Cellular Basis for the Acute Inhibitory Effects of IL-6 and TNF- α on Excitation-contraction Coupling

There is controversy over whether nitric oxide (NO) mediates acute negative inotropic actions of cytokines including tumor necrosis factor-alpha (TNF- α). The reports from established laboratories have appeared inconsistent, which could be due to species differences. Thus, we tried to elucidate the mechanisms underlying negative inotropic actions of interleukin-6 (IL-6) and TNF- α in the same model. We studied the effects of cytokines on [Ca2+]itransients (using indo-1), cell shortening (CS) (using a video motion detector) and the -type Ca2+channel current (ICa) (using the whole cell perforated patch clamp technique) in isolated guinea-pig ventricular myocytes. IL-6 (1000 U/ml) or TNF- α (500 U/ml) decreased both peak systolic [Ca2+]i(IL-6: 0.43±0.01 to 0.40±0.01, n=5, P<0.05; TNF- α: 0.42±0.02 to 0.39±0.02, n=5, P<0.05) and the amplitude of CS (IL-6: 7.5±0.9 to 6.2±0.5 μ m, n=5, P<0.05; TNF- α: 6.7±0.7 to 5.8±0.7 μ m, n=5, P<0.05) without detectable reductions in ICa(IL-6: 0.9±0.1 to 0.9±0.1 nA, n=4, N.S.; TNF- α: 1.1±0.3 to 1.1±0.2 nA,n =4, N.S.) within 5 min. The nitric oxide synthase (NOS) inhibitor, NG-monomethyl- arginine (300 μ mol/l), blocked the effects of IL-6 but not of TNF- α. When pretreated with 20 nmol/l isoproterenol, exposure to IL-6 decreased both ICa(2.8±0.5 to 2.0±0.3 nA) and the amplitude of CS (10.4±2.4 to 7.5±1.9 μ m) within 5 min. TNF- α also clearly depressed ICa(2.9±0.9 to 2.3±0.7 nA) and the amplitude of CS (7.0±1.4 to 5.5±1.3 μ m) inβ -adrenergic stimulated cells. TNF- α significantly increased the content of sphingosine (product of sphingomyelin pathway) in isolated heart. The effects of low dose sphingosine (5 μ mol/l) mimicked those of TNF-α on cardiac myocytes. IL-6 produced an acute negative inotropic effect through a NO-dependent pathway while TNF-α did so via a sphingomyelin-dependent pathway in isolated guinea-pig ventricular myocytes.