Nerve-induced nonadrenergic vasoconstriction and vasodilatation in skeletal muscle

Intravital microscopy was used to study the effect of motor nerve stimulation on microvessel diameters in the rabbit tenuissimus muscle. Stimulation of the motor nerve (0.5-5 ms, 2-20 Hz, 5-15 V) evoked pulse duration- and frequency-dependent constriction of transverse and terminal arterioles. The vasoconstriction induced by low-frequency stimulation (2 Hz) was abolished by the alpha-adrenoceptor antagonist phentolamine, whereas high-frequency stimulation (10-20 Hz) resulted in a response that was only partially inhibited by phentolamine. However, desensitization of the tissue to the vasoconstrictor effects of neuropeptide Y (NPY) changed the response remaining after phentolamine into vasodilatation. Independent of stimulation parameters, pretreatment of the tissue with the adrenergic neuron blocker guanethidine reversed the constriction into dilatation that was resistant to propranolol, atropine, and indomethacin. The results document the functional presence of both vasoconstrictor and vasodilator fibers in the rabbit tenuissimus muscle motor nerve, and they suggest that part of the nerve-induced vasoconstriction at higher stimulation frequencies is caused by neuronally released NPY.