## Abstract

The time course of activation and deactivation and the degree of activation at steady state <latex>$[E_a]/[E_{tot}]$</latex> of adenylate cyclase, in semi-purified rat pancreatic plasma membranes, were compatible with a simple two-state model with three rate constants, so that <latex>$[E_a]/[E_{tot}] = k_{+1}/(k_{+1} + k_2 + k_{-1})$</latex>. The hormone CCK-8 increased <latex>$k_{+1}$</latex> with GTP in a dose-dependent manner, from 0.2 to 10.9 min<latex>$^{-1}$</latex>; <latex>$k_{-1}$</latex> increased from 0.01 to 0.3 min<latex>$^{-1}$</latex>, i.e. in proportion, but <latex>$k_2$</latex> was unaltered at 7 min<latex>$^{-1}$</latex>, so that <latex>$[E_a]/[E_{tot}]$</latex> increased 15-fold, from 4 to 61%. A similar activation was obtained after cholera toxin pretreatment but by a different mechanism. The toxin pretreatment exerted a major inhibitory effect on the value of k<latex>$_2$</latex> and on the corresponding GTPase activity. A pretreatment at the high cholera toxin concentration (30 <latex>$\mu g$</latex>/ml) exerted two additional effects that became evident when p[NH]ppG rather than GTP was used as activating nucleotide: (a) a relatively large increase in <latex>$k_{-1}$</latex> from an unmeasurably low control value to 0.3 min<latex>$^{-1}$</latex>, and (b) a four-fold increase in the p[NH]ppG activation rate, <latex>$k_{+1}$</latex>. This contrasted with the action of CCK-8, which increased <latex>$k_{-1}$</latex> and <latex>$k_{+1}$</latex> in proportion.