Intracellular Titration of Cyclic AMP Bound to Receptor Proteins and Correlation with Cyclic-AMP Levels in the Surviving Rat Diaphragm Lien DO KHAC,Simone HARBON Hubert J. CLAUSER and
lnstitut de Biochimie, Universit6 de Paris-Sud, Orsay (Received April 9/July 17, 1973)
Extracts prepared from rat diaphragms incubated with or without theophylline and/or epinephrine have been tested for their total cyclic AMP content and for their ability to bind exogenously added cyclic ["]AMP. Less cyclic [3H]AMP can be bound inthe extracts after theophylline and/or epinephrine treatment indicating that the rise in cyclic AMP level was accompanied by a n increase in the quantity of cyclic AMP bound intracellularly to the cyclic AMP-dependent protein kinases. Maximum cyclic AMP binding capacities, as measured by total cyclic AMP exchanges, were however identical in all cases. Accurate estimations of intracellular binding of cyclic AMP have been correlated with the level of cyclic AMP in the tissue : the reaction seems to obey simple saturation kinetics, a n apparent intracellular K d for cyclic AMP has been evaluated as 330 nM. The findings are consistent either with a real difference in the intracellular binding constant as compared to that measured in vitro (28 nM) or with the fact that the cyclic nucleotide in the cell may not all be available for the kinase protein receptors. They also suggest that the method described may prove useful for studying any possible intracellular control beyond the step of cyclic AMP synthesis.
Regulation of cellular metabolism by adenosine 3' :5'-monophosphate (cyclic AMP) [I], its mediation through complex protein kinases [2,3] and the mechanism of the activation of these enzymes [4--61 have been well documented within the past years in the eukaryotic cell. Activation has been demonstrated to occur according to Equation (1) through a n interaction of cyclic AMP with the regulatory subunit (R) of the enzyme, leading to a dissociation of this subunit from the catalytic subunit (C) which is thus activated. RC cyclic AMP + R cyclic AMP C . (1)
However completely satisfactory correlations between the levels of intracellular cyclic AMP and its ultimate metabolic effects have been in many cases difficult to obtain. Striking examples for this situation are to be found in the results of Craig et al.  in rat diaphragm, of Stull and Mayer  in rabbit skeletal muscle concerning the regulation of phosphorylase activation, of Schaeffer et al.  and Miller et al. [lo] concerning regulation of glycogen metabolism in adrenalectomized rats, and of Harbon and Clauser [Ill This work is dedicated to Professor E. Lederer for his 65 th anniversary. Abbreviations. Cyclic AMP; adenosine 3': 5'-monophosphate.
in the rat uterus stimulated by prostaglandin El or E,. I n all these cases, cyclic AMP levels may be elevated without eliciting the expected metabolic responses. Two hypotheses have been formulated to explain these obvious discrepancies, either a decrease in the activation of the enzymes mediating cyclic AMP action within the cell, or a compartmentalization of the intracellular nucleotide. Hence it seems necessary to measure directly the degree to which the first step of the activation sequence (Equation 1)reflects the apparent intracellular cyclic AMP concentrations. This might be achieved by establishing in intact cells or tissues, correlations between the levels of intracellular cyclic AMP under welldefined physiological conditions, the extent to which it is bound to the specific receptor protein and the extent to which the complex protein kinases are in the active state. Satisfactory correlations between cyclic AMP levels and protein kinase activation have been recently established in various tissues by Corbin et al. [I21 and Soderling et al. . The present work was to investigate if correlations could also be obtained between intracellular cyclic AMP...