T-tubule uncoupling from the plasma membrane leads to myocardial contractile abnormalities.

The development of autoimmune diabetes in NOD mice results from selective destruction of beta-cells by a T-cell-dependent autoimmune process. However, the mechanisms that control the generation of beta-cell cytotoxic T-cells in vivo are poorly understood. We recently established 8.3-T-cell receptor (TCR)-beta transgenic NOD mice that show a selective acceleration of the recruitment of CD8+ T-cells into the islets of prediabetic animals, resulting in rapid beta-cell destruction and early onset of diabetes. This study was initiated to determine the role of macrophages in the development and activation of diabetogenic CD8+ T-cells in 8.3-TCR-beta transgenic NOD mice. Inactivation of macrophages in these transgenic mice resulted in the complete prevention of diabetes. When splenic T-cells from macrophage-depleted 8.3-TCR-beta transgenic NOD mice were transfused into severe combined immunodeficiency disease (NOD.scid) mice, none of the recipients developed diabetes up to 10 weeks after transfer, while most of the recipients of T-cells from age-matched control 8.3-TCR-beta transgenic NOD mice became diabetic. When intact NOD islets were transplanted under the renal capsule of macrophage-depleted 8.3-TCR-beta transgenic NOD mice, the majority of the grafted islets remained intact, while most of the islets grafted into age-matched, control 8.3-TCR-beta transgenic NOD mice were destroyed within 3 weeks after transplantation. The depletion of macrophages in these mice resulted in a decrease in the Th1 immune response along with an increase in the Th2 immune response because of significant decreases in the expression of macrophage-derived cytokines, particularly interleukin-12, and a decrease in beta-cell-specific T-cell activation, as shown by significant decreases in the expression of Fas ligand (FasL), CD40 ligand (CD40L), and perforin, as compared with control mice. We conclude that macrophages are absolutely required for the development and activation of beta-cell cytotoxic CD8+ T-cells in 8.3-TCR-beta transgenic NOD mice.

Starling's Law and the well-known end-systolic pressure-volume relationship (ESPVR) of the left ventricle reflect the effect of sarcomere length (SL) on stress (sigma) development and shortening by myocytes in the uniform ventricle. We show here that tetanic contractions of rat cardiac trabeculae exhibit a sigma-SL relationship at saturating [Ca2+] that depends on sarcomere geometry in a manner similar to skeletal sarcomeres and the existence of opposing forces in cardiac muscle shortened below slack length. The sigma-SL-[Ca2+]free relationships (sigma-SL-CaR) at submaximal [Ca2+] in intact and skinned trabeculae were similar, albeit that the sensitivity for Ca2+ of intact muscle was higher. We analyzed the mechanisms underlying the sigma-SL-CaR using a kinetic model where we assumed that the rates of Ca2+ binding by Troponin-C (Tn-C) and/or cross-bridge (XB) cycling are determined by SL, [Ca2+] or stress. We analyzed the correlation between the model results and steady state stress measurements at varied SL and [Ca2+] from skinned rat cardiac trabeculae to test the hypotheses that: (i) the dominant feedback mechanism is SL, stress or [Ca2+]-dependent; and (ii) the feedback mechanism regulates: Tn-C-Ca2+ affinity, XB kinetics or, unitary XB-force. The analysis strongly suggests that feedback of the number of strong XBs to cardiac Tn-C-Ca2+ affinity is the dominant mechanism that regulates XB recruitment. Application of this concept in a mathematical model of twitch-stress accurately reproduced the sigma-SL-CaR and the time course of twitch-stress as well as the time course of intracellular [Ca2+]i. Modeling of the response of the cardiac twitch to rapid stress changes using the above feedback model uniquely predicted the occurrence of [Ca2+]i transients as a result of accelerated Ca2+ dissociation from Tn-C. The above concept has important repercussions for the non-uniformly contracting heart in which arrhythmogenic Ca2+ waves arise from weakened areas in cardiac muscle. These Ca2+ waves can reversibly be induced in muscle with non-uniform excitation contraction coupling (ECC) by the cycle of stretch and release in the border zone between the damaged and intact regions. Stimulus trains induced propagating Ca2+ waves and reversibly induced arrhythmias. We hypothesize that rapid force loss by sarcomeres in the border zone during relaxation causes Ca2+ release from Tn-C and initiates Ca2+ waves propagated by the sarcoplasmic reticulum (SR). These observations suggest the unifying hypothesis that force feedback to Ca2+ binding by Tn-C is responsible for Starling's Law and the ESPVR in uniform myocardium and leads in non-uniform myocardium to a surge of Ca2+ released by the myofilaments during relaxation, which initiates arrhythmogenic propagating Ca2+ release by the SR.

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