Electrical and mechanical effects of procaine on mammalian heart muscle

Summary

• 1. Both membrane potential and contraction were recorded in muscle trabeculae of sheep hearts. Addition of 40 mg-% of procaine to the Tyrode solution decreased the rate of rise of the action potential, reduced its duration and decreased the strength of contraction.

• 2. Trabeculae depolarized uniformly by square current pulses across a double sucrose-gap in Tyrode solution gave a mechanical response which was not affected by the presence of procaine (60 to 100 mg-%). With a modified low-sodium Tyrode (5 mM Na/l) however, the mechanical response to the depolarizing current decreased until it reached zero, on addition of procaine to the solution.

• 3. All these effects of procaine were completely reversible up to concentrations of 100 mg-% applied for 1 hour.

• 4. Increasing the calcium concentration to 9 mM/l in the presence of procaine restored the contraction in a low-sodium solution.

  In the absence of procaine, the same increase of the calcium concentration had a large positive inotropic effect in the Tyrode solution but not in the low-sodium solution.

• 5. The addition of 2 mg-% of TTX to the Tyrode solution in presence of 100 mg-% of procaine did not alter the mechanical response.

• 6. These results are consistent with the assumption that in a modified low-sodium Tyrode procaine reduces a slow inward current of calcium ions, causing an electromechanical decoupling.

In the normal Tyrode solution however, the negative inotropic effect of procaine seems to be fully explained by the shortening of the action potential.