FIG. 4.

A qualitative description of latency and adaptation in the signal transduction process under a constant stimulus. The time for activation of G protein is the major component of the latency between stimulus arrival and activation of the adenylate cyclase. Adaptation at the receptor level results from the interaction of ${\text{G}}_{\text{i}}^{\prime }$ with HR_s, which reduces the amount of activated G_s and cyclase. (a) Response curves for HR_s and HR_i under a constant stimulus. T_s1 and T_i1, represent the half-maximal times for the respective components. The dashed line indicates the response for HR_s in the absence of an inhibitory effect. (b) Response curves for the concentrations of ${\text{G}}_{\text{s}}^{\prime }$ and ${\text{G}}_{\text{i}}^{\prime }$. T_s2 and T_i2 represent the half-maximal times for the respective components. The dashed line indicates the response if the inhibitory effect is not present. (c) In the presence of ${\text{G}}_{\text{i}}^{\prime }$, HR_s is diminished due to the formation of ${\text{HR}}_{\text{s}}{\text{G}}_{\text{i}}^{\prime }$. The half-maximal time, T_s3, for HR_s in the presence of ${\text{G}}_{\text{i}}^{\prime }$ is approximately the same as T_s1, whereas T_i3 > T_i2. (d) Response curves for the concentration of ${\text{G}}_{\text{s}}^{\prime }$ and ${\text{G}}_{\text{s}}^{\prime }\text{AC}$. The half-maximal time for ${\text{G}}_{\text{s}}^{\prime }$, T_s4, is approximately equal to T_s2, whereas the half-maximal time for ${\text{G}}_{\text{s}}^{\prime }\text{AC}$, T_s5, is greater than T_s4. The rate of cAMP_i production is proportional to the concentration of G_sAC.