Rapid exchange of actin-bound nucleotide in perfused rat heart

In the rat heart the actin-bound nucleotide contained both ATP and ADP. The ratio of bound ATP to bound ADP depended on the functional state of the heart; it was higher in hearts stopped reversibly in diastole (low Ca(2+), high Mg(2+), or high K(+)), than in stimulated (inotropic agents or pacing) hearts. Immunoblotting and gel electrophoresis showed the existence of G-actin (30% of total actin) in the cytoplasm of the heart. Pure actin was isolated from rat hearts: in G-actin the bound nucleotide readily exchanged with ATP or ADP, and in F-actin the bound nucleotide did not exchange with ATP or ADP. The free and bound nucleotides were separated in the intact heart by extraction with 75% methanol at -15 degrees C. In rat hearts perfused with (32)P-labeled orthophosphate the actin-bound nucleotide rapidly exchanged with the cytoplasmic ATP. The full exchange of the bound ATP was immediate, whereas the full exchange of the bound ADP was slower. The full exchange of the bound ATP was independent of the heartbeat frequency, whereas the full exchange of the bound ADP was frequency dependent. The data suggest that the transformation of actin monomer-ATP to actin polymer-ADP is a part of the normal contraction-relaxation cycle of the rat heart.