Involvement of stretch-sensitive calcium flux in mechanical transduction in visceral afferents

The spinal and vagal visceral innervation to the gastrointestinal tract contains mechanosensitive afferents that are activated by contraction, distension of smooth muscle or movement in the receptive field. The mechanism by which free nerve endings detect changes in smooth muscle tension is not clear. The present study investigated the effects of mechanical stimulation on dorsal root ganglion neurons in vitro. Neurons were cultured using standard techniques and used in experiments after 24-72 h. Intracellular calcium [Ca2+]i was visualized using a video microscopic technique (Attoflour) in Fura-2 loaded neurons. DRG neurons innervating the stomach or colon were identified by the presence of a retrograde tracer, dextran-conjugated Texas Red, injected into the visceral wall 14-28 days previously. Increases in [Ca2+]i were measured in response to transient (0.5 s) mechanical stimulation of the cell soma using a flame polished probe. Approximately 25% of the whole population of DRG neurons (n = 199) were mechanosensitive, showing a transient rise in [Ca2+]i. In labeled afferents (n = 12), approximately 40% of neurons were mechanosensitive. The increase in [Ca2+]i in response to mechanical stimulation was reduced (100 microM) or abolished (250 microM) by superfusion with gadolinium or by removal of extracellular calcium. Cell somata of visceral spinal afferents show a stretch-sensitive calcium flux that may be involved in sensory transduction of mechanical stimuli that lead to autonomic and sensory reflexes.