Fig. 15

Effects of teeth clenching on Gz-induced hypotension. Mean responses of arterial pressure at the level (brain AP) of seven rats without anti-G equipment exposed to +1.5 Gz acceleration for 30 s. Teeth clenching was induced by electrical stimulation of bilateral masseter muscles of rats (20 tetanic contractions for 30 s, at a rate of 1 per s at 50 Hz for 0.4 s with pulse duration 1 ms). To measure clenching force, a 0.65-mm-thick pressure sensor (PSM-type, Kyowa Electric Instruments, Tokyo, Japan) was placed on the right lower molar teeth of rats. The electrical stimulation produced a teeth clenching force of about 5 N. Acceleration of +1.5 Gz for 30 s decreased brain AP by 18.3 ± 2.0 mmHg in seven rats without anti-G equipment when without electrical stimulation of masseter muscles (no clenching), whereas it did not significantly decrease brain AP (1.9 ± 2.0 mmHg) in the same seven rats without anti-G equipment when stimulated of masseter muscles (teeth clenching), indicating that the teeth clenching can protect the animal from the 1.5 Gz-induced hypotension. Then, dantrolene, a postsynaptic skeletal muscle relaxant, was administered to reduce masseter muscle contraction even when electrically stimulated in the same seven rats. Dantrolene markedly reduced clenching force and abolished the teeth clinching-evoked protection from 1.5-Gz-induced hypotension (decrease by 18.9 ± 2.6 mmHg for the teeth clenching with Dantrolene group) [37, 48, 49]