Changes in enzymatic and structural properties of
ventricular myosin in thyrotoxic rabbit hearts have been investigated extensively. However, there is little information regarding the effect of
thyroid hormone on the
atrial myosin. In this study, we have compared enzymatic and structural changes of ventricular and
atrial myosin from euthyroid, hypothyroid, and
hyperthyroid rabbits. In euthyroid rabbits, Ca++- and
actin-activated ATPase activities of
atrial myosin were 2-fold greater than those of the
ventricular myosin. The Ca++- and
actin-activated ATPase activities of
atrial myosin from hypothyroid and
hyperthyroid rabbits were identical with the values for
atrial myosin from euthyroid rabbits. The same
ATPase activities of
ventricular myosin decreased in hypothyroid hearts but increased in
hyperthyroid rabbits. The K+ (
EDTA)-ATPase activities of all
myosins were the same, irrespective of the thyroid status of the animal.
Pyrophosphate-
polyacrylamide gel electrophoresis patterns showed two
isoenzymes (designated as A1 and A3) of
atrial myosin in euthyroid hearts. The same electrophoretic patterns also showed in
atrial myosin from hypothyroid and
hyperthyroid hearts. The
ventricular myosin from euthyroid hearts also exhibited two
isoenzymes (designated as V1 and V3) but each with slower electrophoretic mobilities than the corresponding
atrial myosin. In hypothyroid hearts, only V3
isoenzyme was seen, whereas, in
hyperthyroid hearts, only V1
isoenzyme was seen. These results suggest that
thyroid hormone controls
ventricular myosin ATPase activity by controlling synthesis of a specific ventricular
isoenzyme, whereas
thyroid hormone does not affect
atrial myosin ATPase, possibly due to its inability to control
atrial myosin synthesis.