The effect of acetazolamide, amiloride, bumetanide and SITS on secretion of fluid and electrolytes by the parotid gland of common wombats, Vombatus ursinus.
Mechanisms of saliva formation by wombat parotid glands were investigated in anaesthetized wombats at two levels of cholinergically-stimulated flow viz. mid-range (30-40% maximum flow) and maximum flow using ion-transport and carbonic-anhydrase inhibitors. Bumetanide (0.005-0.1 mmol l-1 carotid plasma) progressively reduced mid-range flow by 52 ± 3.4% (mean ± SEM). Concurrently, saliva [Cl] decreased, [Na] and [HCO3] increased but HCO3 excretion was unaltered. Salivary flow during high-rate cholinergic stimulation was 31 ± 1.1% of the pre-bumetanide maximum. During mid-range stimulation, SITS (0.075 mmol l-1) was without effect whereas 0.75 mmol l-1 stimulated transient increases in fluid output. The higher SITS concentration caused no alterations to flow or electrolyte concentrations during maximal stimulation. Carotid plasma [amiloride] (0.05 mmol l-1) caused immediate falls in flow rate of 20-30% followed by progressive recovery over 25 min to levels above pre-amiloride flow rates despite plasma [amiloride] increasing tenfold. Concurrently, salivary [Na] and [Cl] rose to equal plasma concentrations and [K] fell by 50% indicating blockade of acinar Na/H exchangers and luminal Na channels in the ducts. Increased salivary osmolarity caused the flow recovery. Saliva flow during maximum cholinergic stimulation was reduced by 38-46%. The depression of flow was interpreted as resulting from competition between amiloride and acetylcholine for access to the muscarinic receptors. Plasma [acetazolamide] (0.35-2.5 mmol l-1) did not alter saliva outflow during mid-range or maximum flow regimes whereas salivary [Cl] increased and [HCO3] decreased consistent with reduced anion exchange resulting from inhibition of carbonic anhydrase. Combined with bumetanide, acetazolamide (1.5 mmol l-1) reduced flow by an additional 18-22% relative to bumetanide alone thereby demonstrating that acinar HCO3 synthesis supported a limited proportion of saliva formation and that some HCO3 secretion was independent of carbonic anhydrase activity.