Response of the microcirculation in rat cremaster muscle to peripheral and central sympathetic stimulation.

Microvascular diameter and flow responses to peripheral and central sympathetic stimulation were measured in different segments of the arteriolar network in rat cremaster muscle. For peripheral stimulation, a bipolar electrode was placed on the internodal segment of the lumbar sympathetic chain between ganglia L1 and L2. For central stimulation, a bipolar electrode was stereotaxically implanted in the posterior hypothalamus. Inside vessel diameter, red blood cell velocity, and volumetric flow rate were recorded in response to electrical stimulations of varying magnitude in four series-coupled segments of the arteriolar network: 1A, 2A, 3A, and 4A. Systemic arterial pressure was also monitored. The vasoconstriction and flow reduction produced by stimulation of the lumbar chain was graded with the frequency of stimulation over the range of 0.5-16.0 Hz in all arteriolar segments. Examination of the relation between stimulation frequency and vasoconstrictor response measured as percent of control diameter indicated a sequence of responsiveness to peripheral stimulation where 4A = 3A greater than 2A = 1A. No changes in diameter were recorded in the venous microcirculation at any level of stimulation. Stimulation of the posterior hypothalamus with currents of 38-300 microA for 60 seconds produced graded vasoconstriction in only 3A and 4A vessels. Fluorescence histochemistry for biogenic amines was used to examine the distribution of innervation to the microvasculature. All segments of the arteriolar network from 1A to 4A possessed an adrenergic innervation; no vessels of the venous network were found to be innervated. The results indicate that the pattern of response of the arteriolar network in rat cremaster muscle to peripheral and central sympathetic stimulation is segmentally differentiated and consistent with the distribution of the vasomotor innervation.