Cyclic 3',5'-AMP relay in Dictyostelium discoideum III. The relationship of cAMP synthesis and secretion during the cAMP signaling response.

Refinement of a perfusion technique permitted the simultaneous measurement of cAMP-elicited [3H]cAMP secretion and intracellular [3H]cAMP levels in sensitive D. discoideum amoebae. These data were compared with measurements of the rate of [32P]cAMP synthesis by extracts of amoebae sonicated at different times during the cAMP signaling response. cAMP stimulation of intact cells led to a transient activation of adenylate cyclase, which was blocked if 10(-4) M NaN3 was added with the stimulus. During responses elicited by 10(-6) M cAMP, 10(-8) M cAMP, and an increment in cAMP from 10(-8) M to 10(-7) M, the rate of cAMP secretion was proportional to the intracellular cAMP concentration. Removal of a 10(-6) M cAMP stimulus 2 min after the initiation of the response led to a precipitous decline in intracellular cAMP. This decline was more rapid than could be accounted for by secretion alone, suggesting intracellular phosphodiesterase destruction of newly synthesized cAMP. Employing these data and a simple rate equation, estimates of the time-course of the transient activation of adenylate cyclase and the rate constants for cAMP secretion and intracellular phosphodiesterase activity were obtained. The calculated rate of cAMP synthesis rose for approximately 1 to 2 min, peaked, and declined to approach prestimulus levels after 3 to 4 min. This time-course agreed qualitatively with direct measurements of the time-course of activation, indicating that the activation of adenylate cyclase is a major in determining the time-course of the cAMP secretion response.