Differences in regulatory mechanisms of atrial and ventricular muscle contraction in bovine heart

The purpose of this study was to characterize the regulatory mechanisms of atrial muscle contraction. Natural actomyosin (NAM) and tropomyosin-troponin (TM-TN) complex were prepared from atrial and ventricular muscle of the same bovine heart. The results were as follows: (1) Atrial NAM was more sensitive to Ca2+ than was ventricular NAM: the pCa required for 50% ATPase activation was 5.96 +/- 0.10 vs. 5.63 +/- 0.07, (mean +/- SE; n = 6; p less than 0.01); (2) reconstitution of desensitized actomyosin of rabbit skeletal muscle plus atrial or ventricular TM-TN complex produced higher Ca2+ sensitivity in atrial muscle than in ventricular muscle: the pCa required for 50% ATPase activation was 6.48 +/- 0.10 vs. 6.23 +/- 0.15 (n = 3; p less than 0.05); (3) the amount of inorganic phosphate covalently bound to atrial NAM was equivalent to that bound to ventricular NAM; (4) SDS-polyacrylamide gel electrophoresis of the two NAMs revealed several protein bands of different mobility from 16,000 to 30,000 daltons; and (5) the superprecipitation response of atrial NAM was characterized by a stepwise change in turbidity after the addition of MgATP, in contrast to the biphasic pattern of ventricular NAM. These data suggest that the free Ca ion concentration required for atrial muscle contraction is lower than that required for ventricular muscle contraction and that the difference is attributable to differences in atrial and ventricular regulatory proteins.