Actions of adrenergic agonists on isolated excretory ducts of submandibular glands

The effects of norepinephrine and isoproterenol on the transepithelial potential difference (PD) and the net transepithelial fluxes of Na, K, Cl, and HCO3 of the main ducts of the submandibular glands of rats and rabbits have been studied by microperfusion of ducts incubated in vitro in an artifical bathing medium. In the rabbit duct, both catecholamines caused depolarization and reduced transepithelial Na reabsorption at concentrations above 10(-29) M. In the rat duct, norepinephrine reduced PD and net Na reabsorption and, in addition, inhibited net K secretion at concentrations of 10(-7) M and above. Isoproterenol also depolarized the duct and reduced net K reabsorption, but at concentrations of 10(-9) M it stimulated net Na reabsorption, whereas at concentrations of 10(-4) M it inhibited Na reabsorption. The stimulation caused by isoproterenol at low concentrations could be blocked by prior administration of propranolol. The results suggest that electrolyte transport, by both rat and rabbit ducts, may be influenced not only by sympathetic nerve activity but also by physiological changes in the concentrations of circulating catecholamines. Inconsistencies in the literature regarding the sensitivity of the rabbit duct to catecholamines and the response of the rat duct to isoproterenol have now been resolved.