Calcium regulation of muscle spindle mechanosensory afferent function

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 (Ca_v2.1) and L-type (Ca_v1.1-1.4), 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_Ca), with direct evidence for BK (K_Ca1.1), SK (most likely K_Ca2.2) and IK (K_Ca3.1) 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 (Ca_v2.3) 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.