Comparative vascular effects of stimulation continuously and in bursts of the sympathetic nerves to cat skeletal muscle

Recent electrophysiological studies indicate that sympathetic vasoconstrictor fibre dicharge in vivo usually occurs in intermittent high frequency bursts, increased activity being established by raised intra-burst frequency or, more commonly, by shortened burst intervals. The present study describes the responses of the resistance and capacitance vessels in cat skeletal muscle to electrical vasoconstrictor fibre stimulation with characteristics simulating the discharge pattern in vivo. Stimulation in bursts (1 s duration) was applied either at varying intra-burst impulse rates (from 5 to 160 Hz) at constant burst interval (5 or 10 s), or at varying burst intervals (from 32 to 2 s) at constant intra-burst frequency (16 or 32 Hz). For comparison, continuous stimulation was made at rates from 0.5 to 16 Hz. Both types of burst stimulation were capable of evoking maintained constrictions in the resistance and capacitance vessels, graded in relation to the total number of impulses/unit time. Stimulation in bursts at varying intervals was as effective as continuous stimulation with maximal constriction reached at 32 Hz bursts at 4 s intervals. The responses to stimulation in bursts at varying impulse rates peaked at 40 Hz, higher intraburst frequencies being less effective, at least in the resistance vessels. The data suggests that the resistance and capacitance vessels are gradedly responsive to sympathetic discharge rates up to about 40 Hz provided excitation occurs in bursts and, that an effective control can be achieved especially by shortening the burst interval. Sympathetic firing in skeletal muscle may thus well exceed 10 Hz, previously believed to be the upper physiological discharge range.