Blockade of isoprenaline-induced fluid and protein secretion by the mandibular glands of the red kangaroo, Macropus rufus, with selective antagonists

Selective and non-selective β-adrenoceptor antagonists were used to block the increases in fluid and protein secretion caused by sympathomimetic stimulation of the mandibular gland of red kangaroos during intracarotid infusion of isoprenaline. Atenolol or ICI118551 at antagonist:agonist ratios up to 300:1 caused increasing but incomplete blockade of fluid secretion and protein release. Both selective antagonists had equal potency and both antagonists were more effective at blocking protein release than at blocking fluid secretion. Consequently, the mechanisms underpinning fluid secretion are more sensitive to β-sympathomimetic stimulation than those causing protein release. Propranolol at antagonist:agonist ratios of 300:1 was more potent than the selective antagonists, almost totally blocking the increases in fluid secretion and protein release. The data are consistent with the acini of the kangaroo mandibular gland having both β1- and β2-adrenoceptors and with the increased fluid secretion and protein release by isoprenaline being mediated by both receptor subtypes.