Electrical stimulation of rats: A model for evaluating low voltage stimulation parameters

Anaesthetised rats, used either before or after exsanguination, provided a useful preparation with which to measure muscle tension from the M. triceps surae and follow pH changes from the M. longissimus dorsi due to stimulation. With square wave pulses (5 ms duration) the minimum voltage required to elicit a tension response from the M. triceps surae was 0·1 V when applied via the tibial branch of the sciatic nerve and nearly 3 V when applied directly across the muscles. Maximum responses were achieved by 2 V applied to the nerve or 5-8 V across the muscle. The fusion frequency was higher than the ox, occurring at about 30 pulses per second. There was no response (with voltages less than 8 V) from the same preparation when the rats were curarised. Tension responses of curarised rats, approaching those of normal direct stimulation, were achieved around 20-40 V. Stimulation of the whole rat carcass at 20 V resulted in a pH fall in the M. longissimus dorsi and M. triceps surae. most of which occurred in the first 30 s. The rate of fall of pH following stimulation of M. longissimus dorsi was approximately 0·6 pH units per hour, being considerably greater than the 0·4 pH units per hour for unstimulated rats. As the ultimate pH of rats was high (pH 6·1-6·3), short stimulation times had to be used to avoid asymptotic portions of the curve. Although responses of rat muscles are different from those of bovine or ovine muscles, they show many of the principles which can be used to evaluate stimulation characteristics.