Contractility of Cardiac Muscle:
Substances such as catecholamines (epinephrine, norepinephrine), digitalis, isoproterenol, various pharmaceutics, and of course Ca++, alter the contractility (inotropicity) of cardiac muscle (Table 3). Twitch height, for example, is increased in the presence of catecholamine, as is the rate of rise (+dP/dt) and fall (-dP/dt) of tension (Figure 12).
In the presence of a positive inotrope (e.g. catecholamine), velocity of shortening is increased at any given afterload, as is also maximal velocity of shortening (Vmax) (Figure 13).
With multiple stimuli to skeletal muscle, multiple contractions occur, which if they occur at a sufficiently high frequency result in tetanus. This cannot occur in cardiac muscle, due to the long refractory period. Yet when cardiac muscle is stimulated at an increasing rate, in the range of 1 per 3 secs to 3 per sec, individual twitches become stronger in a step-wise, or "staircase" fashion. This is the Bowditch effect or Treppe (Figure 14). Thus, cardiac muscle contractility (inotropicity) is influenced by the rate of contraction, an effect resulting from the increased mobilization of intracellular Ca++.
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