Some aspects of the genetic and non-genetic control of the amount and rate of calcium cycled during steady-state activation of papillary muscles from right ventricular rabbit myocardium are presented. Genetic reorganization of the intracellular structure of the myocardium is achieved by producing right ventricular pressure overload and thyrotoxic hypertrophy. The mechanical performance of the pressure overload heart is slowed while time to peak tension is increased. These changes are associated with an increase in myothermal economy. In thyrotoxic hypertrophy the rate of mechanical performance is increased while time to peak tension is decreased. These alterations are associated with a decrease in myothermal economy. Tension-independent heat is used as an index of calcium cycling. In pressure overload hearts the amount and rate of calcium cycling is decreased. In contrast in thyrotoxic hypertrophy the amount of calcium cycled is unchanged while the rate is increased. In the pressure overload hearts there is a decrease in sarcoplasmic reticular (SR) Ca++ ATPase, whereas in the thyrotoxic preparations the message is increased. The change in the rate of calcium uptake in pressure overload and thyrotoxic hearts is correlated with a change in the amount of SR Ca++ ATPase mRNA. Calcium cycling was also altered by non-genetic inotropic intervention. Isoproterenol (1 microM) increases the amount of calcium cycled during each contraction relaxation cycle and the rate at which it is removed. These alterations are associated with an increase in force and a foreshortened twitch. Incubating the papillary muscle in high calcium (11 mM) also increases the force and the amount of calcium released into the cytosol. Under these circumstances the rate of uptake is not significantly increased and, accordingly, the isometric twitch is not foreshortened. In the presence of verapamil (14 microM) the peak twitch force is decreased and the isometric myogram is foreshortened. These changes are associated with a decrease in the amount of calcium released during activation and the rate at which it is removed.
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