Role of Sympathetic and Parasympathetic Nervous Systems in Heart Rate Regulation in Cats

The effects of atropine and β-adrenoceptor blockers on mean HR, wave structure of the cardiac rhythm, and chronotropic reaction to noise stress were examined in cats. Atropine (0.5 mg/kg) increased the mean HR and significantly decreased the spectrum power of HF, LF, and VLF oscillations. The decrease in HF power was most pronounced, which enhanced LF/HF ratio. Propranolol (0.5 mg/kg) decreased the mean HR and slightly increased the power of HF, LF, and VLF oscillations. Atenolol (2 mg/kg) exerted similar but more pronounced effects. β-Adrenoceptor blockers increased HF power to a greater extent than LF and VLF power, which led to a decrease in LF/HF ratio. Atropine markedly decreased the chronotropic reaction to stress. β-Adrenoceptor blockers produced no effect on the amplitude of this reaction, but accelerated restoration of initial HR. It is established that the changes in power spectrum of HR and the phase reflex reactions in cats are mediated by parasympathetic influences; the sympathetic system is involved only in the control of mean HR, probably in response to the level of animal activity. The changes in LF/HF ratio during blockade of sympathetic and parasympathetic systems are caused by opposite influences of these systems on HF oscillations, but not by hypothetic “sympathetic” and “parasympathetic” nature of LF and HF oscillations of the heart rhythm.