Disuse atrophy increases the muscle mechanoreflex in rats.

We investigated the effect of disuse atrophy on the magnitude of the muscle mechanoreflex. The left leg of eight rats (6-7 wk, male) was put in a plaster cast for 1 wk. The rats were decerebrated at the midcollicular level. We recorded the pressor and cardioaccelerator responses to 30-s stretch of the calcaneal tendon, which selectively stimulated the muscle mechanosensitive receptors in the left atrophied and right control triceps surae muscles. Atrophied muscles showed significantly lower mass control muscles (1.0 +/- 0.1 vs. 1.4 +/- 0.1 g; P < 0.05). At the same stretch tension (229 +/- 20 g), the pressor response to stretch was significantly greater in the atrophied muscles than in the control muscles (13 +/- 3 vs. 4 +/- 2 mmHg, P < 0.05). The cardioaccelerator response was not significantly different (8 +/- 4 vs. 4 +/- 2 beats/min). Comparing responses at the same relative tension (57 +/- 6 vs. 51 +/- 8% of maximal tension), the pressor response was still significantly greater in the atrophied triceps surae than in the control (14 +/- 4 vs. 4 +/- 2 mmHg; P < 0.05). These results suggest that disuse atrophy increases the magnitude of muscle mechanoreflex.