Effects of cations and ouabain on thyroid adenyl cyclase

Abstract

The effects of various cations and ouabain on basal thyroid adenyl cyclase activity and on its response to the stimulatory agents, thyroid-stimulating hormone (thyrotropin, TSH) and NaF, were investigated. 1. 1. Basal or F −-induced enzyme activity was not modified by the addition of 0.1 M Na + or K +. However, TSH-stimulated cyclase activity was inhibited ( 49±8%) by 0.1 M Na + and augmented ( 41±9%) by 0.1 M K +. Simultaneous addition of 0.1 M Na + and 0.1 M K + to the incubation effected no change in TSH-induced cyclase activation. 2. 2. Li + inhibits TSH-induced cyclase activation without affecting basal activity; in the presence of 2.5 mM Mg 2+, 50% inhibition occurs at 10–25 mM Li + and greater Li + concentrations are required with higher Mg 2+ concentrations. 3. 3. Ouabain, 1·10 −6-1·10 −4 M abolished TSH activation of adenyl cyclase but did not alter NaF effects thereon. The degree of inhibition of TSH-induced adenyl cyclase activation produced by ouabain decreased with increasing concentrations of added K +. 4. 4. Among several divalent cations tested, only Mg 2+ and Mn 2+ (5 mM) augmented adenyl cyclase activity ( Mn 2+ ⪢ Mg 2+ ). The stimulatory effect of Mn 2+ on basal and F −-stimulated activity was not observed in the presence of TSH. Other divalent cations (Ca 2+, Co 2+, Cu 2+, Zn 2+) markedly reduced or abolished basal, TSH- and F −-induced cyclase activation. None of the monovalent or divalent cation additions modified the assay ATP-regenerating system or phosphodiesterase activity. 5. 5. Conclusions: (a) activity of thyroid adenyl cyclase is modified by cations and ouabain, (b) such alterations may be occasioned by ion binding to the enzyme, and, (c) TSH and F − activate the same cyclase system but do so by different means.