Reflex cardiovascular responses evoked by selective activation of skeletalmuscle ergoreceptors

It is well known that the exercise presser reflex (EPR) is mediated by grou p III and IV skeletal muscle afferent fibers, which exhibit unique discharg e responses to mechanical and chemical stimuli. Based on the difference in discharge patterns of group III and IV muscle afferents, we hypothesized th at activation of mechanically sensitive (MS) fibers would evoke a different pattern of cardiovascular responses compared with activation of both MS an d chemosensitive (CS) fibers. Experiments were conducted in chloralose-uret hane-anesthetized cats (n = 10). Passive muscle stretch was used to activat e MS afferents, and electrically evoked contraction of the triceps surae wa s used to activate both MS and CS muscle afferents. No significant differen ces were shown in reflex heart rate and mean arterial pressure (MAP) respon ses between passive muscle stretch and evoked muscle contraction. However, when the reflex responses were matched according to tension-time index (TTI ), the peak MAP response (67 +/- 4 vs. 56 +/- 4 mmHg, P < 0.05) was signifi cantly greater at higher TTI (427 +/- 18 vs. 304 +/- 13 kg.s, high vs. low TTI, P < 0.05), despite different modes of afferent fiber activation. When the same mode of afferent fiber activation was compared, the peak MAP respo nse (65 +/- 7 vs. 55 +/- 5 mmHg, P < 0.05) was again predicted by the magni tude of TTI (422 +/- 24 vs. 298 +/- 19 kg.s, high vs. low TTI, P < 0.05). T otal sensory input from skeletal muscle ergoreceptors, as predicted by TTI and not the modality of afferent fiber activation (muscle contraction vs. p assive stretch), is suggested to be the primary determinant of the magnitud e of the EPR-evoked cardiovascular response.