[en] When cardiac sarcolemmal vesicles were incubated at 37⁰C in 160 mM NaCl containing 0.5 mM CaCl₂ and subsequently assayed for Na-Ca exchange activity, they exhibited a threefold increase in the initial rate of ⁴⁵Ca²⁺ uptake compared with vesicles incubated without added CaCl₂. Removal of endogenous Ca²⁺ by incubation of the vesicles with 0.1 mM ethylene-bis(β-aminoethylether)-N,N'-tetraacetic acid (EGTA) resulted in a 35% inhibition in exchange activity. The pretreatment with CaCl₂ produced an acceleration of Na-Ca exchange activity rather than an increase in Ca²⁺ uptake due to Ca-Ca exchange. Pretreatment of the vesicles with CaCl₂ lowered the apparent K/sub m/ of the exchange system for Ca²⁺. The effects of the Ca treatment were reversed by subsequently incubating the vesicles with EGTA. In contrast to the effects of intravesicular Ca²⁺ on Na/sub i/-dependent Ca²⁺ uptake, external Ca²⁺ had no effect on Na₀-dependent Ca²⁺ efflux. The results suggest that an understanding of the kinetics of the Na-Ca exchange system may be hampered by the autoacceleration of exchange activity that occurs during initial rate measurements as Ca²⁺ accumulates within the vesicles. This phenomenon may contribute to the variability that exists among different vesicle preparations in their apparent K/sub m/ values for Ca²⁺

[en] Quinacrine either inhibited or stimulated Na-Ca exchange in cardiac sarcolemmal vesicles, depending on the experimental conditions. When present in the assay medium for Na-Ca exchange, quinacrine inhibited both Na/sub i/-dependent Ca²⁺ uptake (K/sub i/ = 50 μM) and Na₀-dependent Ca²⁺ efflux. Quinacrine's inhibition of Ca₂⁺ efflux was attenuated by high concentrations of Na⁺. Quinacrine also blocked Na-Na and Ca-Ca exchange activities in the vesicles. The inhibitory effects of quinacrine on Na-Ca exchange activity are qualitatively similar to those reported previously for amiloride derivatives. When Na-loaded vesicles were preincubated with quinacrine and then assayed for Na-Ca exchange in a quinacrine-free medium, stimulation of exchange activity was observed. This stimulation was reversible on the removal of bound quinacrine and involved a reduction in the apparent K/sub m/ for extravesicular Ca²⁺. Stimulation of exchange activity under these conditions was also observed with the lipophilic cations tetraphenylphosphonium. Since Ca²⁺, quinacrine and tetraphenylphosphonium all bind strongly to sarcolemmal membranes it is suggested that the observed stimulation of exchange activity involves a local electrostatic effect of the bound cations in accelerating a rate-limiting step in the reaction mechanism for Na-Ca exchange

[en] The effects of the thymic microenvironment on autoantibody production in (NZB X NZW)F1 mice were studied. Neonatally thymectomized male and female F1 mice reconstituted with a parental or F1-irradiated thymic lobe were compared to nonreconstituted and sham-thymectomized controls. While maleness retarded the spontaneous production of ss- and ds-DNA antibodies, thymic grafts did not suppress antibodies to ss-DNA in either sex, but did suppress the production of antibodies to ds-DNA in female mice. A unique property of NZB thymic grafts was the inability to suppress anti-RBC antibodies in male mice. Thus, (i) the gender of the F1 recipient was the most important determinant of production of antibodies to ss-DNA, (ii) either maleness or the thymic microenvironment could retard production of anti-ds-DNA antibodies, and (iii) both gender and the thymic microenvironment were important in the regulation of anti-RBC antibody production. Since the administration of thymosin did not suppress autoantibody production, the effects of the thymic grafts was not solely via thymic hormone production. These studies suggest that sex hormones and/or the thymic microenvironment can exert a suppressive effect on autoantibody production and that autoantibodies differ in their susceptibility to such suppression

[en] The mechanism by which terminal guanidino nitrogen substituted analogues of amiloride inhibit Na-Ca exchange in purified cardiac sarcolemmal membrane vesicles has been investigated. These inhibitors block both Na/sub i/-dependent Ca²⁺ uptake and Na₀-dependent Ca²⁺ efflux. Inhibition of Na-Ca exchange monitored in K⁺ is noncompetitive vs ⁴⁵Ca²⁺ but competitive vs ²²Na⁺. Substitution of sucrose for K⁺ results in mixed kinetics of inhibition vs ⁴⁵Ca²⁺, suggesting a complex interaction between inhibitor and carrier under this condition. Amiloride derivatives also block two other modes of carrier action: Na-Na exchange is inhibited in a competitive fashion with Na⁺ and kinetics of Ca-Ca exchange inhibition are mixed vs Ca²⁺ in either sucrose or K⁺. However, Ca-Ca exchange inhibition can be alleviated by increasing K⁺ concentration. Dixon analyses of Na-Ca exchange block with mixtures of inhibitors suggest that these agents are interacting at more than one site. In addition, Hill plots of inhibition are biphasic with Hill coefficients of 1 and 2 at low and high inhibitor concentration, respectively. These results indicate that amiloride derivatives are mechanisms-based inhibitors that interact at two classes of substrate-binding sites on the carrier; at low concentration they bind preferentially to a site that is exclusive for Na⁺, while at higher concentration they also interact at a site that is common for Na⁺, Ca²⁺, and K⁺