Abstract

The rate of adenylate cyclase activation via agonist-bound receptors in intact cells can be partly dependent on the rate of turnover of occupancy by agonist with respect to individual receptors. For instance, low occupancy of the full complement of receptors by epinephrine in intact S49 cells has been shown to promote a rate of activation that is substantially greater than that for high occupancy of a small number of receptors for which the concentration of epinephrine-bound receptors is the same. According to the encounter coupling model, a partial dependence of the relationship between receptor occupancy and adenylate cyclase activity on the agonist binding frequency can in principle be explained by episodic interactions of finite duration (encounters) between individual pairs of receptor and GTP-binding protein. The mean lifetime of the agonist-receptor complex and the frequency of binding relative to the mean duration of such encounters dictate whether there is variation of the state of the receptor during an encounter and the extent to which the overall rate of GTP-binding protein activation can be dependent on binding frequency. We present here a quantitative analysis of agonist concentration versus cyclase response curves in terms of the encounter coupling model that explicitly includes agonist binding frequency, the encounter frequency, and the encounter duration as parameters. The essential result is that the model is quantitatively consistent with concentration versus response curves for receptor-mediated activation of adenylate cyclase in S49 cells. It is also shown that the model is consistent with data on the differential effects of antagonists to inhibit agonist-stimulated cyclase activation in a manner that is dependent on the antagonist binding frequency.