McHardy Smith - Muscarinic cholinergic receptor-mediated attenuation of adenylate cyclase activity in rat heart membranes.

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      Smith, M. M., Harden, T. K. 10 197-210-

      Abstract: The regulation of adenylate cyclase by   muscarinic cholinergic receptors has been studied in rat heart   membranes. In the presence of isoproterenol the K0.5 for   Mg2+-mediated activation of adenylate cyclase was 0.1 mM; the   addition of the muscarinic receptor agonist oxotremorine   increased the K0.5 for Mg2+, and activation by Mg2+ no longer   followed mass action kinetics for a single site interaction. The   extent of oxotremorine-mediated attenuation of adenylate cyclase   exhibited a Mg2+ concentration dependence: in short-time assays   at 0.25 mM free Mg2+ the attenuation of enzyme activity was 55   percent, whereas at 20mM Mg2+, only 20 percent inhibition was   observed. The increase in the apparent K0.5 for Mg2+ in the   presence of oxotremorine was less in 20 min assays than in 4 min   assays. The effects of oxotremorine on the rates of activation   and deactivation of adenylate cyclase also were examined.   Oxotremorine increased the rate of deactivation of adenylate   cyclase by two-fold. At low free Mg2+ concentrations,   oxotremorine also decreased the rate of activation of adenylate   cyclase in the presence of either 0.1 uM GTP gamma S or 1 uM GTP.   An oxotremorine-mediated decrease in the rate of activation of   adenylate cyclase activity was not detectable in the presence of   greater than 3 mM Mg2+.

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