamido-4-isothi
ocyanatostilbene-2,2'disulphonic acid administration. It is concluded
that secretion of primary fluid by the kangaroo parotid is initiated m
ainly (> 90%) by secretion of bicarbonate which is formed in the endpi
ece cells from CO2 delivered by the circulation. No evidence was found
for initiation of fluid secretion by chloride transport involving bas
olateral Na+-K+-2Cl(-) symports, Na+-Cl- symports or Cl-1/HCO3- antipo
rts.