Time dependence of frequency potentiation in the isolated guinea-pig's atrium

The effect of changes in cardiac frequency on left and right ventricular dP/dt max at different contractile states of the myocardium

In 17 canine heart-lung preparations the dependence of frequency potentiation of the right and left ventricular myocardium on the basic inotropic state of the heart was investigated. The effect of unipolar stimulation of the right atrium on dP/dt max in both ventricles was measured. The aortic pressure was maintained constant. Shortly after isolation of the heart, a stepwise increase of rate from 140 to 200 beats/min only had a very weak influence on left ventricular dP/dt max. With deterioration of the myocardium the frequency potentiation of dP/dt max increased considerably. End-diastolic pressure regularly decreased with rising cardiac frequency. Since the real positive inotropic effect is masked by the concomitant fall in diastolic loading, the end-diastolic pressure was maintained constant in a second group of 8 hearts during rate variation. The most pronounced inotropic effect was now found shortly after isolation of the heart. A rate increase of 30 beats/min resulted in a 20% rise of dP/dt max. The frequency potentiation decreased with deterioration of the heart resulting in a 12% dP/dt max increase at an estimated inotropic state of 50% of control. When the contractile state of the heart was improved above the control state by calcium application the frequency potentiation of the myocardium decreased. In the right ventricle similar results were obtained except for the fact that no significant correlation between the steepness of the frequency characteristics and the contractile state of the heart could be found when the end-diastolic pressure was kept constant.

On the action of nifedipine under conditions of variable stimulation patterns and [Ca2+]0 in guinea-pig atrium

The effect of nifedipine is studied in perfused isolated guinea-pig left atrium at a temperature of 37 degrees C and a stimulation rate between 60 and 240/min. Rhythmical stimulation at 240/min was interrupted by interposed stimulus intervals (test intervals) in a range of 0.1-4 s. The pressure amplitudes developed during subsequent test contractions rise up to a maximum with increasing test intervals. This time course is described as restitution in contractile response. Results. Restitution is somewhat faster in earlier than in later stages of perfusion. A rise in [Ca2+]0 accelerates restitution without changing maximal pressure development. Lowering [Ca2+]0 depresses pressure amplitudes over the whole range of test intervals. Nifedipine slows down restitution but has (in concentrations up to 10(-6) M) no effect on the maximum achieved after long test intervals following stimulation at 240/min. The negative inotropic effect of nifedipine on rhythmical pressure development rises with increasing stimulation frequency in the range from 60-240/min. An elevation in [Ca2+]0 does not reestablish control pressure-frequency relationship. Conclusions. In respect to the restitution process and also to the pressure-frequency relationship the effect of nifedipine can neither be simulated by a reduction in [Ca2+]0 nor abolished by an elevation in [Ca2+]0.

The effect of isolation on myocardial properties

Denervation deprives the heart of its normal adrenergic and cholinergic control via the sympathetic and parasympathetic pathways. In a heart which is blood supplied by a donor animal of the same species, normal contractility is maintained, probably by blood borne catecholamines or possibly by unknown inotropic agents of the donor. A heart receiving blood oxygenated by isolated lungs is in a state of failure. Substitution of blood by a cell and protein free solution diminishes oxygen availability in cardiac muscle, both in the perfused and bathed preparation. In the unphysiological environment, myocardial cells lose K+ and gain Na+. Under best possible conditions of oxygen supply but in a later stage of perfusion, contractility during rhythmical stimulation is depressed more at lower than at higher rates. Frequency potentiation and the inotropic effectiveness of noradrenaline is more pronounced in vitro than in situ. In excised papillary muscles and ventricular and atrial strips, the disarrangement and a more or less severe lesion of individual fibres accelerates the decay in mechanical performance. The role of endogenous catecholamines for the maintenance of normal contractility in situ and in vitro is still a matter of discussion.

Time- and rate-dependence of the inotropic action of noradrenaline in the isolated guinea-pig's atrium

The inotropic action of noradrenaline (10-6 mol/l)) was studied on a left guinea-pig's atrium which was isolated functionally by ligations and perfused during artificial stimulation (basal rhythm: 120/min) over a period of 5 to 10 hrs. Each half hour the rate was varied within a range from 60 to 300 imp/min.

RESULTS

When modernaline is applied continuously throughout the experienment, contractility in terms of amplitude and rate of pressure development is almost maintained on the control level at the stimulation rates over at least 4 hrs of perfusion. When noradrenaline is applied at various stages of perfusion it compensates for the foregoing decay of contractility even after 8 hrs. Since contractility during normal perfusion decreases much more at a lower than at a higher rate the inotropic action of noradrenaline is minimal at a high rate in the fresh preparation (this means: under physiological conditions) and is fully developed only during later stages of perfusion at a low rate (which is far below the physiological range.