EFFECT OF CYCLOPIAZONIC ACID, AN INHIBITOR OF SARCOPLASMIC-RETICULUM CA2+-ATPASE, ON THE FREQUENCY-DEPENDENCE OF THE CONTRACTION-RELAXATION CYCLE OF THE GUINEA-PIG ISOLATED ATRIUM

1 The relevance of a functional sarcoplasmic reticulum (SR) membrane system to the contraction-relaxation cycle and to the force-frequency relationship of guinea-pig atrial tissue was investigated. Cyclopiazonic acid (CPA) was used to inhibit selectively the activity of the SR Ca2+-ATPase. IC50 values of 0.2 mu M or 1.0 mu M were measured in guinea-pig isolated SR membranes in the absence or presence of millimolar ATP, respectively. CPA (0.3-30 mu M) did not inhibit the activity of the sarcolemmal Na+-Ca2+-exchanger as measured in isolated cardiac cell membrane preparations. 2 In guinea-pig isolated left atrium paced at 2.5 Hz (30 degrees C), CPA (1-l00 mu M) produced a concentration-dependent reduction in developed tension and a fall in the maximum rate of tension increase (+dT/dt(max)) and decrease (-dT/dt(max)). The twitch duration was markedly increased due to a prolongation of the time to peak tension, and in particular, the relaxation phase. 3 The contraction-relaxation cycle of the left atrium showed a marked dependence on the frequency of stimulation. The developed tension and +dT/dt(max) showed a progressive increase from 0.5 Hz, reaching peak values at a stimulation rate of 1.5-2.5 Hz, the positive staircase phenomenon. Higher frequencies of stimulation caused a fall in these parameters. Resting tension was unaffected. The time-course of the contraction-relaxation cycle was also frequency-dependent, with both time to peak tension and relaxation time showing a progressive fall from 2.0-3.5 Hz. 4 The addition of CPA (30 mu M) caused marked alterations in the frequency-dependence of the contraction-relaxation cycle. The frequency-dependence of developed tension, +dT/dt(max) and -dT/dt(max), was shifted downwards, particularly at higher frequencies, and the frequency at which peak values of +dT/dt(max) and -dT/dt(max) were reached was shifted leftwards. The resting tension of the tissues in the presence of 30 mu M CPA was increased markedly at frequencies greater than 2 Hz. The time-course of the contraction-relaxation cycle was markedly prolonged between 1.0 and 3.5 Hz, due to an effect on both time to peak tension and relaxation time. 5 In conclusion, these results show that CPA is a highly selective inhibitor of the cardiac SR Ca2+-ATPase, without effect on the sarcolemmal Na+-Ca2+-exchanger, and suggest that a functional SR Ca2+-ATPase is necessary for the normal contraction-relaxation cycle of guinea-pig cardiac tissue. Additionally, the results suggest an increasing dependence of tension development on SR Ca2+-ATPase with increasing frequency, which may reflect either a frequency-dependent activation of this enzyme or the diminished contribution of the Na+-Ca2+ exchanger. These results also provide novel support for the mechanism of the depressed force-frequency relation found in cardiac tissue of heart failure patients, in which there is a reduced expression of Ca2+-ATPase.