<Li> 5 . An abrupt increase in afterload can cause a small increase in inotropy (Anrep effect) by a mechanism that is not fully understood . </Li> <Li> 6 . An increase in heart rate also stimulates inotropy (Bowditch effect; treppe; frequency - dependent inotropy). This is probably due to the inability of Na / K - ATPase to keep up with the sodium influx at the higher frequency of action potentials at elevated heart rates </Li> <Li> 7 . Drugs . Drugs like digitalis can act as a positive inotropic agent by inhibiting the Na / K pump . High Na concentration gradient is necessary to pump out sarcoplasmic calcium via the Na / Ca antiporter . Inhibition of the Na / K causes extra sodium to accumulate inside the cell . The buildup the Na concentration inside the cell will cause the gradient from inside the cell to the outside of the cell to decrease slightly . This action will make it more difficult for calcium to leave the cell via the Na / Ca antiporter . </Li> <Li> 8 . Increase the amount of calcium in the sarcoplasm or in the T - tubules . More calcium available for Troponin to use will increase the force developed . </Li>

The force and velocity of contraction of cardiac muscle cells can be increased by