GROWTH OF ARTERIOLES IN CHRONICALLY STIMULATED ADULT-RAT SKELETAL-MUSCLE

Objective: The purpose of this study was to test the hypothesis that c apillary growth induced by chronic electrical stimulation of skeletal muscle is accompanied by the growth Of small arterioles. Methods: Lowe r limb flexor muscles of Sprague-Dawley rats were stimulated by electr odes implanted in the vicinity of the peroneal nerve at 10 Hz for 8 h/ d for 2 and 7 days. Cryostat sections from the proximal, middle, and d istal regions of the extensor digitorum longus muscle (EDL) were fluor escently immunolabeled with alpha-smooth muscle actin (alpha SMA) and myosin heavy chain (MHC) to identify mature (alpha SMA and MHC-positiv e) and immature (alpha SMA-positive, MHC-negative) arterioles. The flu orescent derivative of the lectin Griffonia simplicifolia I (GSI) was used to identify all microvessels, including arterioles; capillaries, and venules. Results: The number of vessels positive for GSI or alpha SMA surrounding muscle fibers was similar in all three muscle regions (proximal, middle; distal). The mean values +/- SEM for GSI-positive v essels from all regions were similar in control (4.3 +/- 0.07) and 2-d ay stimulated (4.7 +/- 0.08) but higher in 7-day stimulated muscles (6 .7 +/- 0.1, p < 0.05), thus confirming the previous findings on capill ary growth. A similar increase was found in the number of alpha SMA-po sitive vessels less than or equal to 10 mu m outer diameter (1.3 +/- 0 .09 versus 0.4 +/- 0.03 around muscle fibers in controls). The density of terminal arterioles (less than or equal to 10 mu m) was slightly b ut not significantly higher after 2 days of stimulation (19.5 +/- 4 ve rsus 15.6 +/- 2 profiles/mm(2) in control muscles) and significantly h igher after ? days (33 +/- 7). While a similar increase was observed i n the density of preterminal arterioles >10 mu m (17 +/- 3 control, 22 +/- 3 at 2 days and 40 +/- 5 at 7 days), the density of MHC-positive vessels muscles stimulated for 7 days was unchanged. Seven-day stimula ted muscle also had a fivefold higher density of microvessel profiles less than or equal to 10 mu m that were only partially surrounded by a lpha SMA. This considerably exceeds the relative increase in the numbe r of capillaries and thus supports the concept of arteriolar growth by transformation from capillaries. Conclusions: Chronic electrical stim ulation results in an early increase in the number of immature (MHC-ne gative), but not mature (MHC-positive) arterioles, a process that acco mpanies the increase in capillarization. The great increase in the num ber of microvessels only partially covered bs alpha SMA suggests arter iolization of capillaries as a contributing mechanism in this growth.