Reflex cardiovascular responses evoked by selective activation of skeletal muscle ergoreceptors

It is well known that theexercise pressor reflex (EPR) is mediated by group III and IV skeletalmuscle afferent fibers, which exhibit unique discharge responses tomechanical and chemical stimuli. Based on the difference in dischargepatterns of group III and IV muscle afferents, we hypothesized thatactivation of mechanically sensitive (MS) fibers would evoke adifferent pattern of cardiovascular responses compared with activationof both MS and chemosensitive (CS) fibers. Experiments were conductedin chloralose-urethane-anesthetized cats (n = 10).Passive muscle stretch was used to activate MS afferents, andelectrically evoked contraction of the triceps surae was used toactivate both MS and CS muscle afferents. No significant differenceswere shown in reflex heart rate and mean arterial pressure (MAP)responses between passive muscle stretch and evoked muscle contraction. However, when the reflex responses were matched according totension-time index (TTI), the peak MAP response (67 ± 4 vs.56 ± 4 mmHg, P < 0.05) was significantly greaterat higher TTI (427 ± 18 vs. 304 ± 13 kg · s, highvs. low TTI, P < 0.05), despite different modes ofafferent fiber activation. When the same mode of afferent fiberactivation was compared, the peak MAP response (65 ± 7 vs. 55 ± 5 mmHg, P < 0.05) was again predicted bythe magnitude of TTI (422 ± 24 vs. 298 ± 19 kg · s,high vs. low TTI, P < 0.05). Total sensory input fromskeletal muscle ergoreceptors, as predicted by TTI and not the modalityof afferent fiber activation (muscle contraction vs. passive stretch),is suggested to be the primary determinant of the magnitude of theEPR-evoked cardiovascular response.