Abstract 5305: Knockout of the δ Isoform of CaMKII does not Negatively Influence Cardiomyocyte Excitation-Contraction Coupling in Mice
CaMKII is critically involved in excitation-contraction coupling (ECC) in the heart. Overexpression of the most prominent isoform CaMKIIδC leads to altered ECC and heart failure. Specific inhibition of CaMKIIδ may be a novel therapeutic approach in heart failure. Whether CaMKIIδ knockout (KO), leaving the function of other CaMKII isoforms intact, affects ECC under physiological conditions is unknown. Therefore, CaMKIIδ KO mice were further studied. Ventricular myocytes were isolated from age- and sex-matched mice. Cell shortening and Ca fluorescence (fluo-3 or fura-2) were studied simultaneously under physiological conditions (35°C, 1 mM [Ca]o). Heart and body weight as well as cell dimensions were similar in both groups. Fractional shortening at 1 Hz was 3.02±0.16% for WT (n=57) and slightly higher 3.55±0.23% for KO (n=61, p=0.07). Similarly, Ca transient amplitude was higher in KO (F/F0 2.33±0.07, n=61 vs WT F/F0 2.15±0.05, n=57, p=0.04). This was not associated with significant changes in SR Ca content (caffeine 10 mM). Interestingly, diastolic Ca concentration as indicated by fura 2 ratio was higher in KO (0.44±0.01, n=52) than in WT (0.36±0.01, n=39). This was associated with significantly slower elimination of systolic free Ca (rt50, p<0.01). However, this extends only to frequencies up to 2 Hz, while at higher stimulation rates, there was no effect, demonstrating intact frequency-dependent acceleration of relaxation. Even under β-adrenergic stimulation (isoproterenol 4 x 10–9 M) this effect was still evident. Investigation of SR Ca leak (Ca sparks, fluo-4, 3 mM [Ca]o) showed that RyR function was not impaired by loss of CaMKIIδ (1.66±0.11 sparks/100 μm/s in WT vs 1.49±0.13 in KO myocytes). This is further corroborated by our findings that fractional SR Ca release at 1 Hz was not different between groups. Moreover, recovery during acidosis (pH 6.5) which is largely CaMKII-dependent is still present in KO myocytes. In summary, these data show that CaMKIIδ KO does not negatively influence cardiac myocyte function under physiological conditions. Further research will show whether specific inhibition of CaMKIIδ can become a novel, more targeted therapeutic approach in heart failure that does not compromise physiologic function.