Phospholamban levels regulate cardiac sarcoplasmic reticulum Ca2+ pump activity and myocardial contractility. To determine whether and to what extent phospholamban modulates the force-frequency relation and ventricular relaxation in vivo, we studied transgenic mice overexpressing phospholamban (PLBOE), gene-targeted mice without phospholamban (PLBKO), and isogenic wild-type controls. Contractility was assessed by the peak rate of left ventricular (LV) isovolumic contraction (+dP/d t max), and diastolic function was assessed by both the peak rate (−dP/d t max) and the time constant (τ) of isovolumic LV relaxation, using a high-fidelity LV catheter. Incremental atrial pacing was used to generate heart rate vs. −dP/d t max(force-frequency) relations. Biphasic force-frequency relations were produced in all animals, and the critical heart rate (HRcrit) was taken as the heart rate at which dP/d t max was maximal. The average LV +dP/d t maxincreased in both PLBKO and PLBOE compared with their isogenic controls (both P < 0.05). The HRcrit for LV +dP/d t max was significantly higher in PLBKO (427 ± 20 beats/min) compared with controls (360 ± 18 beats/min), whereas the HRcrit in PLBOE (340 ± 30 beats/min) was significantly lower compared with that in isogenic controls (440 ± 25 beats/min). The intrinsic heart rates were significantly lower, and the HRcrit and the ±dP/d t max at HRcrit were significantly greater in FVB/N than in SvJ control mice. We conclude that 1) the level of phospholamban is a critical negative determinant of the force-frequency relation and myocardial contractility in vivo, and 2) contractile parameters may differ significantly between strains of normal mice.
Dynamic control of maximal ventricular elastance via the baroreflex and force-frequency relation in awake dogs before and after pacing-induced heart failure
