TNFalpha is a
cytokine wit pleiotropic functions in many organs. In the heart increased
TNFalpha levels are not only associated with
heart failure, but also, paradoxically, with protection from ischemic damage. To test whether the protective role of
TNFalpha in the heart is concentration-dependent, we studied two mouse heart models with low (two- to threefold) over-expression of endogenous
TNFalpha: mice deficient in a translational repressor of
TNFalpha mRNA, TIA-1(-/-), and mice over-expressing human
TNFalpha. Hearts lacking TIA-1 were characterized for their endogenous
TNFalpha over-expression during normal Langendorff perfusion. To define which
TNFalpha receptor mediates cardiac protection, we also used mice lacking the
TNFR1 receptor. Contractile function was assessed in isolated hearts perfused in the isovolumic Langendorff mode during and following global no-flow ischemic stress and in response to varying extracellular [Ca(2+)] to determine their contractile response and Ca(2+) sensitivity. All hearts with low over-expression of
TNFalpha, independent of human or murine origin, have improved contractile performance and increased Ca(2+) sensitivity (by 0.2-0.26 pCa). Hearts lacking
TNFR1 have contractile performance equal to wild type hearts. Recovery from
ischemia was greater in TIA-1(-/-) and was diminished in
TNFR1(-/-). Better contractile function in
TNFalpha over-expressing hearts is not due to improved cardiac energetics assessed as [
ATP] and
glucose uptake or to differences in expression of SERCA2a or
calmodulin. We suggest that low levels of
TNFalpha increase the Ca(2+) sensitivity of the heart via a TNFR1-mediated mechanism.