Response to stimulation-evoked eccentric muscle contractions in hypertensive rats

Purpose: The purpose of this study was to determine whether the functional deficits observed in the skeletal muscles of adult, spontaneously hypertensive rats (SHR) arise because of an inability of injured muscles to regenerate normally in the hypertensive environment, Methods: Force decline and recovery were evaluated in SHR tibialis anterior (TA) at various times after a series of 192 eccentric contractions (TC). EC were produced by supramaximal electrical stimulation of the sciatic nerve in anesthetized rats. Experiments compared TA muscles in 3- and B-month-old SHR with TA in age-matched, normotensive Wistar-Kyoto rats (WKY) after one or three exposures to the EC protocol. The repeat exposures were separated by 10 d. Results: TA in SHR and WKY rats experienced a similar decline in strength and a similar level of recovery after one or three exposures to 192 EC. TA in both strains showed a similar 10-15% increase in dry weight and cross-sectional area after three exposures to the EC protocol. Contractile strength increased by 16-28% in WKY TA after three exposures to EC, but the increase was limited to 7% in 3-month SHR TA and was nor evident in 6-month SHR TA, despite the 15% increase in muscle mass. Conclusions: The data indicate that muscle mass and strength can increase in response to electrically evoked EC and that an increase in strength can be significantly greater than an increase in mass after the first few exposures to EC in normotensive animals. Maintained hypertension does not increase the loss of contractile strength after vigorous EC but limits or prevents the EC-induced increase in muscle strength that accompanies repeated exposure to the protocol.