Exchange of the actin-bound nucleotide in intact arterial smooth muscle

The actin-bound ADP was separated from cytoplasmic nucleotides by treatment of intact arterial smooth muscle with 50% ethanol. In P-32-labeled smooth muscle the actin-bound ADP and phosphate readily exchanged with the cytoplasmic [gamma,beta-P-32]ATP; the specific radioactivity of actin-bound ADP was equal to that of the beta -phosphate of cytoplasmic ATP and the specific radioactivity of actin-bound phosphate was equal to that of the gamma -phosphate of cytoplasmic ATP. In contrast, the exchange of the actin-bound ADP in skeletal muscle was very slow. The presence of cytoplasmic ATP was required for the exchange of the actin-bound ADP and phosphate; if ATP synthesis was inhibited the exchange was also inhibited. The extent of exchange was reduced in muscles contracted by histamine or K+, as compared with resting muscles. The exchange was also shown in other mammalian smooth muscles, uterus, urinary bladder, and stomach. The data indicate a dynamic state of actin in smooth muscle. The data also suggest that polymerization-depolymerization of actin is part of the contraction-relaxation cycle of smooth muscle.