The divalent cations Mg2+ and Mn2+ in excess of the concentrations required to complex with ATP (excess Mg2+ or Mn2+) modulate the activity of adenylate cyclase. As a substrate, Mn∙ATP was at least as effective as Mg∙ATP in supporting adenylate cyclase activity in white and brown adipose tissue membranes. Both excess Mg2+ and Mn2+ had quantitatively different effects on the enzyme of lean and ob/ob mice and qualitatively different effects in white and brown adipose tissue. In white adipocyte membranes excess Mg2+ increased basal activity, as well as activity owing to guanylylimidodiphosphate (Gpp(NH)p) (with or without isoproterenol) and NaF. Maximal activation by Gpp(NH)p + isoproterenol required a higher concentration of Mg2+ in tissue from ob/ob than lean mice. Excess Mn2+ prevented the activation of the enzyme by Gpp(NH)p or Gpp(NH)p + isoproterenol in a dose-dependent manner. Mn2+ inhibited even in the presence of maximally effective Mg2+ concentrations. The enzyme of the ob/ob mouse membrane required a significantly higher dose of Mn2+ to achieve 50% inhibition. In brown adipose tissue, specific activities of the isoproterenol + Gpp(NH)p and NaF stimulated enzyme were significantly lower in the obese mice under all conditions studied. Except that NaF-stimulated activity was increased significantly more in the membranes of lean mice by the combination of Mg2+ + Mn2+, these cations did not produce significantly different dose-dependent effects in membranes from lean and ob/ob mice. Maximal activation occurred at lower concentrations of MgCl2 (3–5 versus 10–20 mM) and required higher concentrations of MnCl2 (3–5 versus 1 mM) in brown than in white adipose tissue membranes. Furthermore, Mn2+ in excess of the concentration required to activate maximally produced little or no inhibitory effect on the brown adipose tissue enzyme. These studies illustrate the diversity of adenylate cyclase modulation in different tissues. Further studies with bacterial toxins will be necessary to verify whether there are differences in the equilibrium of association of the stimulatory and inhibitory components of guanine nucleotide regulatory proteins in tissues of lean and obese mice.
Light and Electron Microscopic Studies on the Brown Adipose Tissue in the Bat
