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

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Abstract
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. This data was 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 this 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 .apprx. 1-2 min, peaked and declined to approach prestimulus levels after 3-4 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 element in determining the time-course of the cAMP secretion response.