Frequency-force relationship in guinea-pig ventricular myocardium as influenced by magnesium

1. In guinea-pig papillary muscle, the characteristic relation between force of contraction and frequency is changed by the sithdrawal of magnesium from the incubation medium. In magnesium-free solution, reduction of contraction frequency below 0.1 Hz leads to an increase in force of contraction which reaches its maximum at a frequency of 0.00166 Hz (i.e., one contraction every 10 min). After magnesium withdrawal, the frequency-force relationship in guinea-pig ventricular muscle resembles that of guinea-pig atrial muscle in magnesium-containing solution. 2. The increase by magnesium withdrawal in contractile force of guinea-pig papillary muscles contracting at low frequencies is the result of an increase in contraction velocity. The time to peak force is shortened and the relaxation time is prolonged. 3. After obtaining steady-state values of contractile force at 1 Hz contraction frequency, stimulation was terminated and the time course of changes in the inherent contractile activity of the muscle was determined by eliciting single contractions at time intervals of between 0.5 and 10 min duration. After cessation of stimulation, the contractile activity declines exponentially in solution containing 1.2 mM Mg2+; in magnesium-free solution an initial decline is followed by a slowly developing increase. This rise in contractile activity is reduced at 3.2 mM Ca2+ by the presence of 0.075 mM Mg2+ and is prevented by 0.3 mM Mg2+. 4. The increase in contractile activity obtained in the papillary muscle during rest by magnesium withdrawal depends in its magnitude on [Ca2+]0. 5. The rested-state contractile activity of ventricular muscle in magnesium-free solution is reduced by 75% with the first and by 90% with the second contraction after onset of 1 Hz stimulation. 6. Possible mechanisms are discussed by which Mg2+ inhibits the development of rested-state contractile activity in the guinea-pig ventricular myocardium.