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-alpha). 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-alpha in the same model. We studied the effects of cytokines on [Ca2+](i) transients (using indo-1), cell shortening (CS) (using a video motion detector) and the L-type Ca2+ channel current (I-Ca) (using the whole cell perforated patch clamp technique) in isolated guinea-pig ventricular myocytes. IL-6 (1000 U/ml) or TNF-alpha (500 U/ml) decreased both peak systolic [Ca2+], (IL-6: 0.43 +/- 0.01 to 0.40 +/- 0.01, n = 5, P < 0.05; TNF-alpha: 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 mu m, n = 5, P < 0.05; TNF-alpha: 6.7 +/- 0.7 to 5.8 +/- 0.7 mu m, n = 5, P < 0.05) without detectable reductions in I-Ca (IL-6: 0.9 +/- 0.1 to 0.9 +/- 0.1 nA, n = 4, N.S.; TNF-alpha: 1.1 +/- 0.3 to 1.1 +/- 0.2 nA, n = 4, N.S.) within 5 min. The nitric oxide synthase (NOS) inhibitor, N-G-monomethyl-L arginine (300 mu mol/l), blocked the effects of IL-6 but not of TNF-alpha. When pretreated with 20 nmol/l isoproterenol, exposure to IL-6 decreased both I-Ca (2.8 +/- 0.5 to 2.0 +/- 0.3 nA) and the amplitude of CS (10.4 +/- 2.4 to 7.5 +/- 1.9 pm) within 5 min. TNF-alpha also clearly depressed I-Ca (2.9 +/- 0.9 to 2.3 +/- 0.7 nA) and the amplitude of CS (7.0 +/- 1.4 to 5.5 +/- 1.3 mu m) in beta-adrenergic stimulated cells. TNF-alpha significantly increased the content of sphingosine (product of sphingomyelin pathway) in isolated heart, The effects of low dose sphingosine (5 mu mol/l) mimicked those of TNF-alpha on cardiac myocytes. IL-6 produced an acute negative inotropic effect through a NO-dependent pathway while TNF-alpha did so via a sphingomyelin-dependent pathway in isolated guinea-pig ventricular myocytes. (C) 1999 Academic Press.