A. G. Schmidt, V. J. Kadambi, N. Ball, Y. Sato, R. A. Walsh, E. G. Kranias and B. D. Hoit. Cardiac-specific Overexpression of Calsequestrin Results in Left Ventricular Hypertrophy, Depressed Force–frequency Relation and Pulsus Alternans In Vivo.Journal of Molecular and Cellular Cardiology (2000) 32, 1735–1744. Cardiac-specific overexpression of calsequestrin has been shown to result in significant decreases in contractile parameters and intracellular Ca²⁺transients in vitro. Therefore, the purpose of the present study was to determine the effects of calsequestrin overexpression on basal cardiac function and the force–frequency relation in vivo. Calsequestrin overexpression mice (CSQ-OE, n=20) and their isogenic controls (WT) were studied with an integrative approach using transthoracic echocardiography, stress-shortening relations, and invasive hemodynamics in intact closed-chest mice. M-mode echocardiography indicated that calsequestrin overexpression resulted in concentric hypertrophy (+52 %) and an increase in LV ejection phase indices. However, mean end-systolic stress-shortening coordinates revealed that at matched end-systolic wall-stress, fractional shortening was depressed in CSQ-OE mice. This was confirmed by depressed indices of LV isovolumic contraction and relaxation in CSQ-OE v WT mice. Furthermore, overexpression of calsequestrin resulted in a downward and leftward shift of the biphasic force–frequency relation; thus, the critical heart (HR_crit) was significantly lower in calsequestrin-overexpression mice (264±15 bpm) than in wild-type controls (365±21 bpm). Surprisingly, calsequestrin overexpression was associated with the induction of pulsus alternans in every animal (at an average heart rate of 428±26 bpm), whereas none of the wild-type controls displayed this phenomenon. We conclude that: (i) although increased levels of calsequestrin result in decreased myocardial contractility and a depressed force–frequency relation, LV wall stress is reduced and chamber function is normal, and (ii) an increase in SR Ca²⁺storage capacity induces pulsus alternans in the intact anesthetized mouse.