Simon Anna, Lofthouse Richard A, Miti Philip, Giuraniuc Claudiu V, Banks Robert W, Bewick Guy S
School of Medicine, Medical Sciences & Nutrition, University of Aberdeen, Aberdeen, UK.
Department of Physics, University of Aberdeen, Aberdeen, UK.
Exp Physiol. 2025 Oct;110(10):1457-1472. doi: 10.1113/EP092558. Epub 2025 Jun 27.
Extracellular calcium is crucial for the normal function of muscle spindle sensory afferents. They express multiple calcium buffering proteins. Extracellular calcium is essential for recycling of synaptic-like vesicles (SLVs) in the terminals and for the stretch-evoked inward calcium current of the receptor potential. Conversely, removal of calcium from the extracellular medium abolishes stretch-evoked action potentials (APs). However, the calcium channel(s) involved and mechanism(s) of action are unknown. This study begins identifying the channels involved and their actions. Specific calcium channel toxins, agonists and antagonists were examined for effects on stretch-evoked muscle spindle afferent discharge, and live spindle sensory terminal labelling with FM1-43 was used to monitor SLV recycling in adult rat lumbrical muscle. Voltage-gated calcium channels, particularly P/Q-type (Ca2.1) and L-type (Ca1.1-1.4), strongly regulated the firing frequency of APs in response to a standard stretch, probably by regulating the opening of 'big', 'intermediate' and 'small' calcium-activated potassium channels (K), with direct evidence for BK (K1.1), SK (most likely K2.2) and IK (K3.1) involvement. Moreover, calcium from two different sources regulated separate aspects of SLV recycling. Thus, L-type channel blockers inhibited FM1-43 release, while TRPV4 (transient receptor potential, vanilloid, type 4) channel blockers entirely inhibited FM1-43 uptake. No role in SLV recycling was found for P/Q type channels, and no role at all was found for N-type (Ca2.3) channels. Overall, these studies pinpoint multiple different aspects of calcium signalling, through different channel families, and produce the first evidence of a role for a mechanosensory TRPV4 channel in muscle spindle sensory terminal function.
细胞外钙对于肌梭感觉传入纤维的正常功能至关重要。它们表达多种钙缓冲蛋白。细胞外钙对于终末中突触样小泡（SLV）的再循环以及受体电位的牵张诱发内向钙电流至关重要。相反，从细胞外培养基中去除钙会消除牵张诱发的动作电位（AP）。然而，所涉及的钙通道及其作用机制尚不清楚。本研究开始确定所涉及的通道及其作用。研究了特定的钙通道毒素、激动剂和拮抗剂对牵张诱发的肌梭传入放电的影响，并使用FM1-43对成年大鼠蚓状肌的活梭形感觉终末进行标记，以监测SLV的再循环。电压门控钙通道，特别是P/Q型（Ca2.1）和L型（Ca1.1-1.4），强烈调节AP对标准牵张的发放频率，可能是通过调节“大”、“中”和“小”钙激活钾通道（K）的开放，直接证据表明BK（K1.1）、SK（最可能是K2.2）和IK（K3.1）参与其中。此外，来自两种不同来源的钙调节SLV再循环的不同方面。因此，L型通道阻滞剂抑制FM1-43释放，而瞬时受体电位香草酸亚型4（TRPV4）通道阻滞剂完全抑制FM1-43摄取。未发现P/Q型通道在SLV再循环中起作用，也未发现N型（Ca2.3）通道有任何作用。总体而言，这些研究通过不同的通道家族确定了钙信号传导的多个不同方面，并首次证明了机械感觉TRPV4通道在肌梭感觉终末功能中的作用。
