Mechanisms of primary fluid formation by macropodine mandibular glands were investigated in anaesthetized red kangaroos using ion-transport and carbonic anhydrase inhibitors. Bumetanide at carotid plasma concentrations of 0.005-0.1 mmol/l progressively reduced a stable, acetylcholine-evoked flow rate of 1.02 +/- 0.024 ml/min to 0.16 +/- 0.016 ml/min (mean +/- SEM). Concurrently, saliva [Na], [Cl] and osmolality decreased, [K] and [HCO3] increased and HCO3 excretion was unaffected. High-rate cholinergic stimulation was unable to increase salivary flow above 12 +/- 1.5% of that for equivalent pre-bumetanide stimulation. Furosemide (1.0 mmol/l) and ethacrynate (0.5 mmol/l) caused depression of salivary how and qualitatively similar effects on ion concentrations to those of bumetanide. Amiloride (up to 0.5 mmol/l) caused no reduction in salivary flow rates or [Na] but decreased [K] and [Cl] and increased [HCO3]. When compared with bumetanide alone, amiloride combined with bumetanide further augmented [K] and [HCO3] and lowered [Cl], but had no additional effects on Na or flow. At the higher level, 4-acetamido-4'-isothiocyanatostilbene-2,2'disulphonic acid (SITS) (0.05 and 0.5 mmol/l) stimulated fluid output, increased [HCO3] and [protein], and depressed [Na], [K] and [Cl]. Relative to bumetanide alone, SITS given with bumetanide had no additional effects on salivary flow or electrolytes. Methazolamide (0.5 mmol/l) in combination with bumetanide curtailed the decrease in [Cl] and the increases in [K] and [HCO3] associated with bumetanide. The residual methazolamide-resistant HCO3 excretion was sufficient to support 2-6% of primary fluid secretion. It was concluded that secretion of primary fluid by the kangaroo mandibular gland is initiated mainly (>90%) by Cl transport resulting from Na-K-2Cl symport activity. A small proportion of the fluid secretion (up to 6%) appears to be supported by HCO3 secretion. No evidence was found for fluid secretion being dependent on Ci transport involving Na/H and CI:HCO3 antiports or on HCO3 synthesis involving carbonic anhydrase. (C) 1997 Published by Elsevier Science Ltd.
