Inhibition of rat salivary gland adenylate cyclase by glycosaminoglycans and high molecular weight polyanions

Abstract

The effects of hyaluronic acid, chondroitin-4 and -6 sulphate, dermatan sulphate, heparin and the sulphated, polysugar dextran sulphate on membrane-associated adenylate cyclase were investigated in a plasma-membrane fraction derived from the glands. Adenylate-cyclase activity was inhibited in a dose-dependent fashion by all. The potency (concentration causing a 50 per cent inhibition (Ki) of adenylate cyclase activity) of each molecule varied with the degree of sulphation of the agent tested. The GTP and Gpp(NH)p activation of adenylate cyclase as well as basal-enzyme activity and of adenylate cyclase by isoproterenol (a beta-adrenergic agonist) were inhibited; however, little effect on hormone binding (assessed by [3H]-dihydroalprenolol binding) was observed at Ki concentrations. Inhibition of NaF-activation of adenylate cyclase was not as pronounced as the other inhibitory effects. The findings suggested that the agents inhibited enzyme activity by action at or near the catalytic site; hence the effect of these molecules on forskolin activation of adenylate cyclase was investigated. The polyanions inhibited forskolin stimulation of the enzyme to the same degree that they had previously inhibited basal, GTP and hormonally-stimulated enzyme activity. Thus, these molecules inhibited adenylate cyclase by an action (perhaps charge-related) that directly effected the stabilization of catalytic site of the enzyme. Thus polyanions of the type tested can directly modulate adenylate-cyclase activity and may serve as potential regulatory molecules for various biologic functions which are cyclic-AMP dependent.