Discharge activity of single muscle vasoconstrictor efferents in cats during opposite changes in arterial pressure

Signals of individual muscle vasoconstrictor efferents of gastrocnemius muscle were recorded in narcotized cat during rise and fall of systemic arterial pressure induced by phenylephrine and sodium nitroprusside, respectively. In control, mean discharge rate of these efferents was 2.0 0.4 Hz. Phenylephrine (45 microg/kg) increased arterial pressure from 120.3 4.2 to 170.7 8.2 mm Hg. This increase was accompanied by a short-term (5-10 sec) decrease in discharge rate of muscle vasoconstrictor efferents to 0.5 0.3 Hz followed by virtually complete recovery of muscle discharge rate against the background of increased arterial pressure. Sodium nitroprusside (30 microg/kg) decreased arterial pressure from 132.8 6.2 to 64.1 4.3 mm Hg. Under these conditions the discharge rate of vasoconstrictor efferents increased to 3.5 0.6 Hz and remained at this level throughout the hypotension period (2-3 min). Unloading of baroreceptors (occlusion of the carotid artery) increased the discharge rate of muscle vasoconstrictor efferents throughout the occlusion period (up to 30 sec). Thus, blood pressure rise and drop induced asymmetric by their duration changes in the discharge responses of muscle vasoconstrictor efferents. Phenylephrine increased asymmetry of the vasoconstrictor component of the baroreflex and induced cumulative rise of discharge rate of muscle vasoconstrictor efferents in response to a series of short-term reversible blood pressure jumps caused by repeated occlusions of the abdominal aorta. Our findings extend our knowledge on the efferent component of the baroreflex regulation and on possible mechanisms of hypertension.