COMPARATIVE EFFECTS OF ISCHEMIA AND ACUTE HYPOXEMIA ON MUSCLE AFFERENTS FROM TIBIALIS ANTERIOR IN CATS

Comparative effects of ischemia and acute hypoxemia (PaO2 = 24 mm Hg) were studied in anesthetized cats on afferents from the tibialis anterior limb muscle. Metaboreceptors (groups Ill and IV fibers) and mechanoreceptors were identified by their activation by an intraarterial injection of lactic acid (LA) or high-frequency vibrations (HFV) applied to the extremity of the muscle tendon, respectively. Ischemia and hypoxemia exerted opposite influences on the two categories of muscle afferents: they depressed the response of mechanoreceptors to HFV, but markedly enhanced the spontaneous tonic activity of metaboreceptors. The effects of hypoxemia were delayed but slightly greater and lasted longer during the recovery period than those exerted by ischemia. The inhibitory action on mechanoreceptors exerted by a reduced oxygen supply to muscle is interpreted as a result from local acidosis. Indeed, under normoxic conditions, a LA bolus injection during the HFV test also reduced the firing rate of these receptors.