Simonides W S, Van Hardeveld C
Laboratory for Physiology, Free University, Amsterdam, The Netherlands.
Biochim Biophys Acta. 1988 Aug 18;943(2):349-59. doi: 10.1016/0005-2736(88)90567-6.
The formation and maintenance of Ca2+-filling levels by sarcoplasmic reticulum vesicles from euthyroid (control) and hypothyroid skeletal muscle were investigated using the Ca2+-indicator quin-2, at [Ca2+] in the medium [( Cao2+]) of 0.05-0.3 microM. Rapid ATP-dependent Ca2+ uptake resulted in a steady-state Ca2+-filling level, Cai2+, within one minute. This Ca2+ gradient was maintained for at least three minutes, during which less than 20% of the ATP was consumed. Cai2+ was maximal (120 nmol/mg) for [Cao2+] greater than 0.3 microM and decreased to 40 nmol/mg at [Cao2+] of 0.05 microM. Preparations from both experimental groups showed qualitatively and quantitatively the same relationship between Cai2+ and [Cao2+] at steady state, despite a significantly lower Ca2+-pump content of hypothyroid sarcoplasmic reticulum, which resulted in a 25% lower maximal (Ca2+ + Mg2+)-ATPase activity. Maintenance of the steady state, at all levels of Cai2+, was associated with net ATP consumption by the Ca2+ pump and cycling of Ca2+, which processes were 30% slower in the hypothyroid group as compared to the control group. Determination of the passive efflux of Ca2+, as well as the fraction of leaky or unsealed sarcoplasmic reticulum fragments, excluded either of these possibilities as an explanation for the relatively high (Ca2+ + Mg2+)-ATPase rates at steady state. On the basis of these and previously reported results, it is concluded that the maintenance of a Ca2+ gradient by sarcoplasmic reticulum under physiological conditions with respect to external [Ca2+] and the concentrations of ATP, ADP and Pi, is associated with the cycling of Ca2+ coupled to net ATP hydrolysis. Using the obtained data it is calculated that the sarcoplasmic reticulum may account for 20% of the resting metabolic rate in skeletal muscle. Consequently, together with the previously reported lower sarcoplasmic reticulum content of skeletal muscle in hypothyroidism, we calculate that about one third of the decrease in basal metabolic rate in this thyroid state can be related to the alterations of the sarcoplasmic reticulum.
利用钙离子指示剂喹啉-2，在培养基中钙离子浓度（[CaO2+]）为0.05 - 0.3微摩尔的条件下，研究了来自甲状腺功能正常（对照）和甲状腺功能减退的骨骼肌肌浆网囊泡中钙离子填充水平的形成和维持情况。快速的ATP依赖性钙离子摄取在一分钟内导致了稳定状态的钙离子填充水平，即胞内钙离子浓度（Cai2+）。这种钙离子梯度维持了至少三分钟，在此期间消耗的ATP不到20%。当[CaO2+]大于0.3微摩尔时，Cai2+达到最大值（120纳摩尔/毫克），而在[CaO2+]为0.05微摩尔时降至40纳摩尔/毫克。尽管甲状腺功能减退的肌浆网中钙离子泵含量显著降低，导致最大（钙离子 + 镁离子）-ATP酶活性降低25%，但两个实验组的制剂在稳定状态下Cai2+与[CaO2+]之间在质量和数量上显示出相同的关系。在所有Cai2+水平下维持稳定状态与钙离子泵消耗净ATP以及钙离子循环有关，与对照组相比，甲状腺功能减退组的这些过程慢30%。对钙离子的被动外流以及渗漏或未封闭的肌浆网片段比例的测定排除了这两种可能性作为稳定状态下相对较高的（钙离子 + 镁离子）-ATP酶速率的解释。基于这些以及先前报道的结果，可以得出结论，在生理条件下，肌浆网相对于外部[Ca2+]以及ATP、ADP和Pi的浓度维持钙离子梯度与钙离子循环以及净ATP水解有关。利用获得的数据计算得出，肌浆网可能占骨骼肌静息代谢率的20%。因此，结合先前报道的甲状腺功能减退时骨骼肌中肌浆网含量较低的情况，我们计算出在这种甲状腺状态下基础代谢率降低的约三分之一可能与肌浆网的改变有关。
