HYPERTROPHY ALTERS EFFECT OF INS(1,4,5)P3 ON CA2+ RELEASE IN SKINNED RAT-HEART MUSCLE

The effects of D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] on the ability of the sarcoplasmic reticulum (SR) to accumulate and release Ca2+ and on the Ca2+ sensitivity of the contractile proteins were investigated using chemically (saponin) skinned cardiac fibers (60-120-mu-m diam) obtained from normal and pressure-overloaded hypertrophied rat left ventricles. Left ventricular pressure overload was induced by partial ligation of the abdominal aorta 3-6 wk before study. Age- and weight-matched normal rats served as controls. Pressure overload increased the left ventricular weight-to-body weight ratio by 45%. Ins(1,4,5)P3 at a concentration of 10-mu-M did not change the Ca2+-tension relationship at Ca2+ concentrations of 10(-7) to 10(-5) M in either normal or hypertrophied fibers. Ins(1,4,5)P3 also did not influence Ca2+ uptake by the SR in either normal or hypertrophied fibers. Ins(1,4,5)P3 did not induce Ca2+ release from the SR directly in either group. However, pretreatment with Ins(1,4,5)P3 enhanced the 5 mM caffeine-induced Ca2+ release by 80.5 +/- 22.7% in normal fibers but not in hypertrophied fibers. We conclude that Ins(1,4,5)P3 enhances, rather than directly induces, SR Ca2+ release in normal rat hearts and that sustained pressure overload diminishes the response of the SR Ca2+-release system to Ins(1,4,5)P3, an action that may be partly responsible for contractile dysfunction in cardiac hypertrophy.