Targeted inhibition of sarcoplasmic reticulum CaMKII activity results in alterations of Ca2+ homeostasis and cardiac contractility.

Transgenic (TG) mice expressing a Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitory peptide targeted to the cardiac myocyte longitudinal sarcoplasmic reticulum (LSR) display reduced phospholamban phosphorylation at Thr17 and develop dilated myopathy when stressed by gestation and parturition (Ji Y, Li B, Reed TD, Lorenz JN, Kaetzel MA, and Dedman JR. J Biol Chem 278: 25063-25071, 2003). In the present study, these animals (TG) are evaluated for the effect of inhibition of sarcoplasmic reticulum (SR) CaMKII activity on the contractile characteristics and Ca2+ cycling of myocytes. Analysis of isolated work-performing hearts demonstrated moderate decreases in the maximal rates of contraction and relaxation (+/-dP/dt) in TG mice. The response of the TG hearts to increases in load is reduced. The TG hearts respond to isoproterenol (Iso) in a dose-dependent manner; the contractile properties were reduced in parallel to wild-type hearts. Assessment of isolated cardiomyocytes from TG mice revealed 40-47% decrease in the maximal rates of myocyte shortening and relengthening under both basal and Iso-stimulated conditions. Although twitch Ca2+ transient amplitudes were not significantly altered, the rate of twitch intracellular Ca2+ concentration decline was reduced by approximately 47% in TG myocytes, indicating decreased SR Ca2+ uptake function. Caffeine-induced Ca2+ transients indicated unaltered SR Ca2+ content and Na+/Ca2+ exchange function. Phosphorylation assays revealed an approximately 30% decrease in the phosphorylation of ryanodine receptor Ser2809. Iso stimulation increased the phosphorylation of both phospholamban Ser16 and the ryanodine receptor Ser2809 but not phospholamban Thr17 in TG mice. This study demonstrates that inhibition of SR CaMKII activity at the LSR results in alterations in cardiac contractility and Ca2+ handling in TG hearts.