Selective stimulation and blockade of β-adrenergic receptors in the mandibular gland of the red kangaroo, Macropus rufus

Abstract

Intracarotid infusions of noradrenaline (0.15 nmol · kg⁻¹ · min⁻¹) either alone or accompanied by phentolamine (1.5 nmol · kg⁻¹ · min⁻¹) caused similar-sized increases in salivary protein, magnesium and bicarbonate, and decreases in osmolality, sodium, potassium and chloride whereas intravenous noradrenaline stimulated much smaller responses. Concurrent infusions of the β₁-antagonist, CGP20712A, blocked these noradrenaline-induced changes in salivary composition more effectively than equimolar infusions of the β₂-antagonist, ICI118551, thereby confirming the presence of β₁-adrenoceptors. Intracarotid infusion of salbutamol at 0.15, 0.3 and 1.5 nmol · kg⁻¹ · min⁻¹ caused increasing but qualitatively similar changes in salivary composition, sodium excepted, to intracarotid noradrenaline with 0.3 nmol being most similar quantitatively. Intravenous infusion of salbutamol caused larger changes in salivary composition than equimolar intravenous noradrenaline thereby indicating that the response to salbutamol may, in part, be mediated by reflex increases in general sympathetic tone triggered by lowered blood pressure. Eliminating this hypotensive effect by concurrent intravenous and intracarotid infusions of β₁-(CGP or atenolol) and β₂-(ICI118551) antagonists with intracarotid salbutamol showed that ICI118551 was more potent than the β₁-antagonists thereby demonstrating the presence of β₂-receptors. It was concluded that the kangaroo mandibular has functional β₁- and β₂-adrenoceptor subtypes in both endpieces and excurrent ducts and that the duct system has two populations of cells, each expressing one receptor subtype.