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 s ystem to the contraction-relaxation cycle and to the force-frequency r elationship of guinea-pig atrial tissue was investigated. Cyclopiazoni c acid (CPA) was used to inhibit selectively the activity of the SR Ca 2+-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 AT P, respectively. CPA (0.3-30 mu M) did not inhibit the activity of the sarcolemmal Na+-Ca2+-exchanger as measured in isolated cardiac cell m embrane preparations. 2 In guinea-pig isolated left atrium paced at 2. 5 Hz (30 degrees C), CPA (1-l00 mu M) produced a concentration-depende nt reduction in developed tension and a fall in the maximum rate of te nsion increase (+dT/dt(max)) and decrease (-dT/dt(max)). The twitch du ration was markedly increased due to a prolongation of the time to pea k 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 stimula tion rate of 1.5-2.5 Hz, the positive staircase phenomenon. Higher fre quencies of stimulation caused a fall in these parameters. Resting ten sion was unaffected. The time-course of the contraction-relaxation cyc le was also frequency-dependent, with both time to peak tension and re laxation time showing a progressive fall from 2.0-3.5 Hz. 4 The additi on of CPA (30 mu M) caused marked alterations in the frequency-depende nce of the contraction-relaxation cycle. The frequency-dependence of d eveloped tension, +dT/dt(max) and -dT/dt(max), was shifted downwards, particularly at higher frequencies, and the frequency at which peak va lues 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 wa s 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 relaxa tion time. 5 In conclusion, these results show that CPA is a highly se lective 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 gu inea-pig cardiac tissue. Additionally, the results suggest an increasi ng 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+ exchange r. These results also provide novel support for the mechanism of the d epressed force-frequency relation found in cardiac tissue of heart fai lure patients, in which there is a reduced expression of Ca2+-ATPase.